Earthquake Report: Madagascar!

Today there was an earthquake in a region that we don’t hear about that often. Madagascar is off the coast of southeastern Africa and the oceanic basin to the west is likely formed as part of the East Africa Rift system, but also due to the post Gondwana plate tectonics. Madagascar was once part of India, back when India was part of Gondwana.

Today’s (and of the last few days) earthquakes are located along the Comoros Archipelago, volcanic islands formed from hotspot volcanism.

There exist fracture zones in this region. Below, we see that these fracture zones have been interpreted as right-lateral strike-slip faults. However, the relative offsets of magnetic anomalies (and spreading ridges) show that these faults are instead left-lateral. So, that is my interpretation for this M 5.8 earthquake, a left-lateral strike-slip earthquake. I placed white dashed lines in the poster below to show where some of these fracture zones may be located, based upon the magnetic anomaly data (EMAG2).

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.8 earthquake.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include some inset figures.

  • In the upper center right is a figure from James Wood and Alex Guth, showing the rift systems in eastern Africa.
  • In the upper left corner is a figure showing the estimated (reconstructed) location of Madagascar within Gondwana (Reeves, 2014).
  • In the lower left corner is a map that shows the East Africa Rift system, along with the offshore branch (associated with the Davie fracture zone). The M 5.8 earthquake is just to the east of the larger scale map (Franke et al., 2015).
  • In the upper right corner is a figure that shows a reconstruction of the position of Madagascar (Phethean et al., 2017). The blue lines are pathway lines showing how Madagascar moved away from Africa. Spreading ridges are shown in red. The offsets of the spreading ridges show left lateral strike slip offsets of these ridges.
  • In the lower right corner is a map that shows the free-air gravity data that Phethean et al. (2017) used to make their reconstruction. Note that they interpret these faults as right lateral strike-slip. This interpretation is in contrast to the relative offsets of the oceanic spreading ridges. I placed a blue star in the general location of today’s M 5.8 earthquake.


USGS Earthquake Pages

Some Relevant Discussion and Figures

  • Here is the plate reconstruction figure from Phethean et al. (2017).

  • Plate tectonic reconstruction of Madagascar’s escape from Africa from the Early Jurassic to the cessation of spreading in the Cretaceous. Madagascar is shown without the remainder of East Gondwana (India, Antarctica, and Australia) attached. (a) Present-day sediment thickness in the Western Somali Basin taken from the CRUST1.0 model. (b–e) The key stages of Madagascar’s motion out of Africa. Modeled flow lines are shown as blue-arrowed lines where the center of symmetry is marked by orange circles. (f) Madagascar’s present-day position, which is reached at around 125 Ma. Flow lines closely match the fracture zone pattern of the basin (additional black lines), and the basin’s predicted final symmetry (orange circles) lies in good agreement with the interpreted extinct mid-ocean ridge system (red lines). Locations of magnetic anomalies used to temporally constrain plate motions shown with symbols as interpreted by Davis et al. [2016].

  • Here is the summary figure, showing their interpretation of the different fracture zones (Phethean et al., 2017).

  • (a) Commonly interpreted basin configuration, where the continent-ocean transition is assumed to follow the DFZ [e.g., Bunce and Molnar, 1977; Coffin and Rabinowitz, 1987; Gaina et al., 2013]. (b) Schematic of the basin configuration suggested in this study, with strike-slip tectonics dominating along the edge of the Rovuma Basin, while much of the Tanzania Coastal Basin should be considered as an obliquely rifted margin. The Davie Fracture Zone is a major ocean-ocean fracture zone, not the continent-ocean transform margin. DFZ, Davie Fracture Zone; DHOW, Dhow Fracture Zone; VLCC, Very Large Crude Carrier Fracture Zone; ARS, Auxiliary Rescue and Salvage Fracture Zone. (c) Free-air gravity overlain with interpretation as for Figure 9b

  • This is the map from Franke et al. (2015) showing the EAR system, including the offshore branch, the Davie Ridge. These authors work offshore and use seismic reflection and bathymetric data to show the extension in the offshore basins, as they respond to EAR extension.

  • General geological overview of the study area. Dark grey lines indicate the position of geophysical profiles acquired during R/V Sonne cruise SO-231 in 2014. Earthquake locations and magnitudes (1973–2014; mb>4.0) are shown as magenta circles according to the National Earthquake Information Center catalog and earthquake moment tensors from the Global Centroid Moment Tensor catalog [Ekström et al., 2012]. The Lurio Belt separates the northern from the southern high-grade metamorphic basement of northern Mozambique [Emmel et al., 2011]. The inlay shows the main faults of the western and eastern branches of the East African Rift System (from Chorowicz [2005] and Macgregor [2015]).

  • Emerick and Duncan (1983) demonstrated the age progression for the volcanic islands in this region. Below is a map showing the paths for the Cororos and Seychelles hotspots.

  • Hotspot paths predicted by African absolute motions [3], which are shown as solid lines connected by circles of 20 m.y. increments, are systematically offset from the observed paths for the Comores (A) and Reunion (B) hotspots, outlined by the 2000 m bathymetric contour. The difference between predicted and observed paths can be used to determine Somali-African relative motion between 0 and 10 m.y.B.P. For the period 10-60 m.y. the predicted paths parallel the observed paths, indicating no significant relative motion prior to about 10 m.y. ago. The reported ages for the Comores trend are from this paper and reference 7; for the Reunion trend, from references 24-26, 37, 38.

  • Here is a plot showing the ages for the rocks studied by Emerick and Duncan (1982).

  • A. Distance from present hotspot activity at Grande Comore, measured along the trend of the Comores Islands to Seychelles Islands lineament, is plotted against ages of initial volcanism at several localities (Table 1, and [7,9]). The solid circles represent best age estimates of initial volcanism, whereas the open circles represent minimum ages of volcanism at each site. A rate of migration of volcanism of 50 mm/yr best fits the new K-Ar ages for shield-building lavas at Grande Comore and Mayotte and the minimum age of volcanism in northern Madagascar. Igneous activity in the Seychelles at about 40 m.y. B.P. is consistent with this trend. Generalized topography from reference 4. B. Reported ;adiometric ages along the Reunion hotspot trend [24,25] yield a rate of migration of volcanism of 44 mm/yr. An early Oligocene age for DSDP site 238 on the southern end of the Chagos-Laccadive Ridge 1261 provides a minimum age for the Nazarene Bank region of the Mascarene Plateau, which was sundered from the Chagos-Laccadive Ridge by spreading across the Central Indian Ridge about 32 m.y. ago.

  • This is a fantastic plot that shows how hotspot volcanism has a finite time at the surface (for any given location) as the plate moves across the hotspot (Emerick and Duncan, 1982).

  • Duration of volcanism at oceanic islands is proportional to the inverse of plate velocity over mantle hotspots, which determines how long magmas are available for eruption. The dashed curve fits the maximum observed eruptive histories. Other data fall below this line due to incomplete sampling or unfinished volcanism.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Social Media

    References:

  • Franke, D., W. Jokat, S. Ladage, H. Stollhofen, J. Klimke, R. Lutz, E. S. Mahanjane, A. Ehrhardt, and B. Schreckenberger (2015), The offshore East African Rift System: Structural framework at the toe of a juvenile rift, Tectonics, 34, 2086–2104, doi:10.1002/2015TC003922.
  • Phethean, Jordan J.J. and Kalnins, Lara M. and van Hunen, Jeroen and Biffi, Paolo G. and Davies, Richard J. and McCaffrey, Ken J.W., 2016. Madagascar’s escape from Africa : a high-resolution plate reconstruction for the Western Somali Basin and implications for supercontinent dispersal in Geochemistry, geophysics, geosystems., 17 (12). pp. 5036-5055.
  • Reeves, C. 2014. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci., http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011

Posted in africa, earthquake, education, geology, Indian Ocean, plate tectonics, strike-slip

Earthquake-Volcanic Eruption Report: Hawai’i

My USGS Earthquake Notification Service email inbox has been going on overtime.

There has been a swarm of earthquakes on the southeastern part of the big island, with USGS volcanologists hypothesizing about magma movement and suggesting that an eruption may be imminent. Here is a great place to find official USGS updates on the volcanism in Hawaii (including maps). I had been following this on social media.

Here is a temblor blogpost that I wrote. Here is a Spanish version.

Hawaii is an active volcanic island formed by hotspot volcanism. The Hawaii-Emperor Seamount Chain is a series of active and inactive volcanoes formed by this process and are in a line because the Pacific plate has been moving over the hotspot for many millions of years.

As these volcanoes grow with time, the flanks of the volcanoes become covered in new volcanic rock. The flanks become unstable and collapse as landslides. There is evidence that some of these landslides trigger some of the largest tsunami ever found.

The seismicity started in the central part of the “East Rift Zone” (ERZ), a region of extension probably caused by flank collapse. This extension lowers pressure in the magma chamber, leading to eruptions. Magma migrates around for various reasons, including changes in pressure in the magma chamber. These motions of magma and fluids can cause earthquakes.

This part of Hawaii is the locus of the most recent volcanism, with the newest volcanic center formed to the southeast of the island.

Southeast of the main Kilauea vent, the Pu‘u ‘Ö‘ö crater saw an elevation of lava into the crater, leading to overtopping of the crater (on 4/30/2018). Seismicity migrated eastward along the ERZ. This morning, there was a M 5.0 earthquake in the region of the Hilina fault zone (HFZ). I was getting ready to write something up, but I had other work that I needed to complete. Then, this evening, there was a M 6.9 earthquake between the ERZ and the HFZ.

