I am catching up on earthquake reports today as I was in the field the past couple of weeks…
Well, these reports are getting too long. So, I have placed the explanatory material on 2 web pages, so one does not need to read through that stuff if they have been here before. I will link those pages in all reports. You’re welcome. ;-)
This will also save me some time and make writing these reports simpler.
On 1 August 2019 there was an earthquake along the convergent plate boundary along the west coast of Chile (a subduction zone forming the Peru-Chile trench). This subduction zone megathrust fault produced the largest magnitude earthquake recorded on seismometers in 1960, the Valparaiso, Chile magnitude M9.5 earthquake that caused a trans-pacific tsunami causing damage and deaths all along the western hemispheric coastline.
https://earthquake.usgs.gov/earthquakes/eventpage/us60004yps/executive
- 2015.09.16 M 8.3 Chile
- 2014.04.01 M 8.2 Chile
- 2010.02.27 M 8.8 Chile
- 1960.05.22 M 9.5 Chile
Here are some of the larger earthquakes along this plate boundary (more listed at the bottom of this page).
This M 6.8 earthquake happened at the overlap of the southern end of the 1985 M8.0 and northern end of the 2010 M8.8 earthquakes. Does this portend that there will be another, larger, earthquake in this area soon? Only time will tell.
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 1919-2019 with magnitudes M ≥ 3.0 in one version.
- I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
- A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
- In the upper left corner is a map showing historic earthquakes along the Chile margin (Rhea et al., 2010). We may visualize the earthquake depths by checking out the color of the dots. To the below is a cross section, cutting into the Earth. Earthquakes that are along the profile D-D’ (in blue on the map) are included in this cross section. I also placed a blue line on the main map in the general location of this cross section. I placed a blue star in the general location of the M=6.8 earthquake (same for the other inset figures).
- To the right is a map that shows a comparison between the USGS modeled intensity (using the MMI scale) with the USGS “Did You Feel It?” reports (results from real people). The model and the reported results are quite similar. See the MMI poster below for a more comprehensive comparison. In addition, I include depth contours of the subducting megathrust slab (Hayes et al., 2016; read more here).
- In the center left bottom, I include a schematic cross section of the subduction zone. This shows where earthquakes may occur, generally. There are subduction zone megathrust earthquakes (the largest of magnitude), crustal earthquakes, slab earthquakes, and outer rise earthquakes.
- In the lower right corner is a map that shows the relative seismic hazard for this plate boundary (Rhea et al., 2010). I plot both 2019 earthquakes.< The numbers (“80”) indicate the rate at which the Nazca Plate is subducting beneath South America. 80 mm/yr = 3 in/yr.
- In the upper right corner is a composite figure from several figures from Metois et al., 2016. On the left is a panel that shows the latitudinal range of earthquake ruptures (I fixed it in places as the original figure did not extend the 2010 rupture sufficiently to the north). The panel on the right shows how much the subduction zone fault is “locked” (or, seismically coupled). Darker colors represent parts of the fault that are storing more energy over time and are possibly places where the fault will slip (compared to parts of the fault that are white or yellow, which may be places where the fault is currently slipping and would not generate earthquakes in the future). This is simply a model and there is not way to really know where an earthquake will happen until there is an earthquake.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- Here is the map with a century’s seismicity plotted, for earthquakes associated with the larger earthquakes from this region (colored relative to time scale, 1960, 1985, 2010, 2015, 2019).
USGS Shaking Intensity
- Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Borth data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
- In the lower right corner is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orand and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California). I plot Santiago relative to distance from the earthquake with a blue arrow (compare with the poster).
USGS Historic Seismicity
- Here is a poster that shows the significant earthquakes along this plate boundary. Note how there are earthquakes in the Nazca plate associated with the 2010 and 2015 megathrust subduction zone earthquakes. These are triggered earthquakes along the outer rise, not additional subduction zone earthquakes.
- In the lower right corner is a figure from Beck (1998) that shows the spatial extent of the known earthquakes. I added the extent of the 2015 and 2010 earthquakes as green arrows.
- In the upper right corner is an excellent figure from Horton (2018) that shows the plate tectonic setting for this area.
Some Relevant Discussion and Figures
- Here is the overview figure from Horton, 2018.