There have been earthquakes this large in this region in the past (e.g. the 1975.1.29 M 7.1 earthquake along the HFZ). This earthquake was also most likely related to magma injection (Ando, 1979). The 1975 M 7.1 earthquake generated a small tsunami (Ando, 1979). These earthquakes are generally compressional in nature (including the earthquakes from today).

Today’s earthquake also generated a tsunami as recorded on tide gages throughout Hawaii. There is probably no chance that a tsunami will travel across the Pacific to have a significant impact elsewhere.

So. This M 6.9 may be the largest earthquake. There may be a larger one in store (the USGS suggests that these fault systems could produce a M 8 earthquake). The eruptions may be done for now. There may be more.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5 in a second poster (and down to M ≥ 3.5 in a third poster).

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.9 earthquake, in addition to some relevant historic earthquakes.

I placed green circles in the locations of the (a) 4/30 lava lake filling event and (b) 5/3-4 fissure eruption.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I include some inset figures.

  • In the upper right corner is a geologic map with color representing the relative age of volcanic deposits (Sherrod et al., 2007). (red = youngest, orange next youngest) I placed a blue circle in the location of the vents that erupted on 5/3-4.
  • In the upper left corner is a map that shows the rift zones (active extensional volcanism) and the region is divided by the major sources for the volcanic rocks (e.g. Mauna Kea, Mauna Loa, and Kilauea; Tilling et al., 2010). I placed a blue circle in the location of the vents that erupted on 5/3-4.
  • In the lower left corner is a visualization showing the magma reservoir hypothesized to be the source of lava along the Southwest and East Rift zones, as well as for Kilauea (Tilling et al., 2010).
  • In the lower right corner is a map that shows the relative severity of volcanic hazard for the island of Hawaii (Tilling, et al., 2010).
  • To the left of the hazard map is a geological cross section showing the subsurface structures in the region (USGS).


  • This version includes earthquakes M ≥ 4.5


  • This version includes earthquakes M ≥ 3.5 (note the seismicity offshore to the south, this is where the youngest Hawaii volcano is).


USGS Earthquake Pages

  • I put together a short video that shows seismicity from the past month. This reveals how the magma may have moved throughout the region between 4/27 and 5/4. Here is a link to download the ~8 MB mp4 video file.

Some Relevant Discussion and Figures

  • Here is the map showing the rifts (Tilling et al., 2010)

  • Shaded relief map of the southeastern part of the Island of Hawai‘i, showing the principal features and localities of Mauna Loa, Kïlauea, and Lö‘ihi Volcanoes discussed in the text.

  • This is the figure that shows an hypothetical configuration of the magma reservoir beneath Kilauea (Tilling et al., 2010).

  • Cut-away view looking deep beneath Kïlauea Volcano, showing the shallow magma reservoir and the principal magma passageways. Areas in yellow are the most favorable zones for magma movement (arrows show direction) and storage. Though greatly generalized, this depiction of Kïlauea’s “plumbing system” is compatible with all known scientific information. (Simplified from technical illustration of Michael P. Ryan, USGS.)

  • Below are a series of plots from tide gages installed at several sites in the Hawaii Island Chain. These data are all posted online here and here.
  • Hilo, Hawaii

  • Kawaihae, Hawaii

  • Here is a plot showing the tsunami from the 1975 M 7.1 earthquake (Ando, 1979). On the left are modeled tsunami wave height based on two different fault models (each with fault dips of 20 degrees, but widths of 20km and 30km).

  • This is a timeline of historic volcanism on Hawaii (Tilling et al., 2010).

  • Graph summarizing the eruptions of Mauna Loa and Kïlauea Volcanoes during the past 200 years. The Pu‘u ‘Ö‘ö- Kupaianaha eruption has continued into the 21st century. Information is sketchy for eruptions before 1823, when the first missionaries arrived on the Island of Hawai‘i. The total duration of eruptive activity in a given year, shown by the length of the vertical bar, may be for a single eruption or a combination of several separate eruptions.

  • Here is the volcanic hazard severity map from Tilling et al. (2010).

  • Map of Island of Hawai‘i showing the volcanic hazards from lava flows. Severity of the hazard increases from zone 9 to zone 1. Shaded areas show land covered by flows erupted in the past two centuries from three of Hawai‘i’s five volcanoes (Hualälai, Mauna Loa, and Kïlauea).

  • Below is a series of maps that shows the recent volcanism in the region (Orr et al., 2012).

  • The first 3½ years of the Pu‘u ‘Öÿö eruption of Kïlauea Volcano (January 1983–June 1986) were dominated by episodic lava fountains that constructed the Pu‘u ‘Öÿö cone and fed ‘a‘ä flows (the less fluid of the two types of Hawaiian lava flows) (USGS photo by J.D. Griggs, June 1984). The map shows lava flows erupted from Kïlauea Volcano in the 19th and 20th centuries (gray). These flows originated from the summit caldera, the East Rift Zone, or the Southwest Rift Zone (not shown). Flows erupted during the first 3½ years of the Pu‘u ‘Öÿö eruption are shown in red. The Island of Hawai‘i (see inset map) is composed of five volcanoes—Kohala, Mauna Kea, Hualälai, Mauna Loa, and Kïlauea


    In 1986, the Pu‘u ‘Öÿö eruption shifted to the Kupaianaha vent. This photo shows lava flows erupted from Kupaianaha entering the community of Kalapana on the Island of Hawai‘i’s southeast coast in May 1990 (USGS photo by J.D. Griggs). Over the following months, Kalapana was almost completely destroyed, and lava filled beautiful nearby Kaimü Bay. The map shows lava flows erupted from Kupaianaha and nearby fissures during 1986–1992 in red. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.


    Lava flows erupt from new vents on the south flank of the Pu‘u ‘Öÿö cone (right side of photo) that opened after Pu‘u ‘Öÿö Crater filled to overflowing in early 2004 (USGS photo by Richard Hoblitt, January 2004). Collapse of the southwest side of the cone formed a scallop-shaped scar, revealing red layers of welded spatter (deposited as clots of molten lava) that under-lie loose tan-colored pyroclastic deposits (hot debris ejected during an eruption). The map shows flows erupted from Pu‘u ‘Öÿö and from fissures in Näpau Crater during 1992–2007 in red. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.


    A lava channel, elevated as much as 150 feet (45 meters) above the adjacent terrain, transports lava away from the Fissure D vent, which opened in July 2007 (USGS photo by James Kauahikaua, October 2007). The “perched” (elevated) lava channel was the main path for lava until November 2007, when lava was diverted from the vent to the southeast. Pu‘u ‘Öÿö is at upper right. The map shows lava flows erupted in Pu‘u ‘Öÿö and from the Fissure D vent between Pu‘u ‘Öÿö and Kupaianaha during 2007–2011 in red. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.


    In March 2011, lava broke to the surface between Pu‘u ‘Öÿö and Näpau Crater marking the start of the Kamoamoa fissure eruption. In this photo lava erupts from the fissure shortly after the beginning of the eruption and pours into a deep, older crack (USGS photo by Tim Orr). The map shows flows erupted during the Kamoamoa eruption and from Pu‘u ‘Öÿö during 2011–2012 in red. The Kamoamoa flows are to the left, flows from the August 2011 Pu‘u ‘Öÿö flank breakout are at center, flows from a fissure high on Pu‘u ‘Öÿö’s northeast flank are to the right. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.

  • This map highlights the seismicity associated with volcanism related to the youngest volcano in the Hawaii Islands (Tilling et al., 2010).

  • Map showing the locations of earthquakes that occurred during the 1970s and in the July–August 1996 period in the vicinity of Lö‘ihi. These earthquake swarms, plus similar occurrences in 1984–85 and the early 1990s, provide seismic evidence that Lö‘ihi is an active submarine volcano.

  • The ERZ and HFZ are also actively deforming between earthquakes. Below are two maps that show (a) regional vertical land motion and (b) results from block modeling to resolve the differential motion across this area (Shirzaei et al., 2013).

  • The linear velocity field in the line of sight of the descending-orbit Envisat satellite (track 200) over the Kilauea south flank from 2003 till 2010. Area of the study, Hilina Fault System (HFS), is outlined by dashed box. Location of GPS stations used is marked by their names next to filled squares colored by the mean rate of motion in the LOS direction. Station PGF1 is the reference for both GPS and InSAR datasets. WHP = western Hilina Pali, HP = Holei Pali.


    Colored panels represent relatively coherently moving blocks based on the InSAR deformation over the HFS according to Figs. 2–4 and traces of mapped faults, which are used to compare with GPS data. Each block is labeled by its average LOS velocity.

Geologic Fundamentals

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great educational video from the USGS that discusses eruptions in 2011, which were similar in type and style of eruptions as the current phase of eruption. Here is a link to the 4 MB mp4 video file.
  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Social Media

UPDATE 5/5

    References:

  • Ando, M., 1979. The Hawaii Earthquake of November 29, 1975: Low Dip Angle Faulting Due to Forceful Injection of Magma in JGR, v. 84, no. B13
  • Orr, T.R., et a., 2012. The Ongoing Pu‘u ‘Ö‘ö Eruption of Kïlauea Volcano, Hawai‘i—30 Years of Eruptive Activity in USGS Fact Sheet 2012-3127, 2013.
  • Sherrod, D.R., Sinton, J.M., Watkins, S.E., and Brunt, K.M., 2007, Geologic Map of the State of Hawaii: U.S. Geological Survey Open-File Report 2007-1089, 83 p., 8 plates, scales 1:100,000 and 1:250,000, with GIS database
  • Tilling, R.I., Keliker, C., and Swanson, D.A., 2010. Eruptions of Hawaiian Volcanoes—Past, Present, and Future, U.S. Geological Survey, General Information Product 117, 72 pp.
  • Torsvik, T.H., et al., 2017. Pacific plate motion change caused the Hawaiian-Emperor Bend in Nature Communications, DOI: 10.1038/ncomms15660

Posted in earthquake, education, geology, pacific, plate tectonics, volcanoes

Earthquake Report: Channel Islands Update #1

Well well.

There was lots of interest in this M 5.3 earthquake offshore of Ventura/Los Angeles, justifiably so. Southern California is earthquake country.

Here is an update. There was lots of information that I was trying to incorporate and I needed an additional report to cover some of this material. That being said, there is still some mystery about this earthquake. My favored interpretation is that this EQ was a left-lateral strike-slip earthquake. There is still room to interpret this as a right-lateral strike-slip (llss) earthquake however.

Below I have prepared some figures that provide additional information that helps us learn about the faulting and basin development in the CA Borderlands here. There is lots of work that has been done here and this is far from a comprehensive analysis.

As I mentioned before (here is my initial Earthquake Report for this EQ), due to the big bend in the San Andreas fault (SAF) in southern CA, there is evidence for compression in the form of thrust faults and uplifted mountains (e.g. Sierra Madre fault and the San Gabriel Mtns). One of these thrust faults (which may also have some strike-slip motion) is the Hollywood fault (recently highlighted by the recent work by the CA Geological Survey).

Also part of the development of the SAF involved the clockwise rotation of a crustal block where the Transverse Ranges are (the mtns to the north of Ventura/Santa Barbara). Along the southern boundary of the Transverse Ranges formed left-lateral strike slip faults. The Santa Cruz Island fault just happens to be a left-lateral strike-slip fault.

The CA Borderlands is a complex region of faulting, inheriting structures from the Tertiary, overprinted by modern tectonics and everything in between. The Hollywood fault trends towards (and turns into?) the Malibu Coast fault, which may turn into the Santa Cruz Island fault (SCIF), a vertical left-lateral strike-slip fault (but may have some vertical motion on it, based upon offsets in vertical uplift rates from marine terrace profiles).

Schindler used seismic reflection profiles in the Santa Cruz Basin area to interpret the tectonic history here. I placed the faults interpreted by them as orange lines in the interpretive poster (labeled as the Ferrelo fault and the East Santa Cruz (ECS) Basin fault system). The ESCBFS is a thrust fault system, with possible oblique motion (strike-slip). My initial interpretation was that this M 5.3 was a llss earthquake associated with this fault. There are some interesting problems that arise considering this fault. To the south, the fault is oriented similar to the San Clemente fault (which may have had a M 5.5 right-lateral strike-slip (rlss) earthquake on 1981.09.04). Due to this, the simple interpretation is that the ESCBFS is right lateral oblique at the southern part of the Santa Cruz Basin. However, along the northern boundary of this basin, the ESCBFS rotates to an east-west strike (orientation). The simple interpretation would be that this part of the fault system would be llss, similar to the SCIF. So, clearly, things are not so simple here. See the Chaytor et al. (2008) figure below.

That being said, if this M 5.3 earthquake was on an east-west fault, it would be llss. There is no evidence for a north-south oriented fault on the western boundary of the Santa Cruz Basin (see Schindler (2007) seismic profile below), supporting the left-lateral interpretation.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.3 earthquake, in addition to some relevant historic earthquakes (including the 1971 Sylmar and 1994 Northridge earthquakes, as evidence for the compression in the region).

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the upper left corner is a cross section from Shaw and Suppe (1994). This cross section location is shown on the interpretive poster as a blue line labeled X-Y. This cross section (from interpretations of offshore seismic reflection profiles) shows the major player here is a thrust fault, the Channel Islands Thrust. Note the SCIF is also shown to rip right through Santa Cruz Island.
  • In the upper right corner is a map that shows the area of this fault ramp of the Channel Islands Thrust (Shaw and Suppe, 1994). Note that this fault ramp area is also shown on the interpretive poster, outlined in light orange.
  • In the center left is a figure from Fuis et al. (2001) that shows a block diagram revealing how the north-south convergence (from the bend in the San Andreas) is accommodated by thrust/reverse faults. The Sierra Madre fault is also labeled on the interpretive poster. A recent earthquake in La Habra is an example of this north-south compression. Here are my report and report update for this M 5.1 La Habra earthquake.
  • In the lower left corner is a seismic reflection profile from Schindler (2007), from her Master’s Thesis. The profile A-A’ is shown on the map as a green line labeled A-A’. Note that there is no faulting on the western boundary of the Santa Cruz Basin. When I first looked at this section, I thought that the ESCBFS were either normal (extensional) or strike-slip faults. After reading her thesis, I learned that these faults did have normal offset (in the Miocene Epoch, part of the Tertiary Period), but have been reactivated as thrust faults in post-Miocene time. The San Clemente fault (labeled on the interpretive poster) turns into the Santa Cruz-Catalina Ridge fault (labeled on this cross section).
  • In the lower right corner is a figure that shows how these faults interact in a complicated manner (Sorlien et al., 2006). This figure was prepared after they interpreted seismic reflection profile data. The upper panel is a low-angle oblique view of the faults in 3-D view. The lower two panels are the cross sections B-B’ and E-E’ (also shown on the interpretive poster as orange lines). These cross sections show how the Malibu Coast fault is more deeply dipping (more close to vertical) compared to the Santa Monica-Dume fault (a shallow dipping thrust fault). Both of these faults appear to join in some way near the coast, where they turn into the Hollywood fault. There are probably some inaccuracies in how I am interpreting how these faults interact beyond the limit of the figures I present here.


  • Here is the same map including the magnetic anomaly data (the red and blue shades).


USGS Earthquake Pages

Some Relevant Discussion and Figures

  • Here is a map that shows where the seismic profile was acquired (Shaw and SUppe, 1994).

  • Epicenters from an earthquake swarm in 1984 (Henyey and Teng, 1985) define the active axial surface (A) of the Offshore Oak Ridge trend. Single-event (C and D) and composite (E and F) focal mechanism solutions from the 1984 seismicity have gentle north dipping (C, D, and E) and horizontal (F) nodal planes (Henyey and Teng, 1985) consistent with folding through the active axial surfaces by bedding parallel slip (see Figure 10B). Cross section traces: X-X’ (Fig. 7); X-Y (Fig. 11). SCIF = Santa Cruz Island fault.

  • Here is the cross section. The upper panel shows the modern configuration and the lower panel shows their interpretation during the Tertiary (Shaw and Suppe, 1994).

  • A balanced geologic cross section across the eastern Santa Barbara Channel and Santa Cruz Island combines subsurface seismic reflection and well-log data (the section trace is in Figs. 1 and 10A). The Channel Islands thrust ramps beneath the Offshore Oak Ridge trend and approaches the surface south of Santa Cruz Island. The kink-band width (A-A’) of the Offshore Oak Ridge trend represents dip slip on the underlying Channel Islands thrust. The shallow fold and fault geometry along the Offshore Oak Ridge and Blue Bottle trends is depicted in Figure 7. Strike-slip motion out of the section plane may occur on the Santa Cruz Island fault; however, moderate displacements on this fault should not significantly effect our area balance and restoration, because the strike-slip fault trace is perpendicular to the section plane (Fig. 10A). SCIF = Santa Cruz Island fault. Horizontal equals vertical scale.

  • For background, here is a timeline for the tectonics along the Pacific-North America plate boundary (Schindler, 2007). The Transverse Ranges block is shown as a green bleb labeled WTR. Note how this block is rotating in a clockwise fasion, and see that there are strike-slip faults that form along the block edge to accommodate this rotation.

  • A simple tectonic model of the evolution of the Pacific-North American plate boundary that includes the Inner and Outer Borderland (IB, OB) and rotation of the western Transverse Ranges (WTR) province (from Nicholson et al, 1994). The model assumes a constant rate and direction of Pacific plate motion and constant rate of western Transverse Ranges rotation. As each partially subducted microplate is captured by the Pacific plate (Monterey, ~19 Ma; Arguello, ~17.5 Ma; Guadalupe and Magdalena, ~12 Ma), this results in a transfer of part of the over-riding North American upper plate to the Pacific plate. The fine gray lines provide a reference grid fixed to North America. ArP-Arguello plate; GP-Guadalupe plate; MtP-Monterey plate; SG-San Gabriel block; JdFP-Juan de Fuca plate; SLB-San Lucia Bank; SMB-Santa Maria basin; SB-southern Borderland;T-AFTosco- Arbreojos fault; MP-Magdalena plate. Red areas are regions of transtension; Purple areas are captured or soon to be captured microplates.

  • Here is the seismic reflection profile from Schindler (2010).

  • Regional seismic line WC82-108 showing the ~50 km wide Santa Rosa Ridge anticlinorium. Parallel bedding of pre-Pliocene strata indicates that this anticlinal structure formed post Miocene. The Cretaceous-Paleogene sedimentary rocks are eroded by the early Miocene unconformity (green) and truncate against basement (black arrows). Mapped reference horizons and faults are shown in color and in black, respectively.

  • This is a fantastic low-angle oblique view of the topography and bathymetry of this region (and the Santa Cruz Basin) from Schindler (2010). The figure caption is embedded in the figure.

  • This is the figure from Schindler (2010) that shows the geometry of the ESCBFS and Ferrlo faults. Red shows the upper part of the faults. These faults dip to the north, northeast, and east.

  • A map view of 3D fault surfaces surrounding Santa Cruz basin in the northern Borderland. Depths down-dip along fault surfaces are shown as changing colors at even kilometer levels. The ESCB fault system is observed to be a gently east- to northeast-dipping, right stepping, en echelon reactivated reverse or oblique-reverse fault that bends to become more northerly dipping as it approaches Santa Cruz Island.