Maps of (A) tectonic framework, (B) topography, and (C) sedimentary basin configuration of South America. (A) Map of plate boundaries, Andean magmatic arc (including the northern, central, and southern volcanic zones), regions of flat slab subduction, modern stress orientations from earthquake focal mechanisms, eastern front of Andean fold-thrust belt, and key segments of the retroarc foreland basin system. Plate velocities are shown relative to stable South American plate (DeMets et al., 2010). (B) DEM topographic map showing the Andes mountains and adjacent foreland region, including the Amazon, Parana, Orinoco, and Magdalena (Mag) river systems. (C) Map of Andean retroarc basins, showing isopach thicknesses (in km) of Cretaceous-Cenozoic basin fill, forebulge axis (from Chase et al., 2009), and locations of 13 sites (8 foreland basins, 5 hinterland basins) considered in this synthesis
- Here is the seismicity map and space time diagram from Métois et al. (2016). The subduction zone fault in the region of Coquimbo, Chile changes geometry, probably because of the Juan Fernandez Ridge (this structure controls the shape of the subduction zone). This figure shows a map and cross section for two parts of the subduction zone (Marot et al., 2014). Note how the subduction zone flattens out with depth here. The M=6.7 quake was shallower than this, but the shape of the downgoing slab does affect the amount of slab pull (tension in the down-dip direction) is exerted along the plate.
Left estimated extent of large historical or instrumental ruptures along the Chilean margin adapted from ME´ TOIS et al. (2012). Gray stars mark major intra-slab events. The recent Mw[8 earthquakes are indicated in red. Gray shaded areas correspond to LCZs defined in Fig. 3. Right seismicity recorded by the Centro Sismologico Nacional (CSN) during
interseismic period, color-coded depending on the event’s depth. Three zones have been defined to avoid including aftershocks and preshocks associated with major events: (1) in North Chile, we plot the seismicity from 2008 to january 2014, i.e., between the Tocopilla and Iquique earthquakes; (2) in Central Chile, we plot the seismicity on the entire 2000–2014 period; (3) in South-Central Chile, we selected events that occurred between 2000 and 2010, i.e., before the Maule earthquake.
- This figure is the 3 panel figure in the interpretive poster showing how seismicity is distributed along the margin, how historic earthquake slip was distributed, and how the fault may be locked (or slipping) along the megathrust fault.
a Histogram depicts the rate of Mw>3 earthquakes registered by the CSN catalog during the interseismic period defined for each zone (see Fig. 2) on the subduction interface, on 0.2° of latitude sliding windows. Stars are swarm-like sequences detected by HOLTKAMP et al. (2011) depending on their occurrence date. Swarms located in the Iquique LCZ and Camarones segment are from RUIZ et al. (2014). Empty squares are significant intraplate earthquakes. b Red curve variations of the average coupling coefficient on the first 60 km of depth calculated on 0.2° of latitude sliding windows for our best model including an Andean sliver motion. Dashed pink curves are alternative models with different smoothing options that fit the data with nRMS better than 2 (see supplementary figure 6): the pink shaded envelope around our best model stands for the variability of the coupling along strike. Green curves coseismic distribution for Maule (VIGNY et al. 2011), Iquique (LAY et al. 2014) and Illapel earthquakes (RUIZ et al. 2016). Gray shaded areas stand for the identified low coupling zones (LCZs). LCZs and high coupling segments are named on the left. The apparent decrease in the average coupling North of 30°S is considered as an artifact of the Andean sliver motion (see Sect. 5.2). c Best coupling distribution obtained inverting for Andean sliver motion and coupling amount simultaneously. The rupture zones for the three major earthquakes are indicated as green ellipses. White shaded areas are zones where we lack resolution.
- This is a figure that shows details about the coupling compared to some slip models for the 2010, 2014, and 2015 earthquakes.