  • There has been lots of work here. Jason Chaytor (now at USGS in Woods Hole) worked on submerged marine terraces in this region. These marine terraces were formed when sea level was lower and are a result of erosion from ocean waves at that time. Dr. Chaytor used radiometric ages and sea level curve data to evaluate the tectonic uplift in the region. Here is a map that shows Jason’s interpretation of the seismic profiles for this region (same seismic data used by Schindler).

  • Preliminary map of geologic structures currently mapped using multichannel sparker, and recently released WesternGeco multichannel seismic-reflection profiles (modified from Chaytor, 2006). SCIF—Santa Cruz Island fault.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Social Media

    References:

  • Chaytor, J.D., Goldfinger, C., Meiner, M.A., Huftile, G.J., Romsost, C.G., Legg, M.R., 2008. Measuring vertical tectonic motion at the intersection of the Santa Cruz–Catalina Ridge and Northern Channel Islands platform, California Continental Borderland, using submerged paleoshorelines in GSA Bulletin, v. 120, no. 7/8, p. 1053-1071, doi: 10.1130/B26316.1
  • Du, X., Hendy, I., Schimmelmann, 2018. A 9000-year flood history for Southern California: A revised stratigraphy of varved sediments in Santa Barbara Basin in Marine Geology, v. 397, p. 29-42, https://doi.org/10.1016/j.margeo.2017.11.014
  • Fuis, G.S., Ryberg, T., Godfrey, N.J., Okaya, D.A., Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1, p. 15-18
  • Legg, M. R., M. D. Kohler, N. Shintaku, and D. S. Weeraratne, 2015. Highresolution mapping of two large-scale transpressional fault zones in the California Continental Borderland: Santa Cruz-Catalina Ridge and Ferrelo faults, J. Geophys. Res. Earth Surf., 120, 915–942, doi:10.1002/2014JF003322.
  • Pinter, N., Lueddecke, S.B., Keller, E.A., Simmons, K.R., 1998. Late Quaternary slip on the Santa Cruz Island fault, California in GSA Bulletin, v. 110, no. 6, p. 711-722
  • Pinter, N., Johns, B., Little, B., Vestal, W.D., 2001. Fault-Related Folding in California’s Northern Channel Islands Documented by Rapid-Static GPS Positioning in GSA Today, May, 2001
  • Schindler, C.S., 2010. 3D Fault Geometry and Basin Evolution in the Northern Continental Borderland Offshore Southern California Catherine Sarah Schindler, B.S. A Thesis Submitted to the Department of Physics and Geology California State University Bakersfield In Partial Fulfillment for the Degree of Masters of Science in Geology
  • Shaw, J.H., Suppe, J., 1994. Active faulting and growth folding in the eastern Santa Barbara Channel, California in GSA Bulletin, v. 106, p. 607-626
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [https://pubs.er.usgs.gov/publication/pp1515].

Posted in collision, earthquake, education, geology, los angeles, pacific, plate tectonics, San Andreas, strike-slip

Earthquake Report: Channel Islands

I was finally getting around to writing a report for the deep Bolivia earthquake (Bolivia report here), when a M 5.3 earthquake struck offshore of the channel islands (south of Santa Cruz Island, west of Los Angeles). As is typical when an earthquake hits a populated region in the USA, the USGS websites stopped working (for the earthquakes in South America I was researching). After about half an hour or so, the websites started working again (the M 5.3 earthquake website never had a problem).

The Los Angeles region is dominated by the tectonics associated with the North America – Pacific transform plate boundary system of the San Andreas fault (SAF). The SAF accommodates the majority of plate motion between these two plates. There are sister faults where some of the plate boundary motion also goes. This plate boundary extends from the Pacific Ocean eastwards to Utah (the Wasatch fault system).

The SAF is considered a “mature” strike-slip fault because it is straight along most of the system. We think that strike-slip faults start out as smaller faults that develop as tectonic strain enters a region that is different from prior strain. As time passes, these smaller faults join each other, to align with the great circle aligned to the euler pole (the axis of rotation for plates).

The SAF does bend in some places, most notably in southern CA. This bend creates complexities in the fault, but also results in north-south compression (and thrust faults) forming the Transverse Ranges north of the LA Basin. Recent work by the California Geological Survey has been focusing on these thrust faults as they strike (trend) through Hollywood. These thrust faults are oriented east-west.

There are also additional faults offshore of LA in what is called the borderlands. Many of these faults are sub-parallel to the SAF. The best example is the Newport Inglewood fault (NIF), the locus of the 1933 Long Beach Earthquake. This fault is offshore, but also extends onshore. The NIF is generally a northwest-southeast striking right lateral strike-slip fault just like the SAF.

Some of the east-west faults also extend offshore. Onshore, they are generally thrust faults, but less is known about what they do offshore (i.e. they could have some strike-slip motion too).

Today’s earthquake happened south of Santa Catalina Island, where there is a major fault system that runs through the island: the Santa Cruz Island fault. This fault is mostly a left-lateral strike-slip fault, with a small portion of reverse (compression) motion (Pinter et al, 1998, 2001).

To the north of SC Island, is the Santa Barbara Basin, an oceanic basin that preserves an excellent record of flood and earthquake triggered sedimentary deposits.

If today’s M 5.3 is possibly related to the faults that form the Santa Cruz Basin. I provide some maps of this region below the interpretive poster. Based upon the work conducted by Schindler for their MS Thesis, Today’s earthquake appears associated with the East Santa Cruz Basin fault system (supporting that this was a left-lateral strike-slip earthquake). This is not included in the USGS active fault and fold database, but today’s earthquake suggests that it could be added.

These sedimentary basins are most likely formed from extension when the orientation of strike slip faults is not parallel to the plate motion. These are called “pull apart” basins and are a result of “transtension.” Do an internet search for more about transtension and how pull apart basins can form.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.3 earthquake, in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include some inset figures.

  • On the right side of the poster are figures from Wallace (1990) and show the main faults associated with the SAF system. I place a blue star in the general location of today’s earthquake (as also in other places on this poster).
  • To the upper left of the Wallace SAF map for California is a figure also from Wallace (1990) that shows more details, including elevation information (color = height or depth).
  • To the lower left of the Wallace SAF map for CA is a figure that shows the high resolution bathymetry (seafloor shape) for the Santa Cruz Basin.
  • In the upper left corner is a seismotectonic map of the CA Borderlands (Legg et al., 2015). They show faults and their sense of motion. There are also focal mechanisms for historic earthquakes.
  • In the lower left corner is a larger scale map of this region, showing the faults as mapped by Schindler (2007).


USGS Earthquake Pages

Some Relevant Discussion and Figures

  • Here is the figure showing the evolution of the SAF since its inception about 29 Ma. I include the USGS figure caption below as a blockquote.

  • EVOLUTION OF THE SAN ANDREAS FAULT.

    This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)

  • Here is a map that shows the shaking potential for earthquakes in CA. This comes from the state of California here. Note how Santa Cruz Island has an increased chance of hazard due to the Santa Cruz Island fault.

  • Earthquake shaking hazards are calculated by projecting earthquake rates based on earthquake history and fault slip rates, the same data used for calculating earthquake probabilities. New fault parameters have been developed for these calculations and are included in the report of the Working Group on California Earthquake Probabilities. Calculations of earthquake shaking hazard for California are part of a cooperative project between USGS and CGS, and are part of the National Seismic Hazard Maps. CGS Map Sheet 48 (revised 2008) shows potential seismic shaking based on National Seismic Hazard Map calculations plus amplification of seismic shaking due to the near surface soils.

  • Here is a map that shows the tectonic provides in this region (Legg et al. (2015). While the region inherits topography and geologic structures from past tectonic regimes, the dominant tectonic control here is currently the North America – Pacific plate boundary.

  • Map of the California Continental Borderland showing major tectonic features and moderate earthquake locations (M >5.5). The dashed box shows area of this study. The large arrows show relative plate motions for the Pacific-North America transform fault boundary (~N40° ± 2°W; RM2 and PA-1 [Plattner et al., 2007]). BP = Banning Pass, CH = Chino Hills, CP = Cajon Pass, LA = Los Angeles, PS = Palm Springs, V = Ventura, ESC = Santa Cruz Basin, ESCBZ = East Santa Cruz Basin fault zone, SCI = Santa Catalina Island, SCL = San Clemente Island, SMB = Santa Monica Basin, and SNI = San Nicolas Island. Base map from GeoMapApp/Global Multi-Resolution Topography (GMRT) [Ryan et al., 2009].

  • This shows the timeline of what has controlled the tectonics in this region (Legg et al., 2015).

  • Chronology of major Cenozoic events in the Southern California region (after Wright [1991] and Legg and Kamerling [2012]). Intensity of tectonic deformation is represented by the curve. Local (Los Angeles Basin) biostratigraphic zonation is shown. The slanted labels for Neogene stages represent the time-transgressive nature of these boundaries.

  • Here is the figure with more details about the tectonic interpretation of the area (Legg et al., 2015)

  • Map showing bathymetry, Quaternary faults, and recent seismicity in the Outer Borderland. Fault locations are based on the high-resolution bathymetry, available high-resolution seismic reflection profiles, and published fault maps [cf. California Geological Survey (CGS), 2010]. The red symbols show magnitude-scaled (M>4) epicenters for seismicity recorded for the period of 1932 to 2013. Seismicity data and focal mechanisms are derived from the Southern California Seismograph Network catalogs, National Earthquake Information Center [2012–2013], and Legg [1980]. Focal mechanism event numbers correspond to Table S2 in the supporting information. The black rectangle shows location of Figure 10. The light blue lines show tracklines of multichannel seismic profiles—the labeled white profiles are shown in Figures 12 (124) and 13 (108 and 126).

  • Here is the summary figure from Legg et al. (2015). This helps us put these faults systems into context.

  • Map showing major active tectonic elements of the northern part of the California Continental Borderland. Major active (Quaternary) faults are shown in red (SAF = San Andreas fault, ABF = Agua Blanca fault, SCF = San Clemente fault, and SCCR = Santa Cruz-Catalina Ridge, Ferrelo). Major strike-slip offsets are shown by shaded areas with estimated displacement (EK = Emery Knoll crater; Tanner Basin near DB = Dall Bank; and SDT = San Diego Trough, small pull-apart near Catalina). Other symbols show oblique fault character including transpressional restraining bends (CAT = Santa Catalina Island, CB = Cortes Bank, and TB = Tanner Bank), uplifts (SRI = Santa Rosa Island, SCz = Santa Cruz Island, SNI = San Nicolas Island, CB = Cortes Bank, TB = Tanner Bank, and SBM = San Bernardino Mountains), and transtensional pull-apart basins (SD = San Diego, ENS = Ensenada, SCB = San Clemente Basin, and SIB = San Isidro Basin). The large arrows show Pacific-North America relative plate motions with the blue dashed line (PAC-NAM) along a small circle for the RM2 [Minster and Jordan, 1978] plate motions model through San Clemente Island (SCL). Boundary between the Inner and Outer Borderland follows the East Santa Cruz Basin fault zone (dotted line; modified from Schindler [2010] and De Hoogh [2012]). Holocene volcanoes exist along the coast (SQ= San Quintín) and within the Gulf of California Rift (CP = Cerro Prieto and Obsidian Buttes, Salton Trough). Dates show year of earthquakes with mapped focal mechanisms (see Table S2 in the supporting information). SB = Santa Barbara, LA = Los Angeles, and PS = Palm Springs.

  • The Santa Barbara Basin to the north has an excellent Holocene record of floods and earthquakes (Du et al., 2018). Here is a plot showing the ages of possible earthquake triggered turbidites (submarine landslide deposits) from the Santa Barbara Basin.

  • Probability density functions (PDFs) for the 19 turbidites (olive layers) in core MV0811-14JC and core SPR090106KC in Santa Barbara Basin generated from Bacon 2.2. Brackets show 95% confidence intervals. Estimate emergence times of the Newport-Inglewood Fault (Leeper et al., 2017) in pink, Ventura- Pitas Point Fault (Rockwell et al., 2016) in green, Ventura blind thrust fault (McAuliffe et al., 2015) in purple, Compton Thrust Fault (Leon et al., 2009) in yellow and the Goleta Slide Complex (Fisher et al., 2005)in gray. Age of slumped material in 14JC is indicated by wavy texture. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

  • As I mentioned, there is some uplift associated with compression along the Santa Cruz Island fault (Pinter et al., 2001). This plot shows uplift across the region in the form of uplifted marine terraces. This plot assumes these marine terraces were formed at the same time, so if there were no differential tectonic uplift, these lines would be horizontal.

  • Cross-sectional profile A-B-C on Santa Rosa Island (see Fig. 3) showing corrected terrace elevations. SRIF shows the locations of the Santa Rosa Island fault. Error bars are the sum of the ±1 s uncertainties in wave-cut platform slope and the GPS measurement errors. Note the change in vertical exaggeration between the lower and upper plots. The green curve was qualitatively fit to the T2 data in order to create the smoothest possible curve that conforms to all points; other curves are scaled versions of the T2 curve. Point spacing is too coarse and error bars too large on the other levels to show deformation details, but the scaled curves show that every measured point is consistent with the pattern measured on T2.

  • This is a diagram that shows how a pull apart basin might form (Wu et al., 2009).

  • General characteristics of a pull-apart basin in a dextral side-stepping fault system. The pull-apart basin is defined to develop in pure strike-slip when alpha = 0 degrees and in transtension when 0 degrees < alpha 45 degrees.

  • This figure shows the results of modeling in clay, showing a pull apart basin form (Wu et al., 2009).

  • Plan view evolution of transtensional pull-apart basin model illustrated with: (a) time-lapse overhead photography; and (b) fault interpretation and incremental basin subsidence calculated from differential laser scans. Initial and final baseplate geometry shown with dashed lines; (c) basin topography at end of experiment.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Social Media

    References:

  • Du, X., Hendy, I., Schimmelmann, 2018. A 9000-year flood history for Southern California: A revised stratigraphy of varved sediments in Santa Barbara Basin in Marine Geology, v. 397, p. 29-42, https://doi.org/10.1016/j.margeo.2017.11.014
  • Legg, M. R., M. D. Kohler, N. Shintaku, and D. S. Weeraratne, 2015. Highresolution mapping of two large-scale transpressional fault zones in the California Continental Borderland: Santa Cruz-Catalina Ridge and Ferrelo faults, J. Geophys. Res. Earth Surf., 120, 915–942, doi:10.1002/2014JF003322.
  • Pinter, N., Lueddecke, S.B., Keller, E.A., Simmons, K.R., 1998. Late Quaternary slip on the Santa Cruz Island fault, California in GSA Bulletin, v. 110, no. 6, p. 711-722
  • Pinter, N., Johns, B., Little, B., Vestal, W.D., 2001. Fault-Related Folding in California’s Northern Channel Islands Documented by Rapid-Static GPS Positioning in GSA Today, May, 2001
  • Schindler, C.S., 2010. 3D Fault Geometry and Basin Evolution in the Northern Continental Borderland Offshore Southern California Catherine Sarah Schindler, B.S. A Thesis Submitted to the Department of Physics and Geology California State University Bakersfield In Partial Fulfillment for the Degree of Masters of Science in Geology
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [https://pubs.er.usgs.gov/publication/pp1515].

Posted in earthquake, education, geology, los angeles, pacific, plate tectonics, San Andreas, strike-slip, Transform

Earthquake Report: Bolivia!

A couple days ago there was a deep focus earthquake in the downgoing Nazca plate deep beneath Bolivia. This earthquake has an hypocentral depth of 562 km (~350 miles).

We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.

As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.

So, we don’t really know what causes earthquakes at the depth of this Bolivia M 6.8 earthquake. Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhtosk Earthquake.

There are lots of examples in this region of South America for deep earthquakes. They are all extensional (normal fault) earthquakes.

One option could be “fluid-assisted faulting,” in which water is released from minerals as they change phases during faulting, thus lubricating the plates, Lay says.

But although this is a common mechanism for earthquakes between 70 and 400 kilometers deep, it’s unlikely to be the cause of this quake because the plate is significantly dewatered by the time it reaches 400 kilometers deep. Minerals releasing carbon dioxide as they are compacted could provide an alternative fluid to lubricate the fault, he says, much like water does at shallower depths.

And another possibility is that a transition in mineral form from low-pressure polymorphs (the form in which a mineral is stable at the surface) to high-pressure polymorphs (a denser form of a mineral that is stable at greater depths), gives the fault a start. According to this model, the plate subducts too quickly for the mineral to slowly transition to its denser form. The mineral will reach depths greater than where it is normally stable, and thus the transformation may be a catastrophic process, causing a jolt at 600 kilometers, which would allow for movement along the fault, Lay says.

There have been a number of deep earthquakes globally in the past several years. These include the 2013 M 8.3 in the Sea of Okhtosk, the 2015 M 7.8 along the Izu-Bonin Arc, and several along the central Andes. I present some interpretive posters for these earthquakes below.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.8 earthquake, in addition to some relevant historic earthquakes.

I include the magnetic anomaly data (EMAG2) which helps reveal the structure of the Nazca plate.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the upper right corner is a plate tectonic map from Hu et al. (2016), which shows the major plate boundaries in the region. The subduction zone is indicated as a black line with triangles (the triangles show the direction that the Nazca plate is subducting below the South America plate). I place a cyan star in the general location of this M 6.8 earthquake (as in other figures).
  • In the upper left corner is a part of the map from Hayes et al. (2015) that shows historic seismicity. Below the map is a cross section showing seismicity. This is the cross section C-C’ shown on the map above in cyan.
  • In the lower right corner is part of the seismic hazard map for South America (Hayes et al., 2015). Color represents the relative amount of shaking a location may experience in the next 50 years (“10% in 50 years peak acceleration”). Yellow areas may experience 1.6-3.2 m/s^2 (gravity is 9.8 m/s^2). Green may experience between 0.8-1.6 m/s^2.

USGS Earthquake Pages


Some Relevant Discussion and Figures

  • Here is an animation from IRIS that reviews the tectonics of the Peru-Chile subduction zone. For the animation, first is a screen shot and below that is the embedded video. This animation is from IRIS. Written and directed by Robert F. Butler, University of Portland. Animation and Graphics: Jenda Johnson, geologist. Consultant: Susan Beck, University or Arizona. Narration: Elayne Shapiro, University of Portland.

  • Here is a download link for the embedded video below (34 MB mp4)
  • The Goes et al. (2017) paper presents an excellent review of the various forces and earthquake types along subduction zones globally. This paper is open source and free to download. Below are some summary figures.
  • This shows the general relations between various forces exerted on a subducting slab.