Left coupling maps (color coded) versus coseismic slip distributions (gray shaded contours in cm) for the last three major Chilean earthquakes (epicenters are marked by white stars). From top to bottom Iquique area, white squares are pre-seismic swarm event in the month before the main shock, green star is the 2005, Tarapaca´ intraslab earthquake epicenter, blue star is the Mw 6.7 Iquique aftershock; Illapel area, green squares show the seismicity associated with the 1997 swarm following the Punitaqui intraslab earthquake (green star); Maule area, green star is the epicenter of the 1939 Chillan intraslab earthquake. Right interseismic background seismicity in the shallow part of the subduction zone (shallower than 60 km depth) for each region (red dots) together with 80 and 90 % coupling contours. White dots are events identified as mainshock after a declustering procedure following GARDNER and KNOPOFF (1974). Yellow areas extent of swarm sequences identified by HOLTKAMP et al. (2011) for South and Central Chile, and RUIZ et al. (2014) for North Chile.
- This is the fault locking figure from Saillard et al. (2017), showing the percent coupling (how much of the plate convergence contributes to deformation of the plate boundary, which may tell us places on the fault that might slip during an earthquake. We are still learning about why this is important and what it means.
Comparison between the uplift rates, interseismic coupling, major bathymetric features, and peninsulas along the Andean margin (10°S–40°S). (a) Uplift rates of marine terraces reported in the literature (we present the average rate since terrace abandonment; Table S1 in the supporting information [Jara-Muñoz et al., 2015]). Each color corresponds to a marine terrace assigned to a marine isotopic stage (MIS). Gray dots are the uplift rates of the central Andean rasa estimated from a numerical model of landscape evolution [Melnick, 2016]. (b) Major bathymetric features and peninsulas and pattern of interseismic coupling of the Andean margin from GPS data inversion (this study). Gray shaded areas correspond to the areas where the spatial resolution of inversion is low due to the poor density of GPS observations (see text and supporting information for more details). The Peru-Chile trench (thick black line), the coastline (thin black line), and the convergence direction (black arrows) are indicated. We superimposed the curve obtained by shifting the trench geometry eastward by 110 km (trench-coast distance of 110 km; blue line) with the curve reflecting the 40 km isodepth of the subducting slab (red line; Slab1.0 from Hayes and Wald [2009]), a depth which corresponds approximately with the downdip end of the locked portion of the Andean seismogenic zone (±10 km) [Ruff and Tichelaar, 1996; Khazaradze and Klotz, 2003; Chlieh et al., 2011; Ruegg et al., 2009; Moreno et al., 2011; Métois et al., 2012]. The two curves are spatially similar in the erosive part of the Chile margin (north of 34°S), whereas they diverge along the shallower slab geometry in the accretionary part of the Chile margin (south of 34°S), where the downdip end of the locked zone may be shallower (Figure 4b). Red arrows indicate the low interseismic coupling associated with peninsulas and marine terraces and evidence of aseismic afterslip (after Perfettini et al. [2010] below the Pisco-Nazca Peninsula; Pritchard and Simons [2006], Victor et al. [2011], Shirzaei et al. [2012], Bejar-Pizarro et al. [2013], and Métois et al. [2013] for the Mejillones Peninsula; Métois et al. [2012, 2014] below the Tongoy Peninsula; and Métois et al. [2012] and Lin et al. [2013] for the Arauco Peninsula). FZ: Fracture zone. Horizontal blue bands are the areas where coastline is less than 110 km (light blue) or 90 km (dark blue) from the trench (see Figure 1).
- The following figures from Leyton et al. (2009) are great analogies, showing examples of interplate earthquakes (e.g. subduction zone megathrust events) and intraplate earthquakes (e.g. slab quakes, or events within the downgoing plate). The first figures are maps showing these earthquakes, then there are some seismicity cross sections.
Maps showing the location of the study and the events used ((a)–(c)). In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones. We used beach balls to plot those events with known focal and circles for those without. White triangles mark the position of the Chilean Seismological Network used to locate the events; those with names represent stations used in the waveform analysis (either accelerometers or broadbands with known instrumental response). Labels over beach balls correspond to CMT codes.
- Here are 2 cross sections showing the earthquakes plotted in the maps above (Leyton et al., 2009).