  • Schematic diagram showing the main forces that affect how slabs interact with the transition zone. The slab sinks driven by its negative thermal buoyancy (white filled arrows). Sinking is resisted by viscous drag in the mantle (black arrows) and the frictional/viscous coupling between the subducting and upper plate (pink arrows). To be able to sink, the slab must bend at the trench. This bending is resisted by slab strength (curved green arrow). The amount the slab needs to bend depends on whether the trench is able to retreat, a process driven by the downward force of the slab and resisted (double green arrow) by upper-plate strength and mantle drag (black arrows) below the upper plate. At the transition from ringwoodite to the postspinel phases of bridgmanite and magnesiowüstite (rg – bm + mw), which marks the interface between the upper and lower mantle, the slab’s further sinking is hampered by increased viscous resistance (thick black arrows) as well as the deepening of the endothermic phase transition in the cold slab, which adds positive buoyancy (open white arrow) to the slab.

    By contrast, the shallowing of exothermic phase transition from olivine to wadsleyite (ol-wd) adds an additional driving force (downward open white arrow), unless it is kinetically delayed in the cold core of the slab (dashed green line), in which case it diminishes the driving force. Phase transitions in the crustal part of the slab (not shown) will additionally affect slab buoyancy. Buckling of the slab in response to the increased sinking resistance at the upper-lower mantle boundary is again resisted by slab strength.

  • Here is a plot showing their summary of observations for various subduction zones globally.

  • Summary of morphologies of transition-zone slabs as imaged by tomographic studies and their Benioff stress state. Arrows on the map indicate the approximate locations of the cross sections shown around the map, with their points in downdip direction. Blue shapes are schematic representations of slab morphologies (based on the extent of fast seismic anomalies that were tomographically resolvable from the references listed). Horizontal black lines indicate the base of the transition zone (~660 km depth). For flattened slabs, the approximate length of the flat section is given in white text inside the shapes. For penetrating slabs, the approximate depth to which the slabs are continuous is given in black text next to the slabs. Circles inside the slabs indicate whether the mechanisms of earthquakes at intermediate (100–350 km) and deep (350–700 km) are predominantly downdip extensional (black) or compressional (white). Stress states are from the compilations of Isacks and Molnar (1971), Alpert et al. (2010), Bailey et al. (2012), complemented by Gorbatov et al. (1997) for Kamchatka, Stein et al. (1982) for the Antilles, McCrory et al. (2012) for Cascadia, Papazachos et al. (2000) for the Hellenic zone, and Forsyth (1975) for Scotia. The subduction zones considered are (from left to right and top to bottom): RYU—Ryukyu, IZU—Izu, HON—Honshu, KUR—Kuriles, KAM—Kamchatka, ALE—Aleutians, ALA—Alaska, CAL—Calabria, HEL—Hellenic, IND—India, MAR—Marianas, CAS—Cascadia, FAR—Farallon, SUM—Sumatra, JAV—Java, COC—Cocos, ANT—Antilles, TON—Tonga, KER—Kermadec, CHI—Chile, PER—Peru, SCO—Scotia. Numbers next to the red subduction zone codes refer to the tomographic studies used to define the slab shapes

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Social Media

Posted in earthquake, education, geology, pacific, plate tectonics, subduction

Earthquake Report: New Britain

Well, those earthquakes from earlier, one a foreshock to a later one, were foreshocks to an earthquake today! Here is my report from a couple days ago. The M 6.6 and M 6.3 straddle today’s earthquake and all have similar hypocentral depths.

There are no major updates in this report compared to the earthquake report from a few days ago. I did add a couple figures from Dr. Stephen Hicks that they posted on their twitter feed. Dr. Hicks is currently working on a paper that uses seismicity to characterize the subduction zone here.

The New Britain region is one of the more active regions in the world. See a list of earthquake reports for this region at the bottom of this page, above the reference list.

In the poster below, the recent seismicity in Papua New Guinea and New Britain is evident.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend).

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for these earthquakes. I also include moment tensors from earthquakes on the Island of New Guinea (2018.02.25) and near New Ireland (2018.03.08).

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the upper left corner are two figures from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” To the right of the map is a cross section showing how the Solomon Sea plate is subducting beneath New Britain. This is from Johnson, 1976. I place a blue star in the general location of today’s earthquake (as also for other inset figures).
  • In the lower right corner is another generalized tectonic map of the region from Holm et al., 2015.
  • In the upper right corner are figures from Dr. Stephen Hicks. On the left is a map that shows seismicity in this region. There is a blue star in the location of the M 6.9 earthquake and a yellow star (and focal mechanism) for the M 6.6 earthquake. Dr. Hicks plots seismicity as a cross section on the right (with the spatial extent oulined in a dashed rectangle on the map).


USGS Earthquake Pages

    Here are some recent reports for other seismicity in the region.

  • On 2018.02.25 there was a M 7.5 earthquake on the island of New Guinea. Here is my report update on that sequence. Below are my original poster and an aftershock update poster.


  • On 2018.03.08 there was an > 6.8 earthquake in New Ireland that also shows up on the map today. Here is my report on this earthquake. Below is my interpretive poster (with and without magnetic anomaly data).


Some Relevant Discussion and Figures

  • This is a map showing the seismicity of this region since 2000 A.D.

  • Earlier, I discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

  • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

  • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

  • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

  • This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012). I include the figure caption below as a blockquote.

  • Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

  • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). The New Britain region is in the map near the A and B sections. I include the figure caption below as a blockquote.

  • Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

Social Media

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Posted in earthquake, education, geology, plate tectonics, subduction

Earthquake Report: New Britain

The New Britain region is one of the more active regions in the world. See a list of earthquake reports for this region at the bottom of this page, above the reference list.

Today’s M 6.6 earthquake happened close in proximity to a M 6.3 from 2 days ago and a M 5.6 from a couple weeks ago. The M 5.6 may be related (may have triggered these other earthquakes), but this region is so active, it might be difficult to distinguish the effects from different earthquakes. The M 5.6 is much deeper and looks like it was in the downgoing Solomon Sea plate. It is much more likely that the M 6.3 and M 6.6 are related (I interpret that the M 6.3 probably triggered the M 6.6, or that M 6.3 was a foreshock to the M 6.6, given they are close in depth). Both M 6.3 and M 6.6 are at depths close to the depth of the subducting slab (the megathrust fault depth) at this location. So, I interpret these to be subduction zone earthquakes.

In the poster below, the recent seismicity in Papua New Guinea is evident.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend).

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for these earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the upper left corner are two figures from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” To the right of the map is a cross section showing how the Solomon Sea plate is subducting beneath New Britain. This is from Johnson, 1976. I place a blue star in the general location of today’s earthquake (as also for other inset figures).
  • In the lower right corner is another generalized tectonic map of the region from Holm et al., 2015.
  • In the upper right corner is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross section ‘C’ is the most representative for the earthquake today. I place the general location of the C-C’ section on the main map as an orange dashed line. I present the map and this figure again below, with their original captions.


USGS Earthquake Pages

    Here are some recent reports for other seismicity in the region.

  • On 2018.02.25 there was a M 7.5 earthquake on the island of New Guinea. Here is my report update on that sequence. Below are my original poster and an aftershock update poster.


  • On 2018.03.08 there was an > 6.8 earthquake in New Ireland that also shows up on the map today. Here is my report on this earthquake. Below is my interpretive poster (with and without magnetic anomaly data).


Some Relevant Discussion and Figures

  • This is a map showing the seismicity of this region since 2000 A.D.

  • Earlier, I discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

  • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

  • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

  • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

  • This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012). I include the figure caption below as a blockquote.

  • Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

  • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). The New Britain region is in the map near the A and B sections. I include the figure caption below as a blockquote.

  • Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

Social Media

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Posted in earthquake, education, geology, pacific, plate tectonics, subduction

Earthquake Report: Gorda plate!

I was at a workshop to develop a unified strategy for research and monitoring in the Klamath River estuary (led by the Yurok Tribe, Andreas Krauss) yesterday and missed feeling the first of two M 4.6-4.7 earthquakes. I was presenting the results from our tectonic geodetic studies as they moderate Sea Level Rise at the mouth of the Klamath River (and control sedimentation there). However, I was home in Manila, CA when the second earthquake hit. I felt a sharp and short motion (1-2 seconds max in duration). But I was exhausted still from the death of my cat Chicken. So, I needed to wait until today to put this report together.

The Cascadia subduction zone (CSZ) is a convergent plate boundary fault formed by the interaction between the downgoing oceanic Gorda plate (GP) beneath the continental North America plate (NAP). More about the CSZ can be found here.

The first M 4.6 was a thrust earthquake within the Gorda plate. The CSZ megathrust fault slab depth is about 15 km at the location of these 2 earthquakes. The M 4.7 earthquake is a strike-slip earthquake. Because of the predominant northeast striking left-lateral faults in the GP, I interpret this earthquake to be left-lateral.

My initial thought was that these two EQs could be related. So, I rummaged the literature to find papers that present research of static coulomb stress changes between earthquakes and faults similar to what we had yesterday. When an earthquake fault slips during an earthquake, the crust deforms elastically. This causes some regions to extend and other regions to compress. These extension/compression changes cause static stresses on faults to change. As the seismic waves travel through the crust, this can cause dynamic changes in stress along faults. Both of these types of stress change (static and dynamic) are very small. If there is a fault that is oriented correctly, has a high stress state (almost ready to slip during an earthquake), and has a sufficiently large enough stress change, the first earthquake may trigger a second earthquake.

Because the second earthquake happened long after the main seismic waves had stopped traveling through the region, the M 4.7 earthquake could not have been dynamically triggered by the earlier M 4.6 earthquake. However, based on my review of the literature, it appears that the M 4.6 may have triggered the M 4.7 earthquake.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5 (and down to M ≥ 4.5 in a second poster).

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 4.6 & 4.7 earthquakes. I include generic fault plane solutions for the other fault systems in the region.