Cross-section at (a) 33.5◦S and (b) 36.5◦S showing the events used in this study. In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones.We used beach balls (vertical projection) to plot those events with knownfocal and circles for those without. In light gray is shown the background seismicity recorded from 2000 to 2006 by the Chilean Seismological Service
- 2010.02.27 M 8.8 Earthquake Review
- 2019.08.01 M 6.8 Chile
- 2019.06.14 M 6.4 Chile
- 2019.05.26 M 8.0 Peru
- 2019.05.12 M 6.1 Panama
- 2019.03.01 M 7.0 Peru
- 2019.02.22 M 7.5 Ecuador
- 2019.01.20 M 6.7 Chile
- 2018.08.21 M 7.3 Venezuela
- 2018.08.24 M 7.1 Peru
- 2018.04.02 M 6.8 Bolivia
- 2018.01.14 M 7.1 Peru
- 2018.01.15 M 7.1 Peru Update #1
- 2017.06.30 M 6.0 Ecuador
- 2017.04.24 M 6.9 Chile
- 2017.04.23 M 5.9 Chile
- 2016.12.25 M 7.6 Chile
- 2016.11.24 M 7.0 El Salvador
- 2016.11.04 M 6.4 Maule, Chile
- 2016.04.16 M 7.8 Ecuador
- 2016.04.16 M 7.8 Ecuador Update #1
- 2015.11.29 M 5.9 Argentina
- 2015.11.11 M 6.9 Chile
- 2015.11.24 M 7.6 Peru
- 2015.11.26 M 7.6 Peru Update
- 2015.09.16 M 8.3 Chile
- 2014.04.01 M 8.2 Chile
- 2010.02.27 M 8.8 Chile
- 1960.05.22 M 9.5 Chile
Chile | South America
General Overview
Earthquake Reports
Social Media
- Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
- Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
- Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
- Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
- Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
- Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
- Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
- Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
- Beck, S., Barrientos, S., Kausel, E., and Reyes, M., 1998. Source Characteristics of Historic Earthquakes along the Central Chile Subduction Zone in Journal of South American Earth Sciences, v. 11, no. 2, p. 115-129, https://doi.org/10.1016/S0895-9811(98)00005-4
- Gardi, A., A. Lemoine, R. Madariaga, and J. Campos (2006), Modeling of stress transfer in the Coquimbo region of central Chile, J. Geophys. Res., 111, B04307, https://doi.org/10.1029/2004JB003440
- Horton, B.K., 2018. Sedimentary record of Andean mountain building< in Earth-Science Reviews, v. 178, p. 279-309, https://doi.org/10.1016/j.earscirev.2017.11.025
- Leyton, F., Ruiz, J., Campos, J., and Kausel, E., 2009. Intraplate and interplate earthquakes in Chilean subduction zone:
A theoretical and observational comparison in Physics of the Earth and Planetary Interiors, v. 175, p. 37-46, https://doi.org/10.1016/j.pepi.2008.03.017 - Marot, M., Monfret, T., Gerbault, M.,. Nolet, G., Ranalli, G., and Pardo, M., 2014. Flat versus normal subduction zones: a comparison based on 3-D regional traveltime tomography and petrological modelling of central Chile and western Argentina (29◦–35◦S) in GJI, v. 199, p. 1633-164, https://doi.org/10.1093/gji/ggu355
- Métois, M., Vigny, C., and Socquet, A., 2016. Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°–18°S) in Pure Applied Geophysics, https://doi.org/10.1007/s00024-016-1280-5
- Rhea, S., Hayes, G., Villaseñor, A., Furlong, K.P., Tarr, A.C., and Benz, H.M., 2010. Seismicity of the earth 1900–2007, Nazca Plate and South America: U.S. Geological Survey Open-File Report 2010–1083-E, 1 sheet, scale 1:12,000,000.
- Ruiz, S. and Madariaga, R., 2018. Historical and recent large megathrust earthquakes in Chile in Tectonophysics, v. 733, p. 37-56, https://doi.org/10.1016/j.tecto.2018.01.015
- Saillard, M., L. Audin, B. Rousset, J.-P. Avouac, M. Chlieh, S. R. Hall, L. Husson, and D. L. Farber, 2017. From the seismic cycle to long-term deformation: linking seismic coupling and Quaternary coastal geomorphology along the Andean megathrust in Tectonics, 36, https://doi:10.1002/2016TC004156.