I include posters that show either M 4.6 or M 4.7 MMI and Did You Feel It data. I also have version that include emag2 magnetic anomaly data. These mag anomaly data nicely show the structure of the oceanic crust formed at the Gorda spreading center (the anomalies are initially parallel to the spreading center; that these anomalies are parallel to the spreading center was some key evidence for the plate tectonic hypothesis prior to it being accepted as a theory).

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (McCrory et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I placed a blue stars in the general location of today’s earthquakes.
  • In the upper left corner is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. This figure shows the predominant tectonic fabric in the GP (northeast striking left-lateral faults). More about this figure can be found below.
  • Below the Chaytor figure is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ. Today’s earthquakes happened in the lower Gorda plate
  • In the lower left corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults. I place a blues star in the general location of today’s earthquakes.
  • To the left of the CSZ map is a figure from Yue et al. (2008) that shows the results of their static coulomb modeling. They model static changes in stress change from source earthquakes (strike-slip in upper panel and thrust faults in lower panel) imparted onto receiver faults. The lower left example (c) shows the stress change imparted following a thrust fault source earthquake imparted onto a left-lateral strike-slip receiver fault. If we rotate this panel counterclockwise (about 25°)to match the orientation of the M 4.6 earthquake, we may observe that the M 4.7 earthquake resides in the quadrant that saw an increase in static coulomb stress (colored red).
  • In the lower right corner is another example of static coulomb stress modeling from Lin et al. (2011). The left panel shows what static stress changes may be imparted from a source thrust fault onto left lateral strike-slip faults (“tear” faults are strike-slip faults that connect thrust faults). These left panels also show an increase in static coulomb stress in the lower left quadrant. I take these two examples as supporting evidence for my hypothesis that the M 4.6 earthquake triggered the M 4.7 earthquake.

M 4.6 MMI/DYFI


M 4.6 MMI/DYFI emag2


M 4.7 MMI/DYFI


M 4.7 MMI/DYFI emag2


USGS Earthquake Pages

  • Here is the Baby Benioff Seismograph from Humboldt State Univ. Dept. of Geology. See social media below.

UPDATE: 2018.03.23 22:10 local time

  • I later noticed that there was a M 4.5 earlier on 2018.03.09 south of these two M 4.6 and 4.7 earthquakes. I here prepare an overlay analysis of the seismicity with the Yue et al. (2008) results compared to these 3 earthquakes in a sequence. The below figure has two panels representing the hypothetical static coulomb stress changes between these three earthquakes. Earthquake order number is labeled in cyan.
  • I orient the Yue et al. (2008) figures relative to the primary nodal plane strike preferred USGS interpretation. In other words, I use the orientation of the USGS preferred fault plane solution to orient the Yue et al. (2008) coulomb stress change figures.
  • These overlays are scaled relative to the published scale.
  • Note how the largest magnitude (M 4.7) earthquake is in a region of increased static coulomb stress from both the prior earthquakes. These stress changes are very small and the magnitudes are probably not scaled appropriately for the space, so this is possibly a conjectural interpretation.
    1. The left panel shows what stress changes might happen on left-lateral strike-slip receiver faults given a left-lateral strike-slip source fault. The M 4.7 EQ is in the region of increased static coulomb stress.
    2. The right panel shows what stress changes might happen on left-lateral striek-slip receiver faults given a thrust fault source earthquake. The M 4.7 EQ is AGAIN in the region of increased static coulomb stress.


Some Relevant Discussion and Figures

  • Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

  • Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.

  • Here is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. The moment tensor (at the moment i write this) shows a north-south striking fault with a reverse or thrust faulting mechanism. While this region of faulting is dominated by strike slip faults (and most all prior earthquake moment tensors showed strike slip earthquakes), when strike slip faults bend, they can create compression (transpression) and extension (transtension). This transpressive or transtentional deformation may produce thrust/reverse earthquakes or normal fault earthquakes, respectively. The transverse ranges north of Los Angeles are an example of uplift/transpression due to the bend in the San Andreas fault in that region.

  • These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
  • Mw = 5 Trinidad Chaytor

  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.

The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. These two aftershocks align on what may be the eastern extension of the Mendocino fault.

There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.

  • Here is the Yue et al. (2008) figure, along with their figure caption below.

  • Coulomb stress change for different combination of faults. The thick while line marks the source fault, and the white arrows indicate the focal mechanism. The black line and the black arrows represent the orientation of the receiving fault and its mechanism, respectively.

  • Here is the figure from Lin et al. (2011), along with their figure caption.

  • Maps showing Coulomb stress changes caused by an M = 7.0 earthquake on adjacent tear faults. The source is the same as in Figure 4. Coulomb stresses are calculated on (a–c) left‐lateral and (e–g) right‐lateral tear faults. Stress is sampled at depth of 1 km (Figures 6a and 6e), 10 km (Figures 6b and 6f), and 19.5 km (Figures 6c and 6g). (d) Cross section at the right end of the source earthquake (cross section position shown in Figure 6a). Note that left‐lateral tear faulting is favored in one position with respect to the thrust, while right‐lateral faulting is favored in the opposite position.

Social Media

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

    References:

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • Lin, J., R. S. Stein, M. Meghraoui, S. Toda, A. Ayadi, C. Dorbath, and S. Belabbes (2011), Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 Mw = 6.9 Zemmouri, Algeria, earthquake, J. Geophys. Res., 116, B03305, doi:10.1029/2010JB007654.
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Yue, H., Zhang, Z., Chen, Y.J., 2008. Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence in Chinese Science Bulletin, v. 53, no. 16, p. 2520-2526
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Posted in cascadia, earthquake, education, geology, gorda, humboldt, plate tectonics, strike-slip

Earthquake Report: Malawi & Mozambique

Busy day today. This is my second earthquake report today.

This report is about a M 5.6 earthquake along the Malawi Rift (MR) system, part of the larger East Africa Rift (EAR) extensional plate boundary. The EAR is currently the locus of extension between the Nubia and Somalia plates. The orientation of extension in this region has changed over time (for more on this, see Castaing, 1991). There are many normal faults that accommodate this extension (forming the rift valleys where so much paleoanthopologic evidence has been archived by rift volcanic deposits, and later exposed due to the extension). As the faults change strike (compass orientation), the type of faulting also changes (there are lots of strike-slip faults that have formed to accommodate the mismatch between fault strike and extension direction).

Topday’s M 5.6 earthquake is extensional, showing extension in the northeast-southwest direction. At first, we might think that this is strange, since the predominant direction of extension is east-west. However, upon further investigation, we learn that the normal faults in the region of today’s earthquake have northwest strike (they are oriented northwest-southeast). So, northeast extension makes sense here.

There have been a number of earthquakes along the EAR and I include these in the poster. Links to the USGS websites are listed below.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include USGS earthquake epicenters from 1918-2018 with magnitudes M ≥ 5.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.6 earthquakes, in addition to some relevant historic earthquakes.

I include the magnetic anomaly data (emag 2).

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include some inset figures.

  • In the upper left corner is a map from Stamps et al. (2018) that shows the relative plate motion across the plate boundaries. The length of these vectors represents relative velocity between the plate systems (designated by color). The EAR has been propagating to the south, and the GPS rates reflect this (faster in then north and slower in the south). I place a blue star in the general location of today’s earthquake (as in other inset maps).
  • In the upper right corner is a map from Hayes et al. (2014) that shows the heightened seismic hazard associated with the EAR.
  • To the left of the seismic hazard map is a map showing faults colored relative to when they formed. This also shows how the EAR is propagating to the south.
  • In the lower right corner is a graphic that illustrates how Castaing (1991) has interpreted the tectonic strain to have evolved through time. Note the lower right panel as this represents the Cenozoic to Recent tectonic setting.


USGS Earthquake Pages

    These are from this current sequence

  • 2018.03.08 M 5.6 Malawi

Some Relevant Discussion and Figures

  • Here is the Stamps et al. (2018) figure. These authors describe their efforts to create the “Sub-Saharan Africa Geodetic Strain Rate Model v.1.0 (SSA-GSRM v.1.0).” They used GPS data to estimate strain rates for the EAR system.

  • Tectonic setting of Africa and the East African Rift System. OR = Okavangu Rift, LR = Luangua Rift, MR = Mweru Rift, EB = Eastern Branch, KP = Kivu Volcanic Province, CVL = Cameroon Volcanic Line. Earthquakes >M4 from the International Seismological Catalog29 are shown in different colors as well as relative plate motions from Saria et al.3, which are used to constrain long-term tectonic rigid plate motions. Figure was created by DSS using the open source software Generic Mapping Tools v5.2.1 supported by the National Science Foundation.

  • This map shows their interpretation of how much of the EAR is experiencing either extension or compression and their comparison with the GSRM existing model. The Stamps et al. (2018) model is on the left. The upper panel shows how their new model is sensitive to additional strain not observed in the GSRM model. The lower panel shows extension in warm colors and compression in cool colors.

  • Geodetic strain rate second invariant and dilatation and comparison with GSRM v.2.1. (A) The second invariant of strain rate for the new long-term tectonic deformation model indicating magnitude. (B) Residual strain rate magnitudes relative to GSRM v2.1. (C) Dilatation indicating the dominantly compressional and extensional regimes. Tensor orientations are overlaid. Red = extension and black = compression. (C,D) Same as (C), but for residual strain rate tensors and dilatation.

  • Here is the Hayes et al. (2014) seismic hazard map.

  • Here is the Castaing (1991) figure that shows how the tectonics of the EAR has changed through time.

  • Stereograms showing successive stress fields in South Malawi

  • This map shows some of the fault mapping in the region of todays earthquake (Castaing, 1991). Today’s earthquake happened due south of Lake Chilwa, possibly associated with the Cholo fault (tho they do not map the Cholo fault to the location of the USGS epicenter).

  • Reactivation of the Shire Valley area by the Recent East African Rift System (modified after Habgood, 1963; Pinna et al., 1987). I = Malawi-Mozambique border; 2 = ante-Cenozoic formations; S = Cenozoic to Recent deposits; 4 = dextral strike-slip faults; 5 = normal faults; 6 = strike-slip fault with normal component.

  • This map shows the more recent faulting in the region (Castaing, 1991).

  • Recent East African Rift System {modified after Chorowicz, 1989; Chorowicz and Mukonki. 1980; Chorowicz et al., 1983, 1987; Daly et al., 1989; Ebinger et al., 1987; Katz, 1987; Kazrnin, 1980; McConnel, 1972; Rach and Rosendahl, 1989; Rosendahl, 1987; Villeneuve, 1983; Wheeler and Karson, 1989). I = Rift boundary normal faults; 2 = pre-transform faults; = Cenozoic and Recent volcanics; 4 = Cenozoic granites; 5 = direction of extension (a = Lengwe and Mwabvi basins-present study and focal mechanism solution of 6 May 1966 earthquake from Shudofsky (1985), b-h = microtectonic observations between Lake Edward and Lake Malawi from Chorowicz (1989) and Chorowicz and Mukonki (1980)); 6 = general extension.

  • Regions of extension (Saemundsson, 2010). I include the original figure captions below them as blockquotes.

  • The Afro-Arabian rift system (continental graben and depressions are shaded) (From: Baker et al., 1972)

  • Fault segments along the EAR, Chorowicz (2005).

  • Hypsographic DEM of the East African rift system. Black lines: main faults; E–W dotted lines: locations of cross-sections of Fig. 3; white surfaces: lakes; grey levels from dark (low elevations) to light (high elevations). The East African rift system is a series of several thousand kilometers long aligned successions of adjacent individual tectonic basins (rift valleys), separated from each other by relative shoals and generally bordered by uplifted shoulders. It can be regarded as an intra-continental ridge system comprising an axial rift.

  • Faults characterized vs. their major sense of motion, Chorowicz (2005).

  • Western branch and part of eastern branch of the East African rift system, on shadowed DEM.

  • Regional tectonic strain, Chorowicz (2005).

  • On-going individualization of the Somalian plate in Eastern Africa. Asthenospheric intrusions (black polygons) show already open lithosphere. White arrows show direction of relative divergent movement.

  • This is an illustration showing how the extension in this region may be accommodated by dextral (right-lateral) strike-slip faults, Chorowicz (2005).

  • Fault and fold zone of the Tanganyika–Rukwa–Malawi segment of the EARS. Folds are developed in stripes between left-stepping en echelon dextral strike-slip faults. This pattern of folds explains why some segment border areas of the Tanganyika rift form low plains instead of the usual high shoulders.

  • Here is the USGS “Seismicity of the Earth” poster for this region (Hayes et al., 2014).

  • This is the latest geologic maps of Africa (Thieblemont, D., 2016). Click on the map for a 67 MB pdf version.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Social Media

    References:

  • Baker, B.H., Mohr P.A., and Williams, L.A.J., 1972: Geology of the Eastern Rift System of Africa in The Geol. Soc. of America. Special Paper, 136, 67 pp.
  • Castaing, C., 1991. lost-Pan-African tectonic evolution of South Malawi in relation to the Karroo and Recent East African Rift Systems in Tectonophysics, v. 191, p. 55-73
  • Chorowicz, J., 2005. The East African rift system in Journal of African Earth Sciences, v. 43., p. 379-410.
  • Hayes, G.P., Jones, E.S., Stadler, T.J., Barnhart, W.D., McNamara, D.E., Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2013 East African Rift: U.S. Geological Survey Open-File Report 2010–1083-P, 1 sheet, scale 1:8,500,000 http://dx.doi.org/10.3133/of20101083P
  • Leseane, K., Atekwana, E.A., Mickus, K.L., Abdelsalam, M.G., Shemanq, E.M., and Atekwana, E.A., 2015. Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana in JGR: Solid Earth, v. 120, doi:10.1002/2014JB011029.
  • Saemundsson, K., 2010. East African Rift System – an Overview presented at Short Course V on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Oct. 29 – Nov. 19, 2010, 10 pp.
  • Stamps, D.S., Saria, E., and Kreemer, C., 2018. A Geodetic Strain Rate Model for the East African Rift System in Scientific Reports, v. 8, DOI:10.1038/s41598-017-19097-w
  • Thieblemont, D. (ed.), 2016. Geological Map of Africa et 1:10M scale, CGMW-BRGM 2016

Posted in africa, earthquake, education, Extension, geology, plate tectonics

Earthquake Report: New Ireland!

We had an M 6.8 earthquake near a transform micro-plate boundary fault system north of New Ireland, Papua New Guinea today. Here is the USGS website for this earthquake.

The main transform fault (Weitin fault) is ~40 km to the west of the USGS epicenter. There was a very similar earthquake on 1982.08.12 (USGS website).

This earthquake is unrelated to the sequence occurring on the island of New Guinea.

Something that I rediscovered is that there were two M 8 earthquakes in 1971 in this region. This testifies that it is possible to have a Great earthquake (M ≥ 8) close in space and time relative to another Great earthquake. These earthquakes do not have USGS fault plane solutions, but I suspect that these are subduction zone earthquakes (based upon their depth).

This transform system is capable of producing Great earthquakes too, as evidenced by the 2000.11.16 M 8.0 earthquake (USGS website). This is another example of two Great earthquakes (or almost 2 Great earthquakes, as the M 7.8 is not quite a Great earthquake) are related. It appears that the M 8.0 earthquake may have triggered teh M 7.8 earthquake about 3 months later (however at first glance, it seemed to me like the strike-slip earthquake might not increase the static coulomb stress on the subduction zone, but I have not spent more than half a minute thinking about this).

Here are the USGS websites for the earthquakes discussed here.

https://earthquake.usgs.gov/earthquakes/eventpage/usp000a3sp#executive

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 (in a second poster). I also prepared these two posters with emag2 magnetic anomaly data (the file sizes are larger for these emag2 posters).

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.8 earthquake, in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity intensity on the map (shows where there is land). These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the top right of the poster are two figures from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” To the right of the map is a cross section showing how the Solomon Sea plate is subducting beneath New Britain. This is from Johnson, 1976 I place a blue star in the general location of the earthquake in these inset figures.
  • In the upper left corner is another generalized tectonic map of the region from Holm et al., 2015.
  • In the lower left corner is a map from Müller et al. (2001) that shows details of the faulting in the Manus and New Ireland basins.

Main Interpretive Poster


Main Interpretive Poster with emag2


Earthquakes M≥ 6.5


Earthquakes M≥ 6.5 with emag2


Some Relevant Discussion and Figures

  • This is a map showing the seismicity of this region since 2000 A.D.

  • Earlier, I discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

  • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

  • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

  • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

  • This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012). I include the figure caption below as a blockquote.

  • Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

  • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). The New Britain region is in the map near the A and B sections. I include the figure caption below as a blockquote.

  • Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

Social Media

    References:

  • Baldwin, S.L., Monteleone, B.D., Webb, L.E., Fitzgerald, P.G., Grove, M., and Hill, E.J., 2004. Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea in Nature, v. 431, p/ 263-267, doi:10.1038/nature02846.
  • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region, Annu. Rev. Earth Planet. Sci., v. 40, pp. 495-520.
  • Cloos, M., Sapiie, B., Quarles van Ufford, A., Weiland, R.J., Warren, P.Q., and McMahon, T.P., 2005, Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff: Geological Society of America Special Paper 400, 51 p., doi: 10.1130/2005.2400.
  • Hamilton, W.B., 1979. Tectonics of the Indonesian Region, USGS Professional Paper 1078.
  • Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
  • Holm, R. and Richards, S.W., 2013. A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea in Australian Journal of Earth Sciences, v. 60, p. 605-619.
  • Holm, R.J., Richards, S.W., Rosenbaum, G., and Spandler, C., 2015. Disparate Tectonic Settings for Mineralisation in an Active Arc, Eastern Papua New Guinea and the Solomon Islands in proceedings from PACRIM 2015 Congress, Hong Kong ,18-21 March, 2015, pp. 7.
  • Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
  • Johnson, R.W., 1976, Late Cainozoic volcanism and plate tectonics at the southern margin of the Bismarck Sea, Papua New Guinea, in Johnson, R.W., ed., 1976, Volcanism in Australia: Amsterdam, Elsevier, p. 101-116
  • Koulali, A., tregoning, P., McClusky, S., Stanaway, R., Wallace, L., and Lister, G., 2015. New Insights into the present-day kinematics of the central and western Papua New Guinea from GPS in GJI, v. 202, p. 993-1004, doi: 10.1093/gji/ggv200
  • Müller, D., Franz, L., Herzig, P.M., and Hunt, S., 2001. Potassic igneous rocks from the vicinity of epithermal gold mineralization, Lihir Island, Papua New Guinea in Lithos, v. 57, p. 163-186
  • Sapiie, B., and Cloos, M., 2004. Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia in GSA Bulletin, v. 116; no. 3/4; p. 277–293
  • Tregoning, P., McQueen, H., Lambeck, K., Jackson, R. Little, T., Saunders, S., and Rosa, R., 2000. Present-day crustal motion in Papua New Guinea, Earth Planets and Space, v. 52, pp. 727-730.
  • Wells, D., l., and Coppersmith, K.J., 1994. New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement in BSSA, vol. 84, no. 4, pp. 974-1002

Posted in earthquake, education, geology, pacific, plate tectonics, strike-slip, Transform