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
Well, these reports are getting too long. So, I have placed the explanatory material on 2 web pages, so one does not need to read through that stuff if they have been here before. I will link those pages in all reports. You’re welcome. ;-) When earthquake faults slip, the surrounding crust and faults change shape and this causes areas of the faults to get imparted increased or decreased amounts of stress. If these faults are almost ready to slip and the change of stress is increased sufficiently, those source earthquakes may trigger earthquakes on the receiver fault (the one with increased stress). This is termed “static coulomb stress triggering.”
* note, i corrected this caption by changing the word “relationships” to “relations.” We don’t really know what the SRL for the M 6.3, but using these empirical relations, the length of the M 6.3 fault is probably between 11-14 km. So, the distance that the M 6.3 could probably trigger another quake is limited to 30 km or so. The westward tip of North America is about 230 km from the M 6.3 epicenter, with the locked zone (the part of the megathrust that might slip during an earthquake) is tens of km even further away (maybe more than 300 km).
Coulomb stress changes imparted by the 1980 Mw = 7.3 earthquake (B) to a matrix of faults representing the Mendocino Fault Zone, the Cascadia subduction zone, and NE striking left‐lateral faults in the Gorda zone. (con’t)
So, now you may have more insight about whether or not a BF earthquake could affect the CSZ megathrust. (If a M 7.8 BF earthquake happened, it would be at the outer limits of beginning to influence the megathrust, but this affect would be quite small) About a year ago, there was a magnitude M 6.2 temblor on the same plate boundary fault system. Here is the earthquake report for that M 6.2 event. Below I include the 2 posters from that Earthquake Report.
(Top) Sea Beam bathymetric map of the Cascadia Depression, Blanco Ridge, and Gorda Depression, eastern Blanco Transform Fault Zone (BTFZ).Multibeam bathymetry was collected by the NOAA R/V’s Surveyor and Discoverer and the R/V Laney Chouest during 12 cruises in the 1980’s and 90’s. Bathymetry displayed using a 500 m grid interval. Numbers with arrows show look directions of three-dimensional diagrams in Figures 2 and 3. (Bottom) Structure map, interpreted from bathymetry, showing active faults and major geologic features of the region. Solid lines represent faults, dashed lines are fracture zones, and dotted lines show course of turbidite channels. When possible to estimate sense of motion on a fault, a filled circle shows the down-thrown side. Inset maps show location and generalized geologic structure of the BTFZ. Location of seismic reflection and gravity/magnetics profiles indicated by opposing brackets. D-D’ and E-E’ are the seismic reflection profiles shown in Figures 8a and 8b, and G-G’ is the gravity and magnetics profile shown in Figure 13. Submersible dive tracklines from sites 1 through 4 are highlighted in red. L1 and L2 are two lineations seen in three-dimensional bathymetry shown in Figures 2 and 3. Location of two Blanco Ridge slump scars indicated by half-rectangles, inferred direction of slump shown by arrow, and debris location (when identified) designated by an ‘S’. CD stands for Cascadia Depression, BR is Blanco Ridge, GD is Gorda Depression, and GR is Gorda Ridge. Numbers on north and south side of transform represent Juan de Fuca and Pacific plate crustal ages inferred from magnetic anomalies. Long-term plate motion rate between the Pacific and southern Juan de Fuca plates from Wilson (1989).
VIDEOS Here is the first animation that first adds the epicenters through time (beginning with the oldest earthquakes), then removes them through time (beginning with the oldest earthquakes).
A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.
Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.
Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson [1989], with Cande and Kent’s [1995] timescale correction.
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. — Jason "Jay" R. Patton (@patton_cascadia) August 29, 2019 — Jason "Jay" R. Patton (@patton_cascadia) August 29, 2019
Around the beginning of the month, I was helping get a fundraiser prepared for a weekend concert series (audio crew for load in and strike; stage manager during the show). So, I was away from the computers when there was a magnitude M6.9 earthquake offshore of Sumatra and Java, Indonesia. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 in one version.
Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003).
Tectonic sketch map of the Sumatra–Java trench-arc region in eastern Indian Ocean Benioff Zone configuration. Hatched line with numbers indicates depth to the top of the Benioff Zone (after Newcomb and McCann13). Magnetic anomaly identifications have been considered from Liu et al.14 and Krishna et al.15. Magnitude and direction of the plate motion is obtained from Sieh and Natawidjaja. O indicates the location of the recent major earthquakes of 26 December 2004, i.e. the devastating tsunamigenic earthquake (Mw = 9.3) and the 28 March 2005 earthquake (Mw = 8.6).
Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.
We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault.
Bathymetry off Sumatra (multibeam bathymetry, where available underlain by satellite derived bathymetry; Smith and Sandwell, 1997). Tectonic setting is after Newcomb and McCann, 1987. Fracture zones (after Kopp et al., 2008) on the incoming plate as well as subduction direction and velocity (after Simons et al., 2007) are indicated by annotated black arrows on Indo-Australian plate. Major Mentawai islands as well as major faults are annotated along the forearc. Dashed lines sub-parallel to the trench mark the updip and downdip limit of the SZ. The seaward trench-parallel dashed line marking the updip limit of the SZ coincides with the slope break. Profiles and regions are marked and annotated, where additional investigations were available to constrain or refute their limits of the SZ.
Bathymetry off Java and the Lesser Sunda islands (multibeam bathymetry (for YK0207 see Soh et al., 2002), where available underlain by satellite derived bathymetry; Smith and Sandwell, 1997). Tectonic setting (after Newcomb and McCann, 1987) on the incoming plate as well as subduction direction and velocity (after Simons et al., 2007) are indicated by annotated black arrows on Indo-Australian plate. Lesser Sunda islands as well as major tectonic features are annotated along the forearc. Dashed lines sub-parallel to the trench mark the updip and downdip limit of the SZ. The seaward trench-parallel dashed line marking the updip limit of the SZ coincides with the slope break. Profiles and regions are marked and annotated, where additional investigations were available to constrain or refute their limits of the SZ.
Map of Southeast Asia showing recent and selected historical ruptures of the Sunda megathrust. Black lines with sense of motion are major plate-bounding faults, and gray lines are seafloor fracture zones. Motions of Australian and Indian plates relative to Sunda plate are from the MORVEL-1 global model [DeMets et al., 2010]. The fore-arc sliver between the Sunda megathrust and the strike-slip Sumatran Fault becomes the Burma microplate farther north, but this long, thin strip of crust does not necessarily all behave as a rigid block. Sim = Simeulue, Ni = Nias, Bt = Batu Islands, and Eng = Enggano. Brown rectangle centered at 2°S, 99°E delineates the area of Figure 3, highlighting the Mentawai Islands. Figure adapted from Meltzner et al. [2012] with rupture areas and magnitudes from Briggs et al. [2006], Konca et al. [2008], Meltzner et al. [2010], Hill et al. [2012], and references therein.
Morphology of the Java margin based on satellite altimetry data (Smith & Sandwell 1997). A large bivergent accretionary wedge is expressed as a continuous bathymetric ridge fronting the Java fore-arc basin offshore western Java. This ridge structure is broken and highly deformed offshore central Java, where the oceanic Roo Rise is colliding with the margin. The eastern Java trench offshore Bali to Sumba is characterized by the subduction of smooth oceanic crust of the Argo Abyssal Plain. The transition from oceanic subduction to continent–island arc collision occurs south of Sumba where the Scott plateau enters the trench. Black lines show wide-angle refraction profiles.
Tomographic images and velocity–depth distribution along seven refraction seismic dip lines crossing the fore-arc between western Java and east of Sumba island. The profiles document the variation from the accretionary domain (a and b) to the erosional seamount/plateau subduction regime off central to eastern Java (c and d). To the east, the transition from oceanic subduction offshore Lombok (e) to continent–island arc collision (f and g) occurs. All profiles west of Sumba show a shallow hydrated upper plate mantle, which limits the downdip extent of the seismogenic zone. Profiles are approximately aligned along the vertical stippled line. Vertical exaggeration in all profiles is 2.5.
Tectonic map of the Lesser Sunda Islands, showing the main tectonic units, main faults, bathymetry and location of seismic sections discussed in this paper.
This plot shows the earthquake localizations on a South-North cross section for the lat -14°/-4° long 114°/124° quadrant corresponding to the Lesser Sunda Islands region. The localizations are extracted from the USGS database and corresponds to magnitude greater than 4.5 in the 1973-2004 time period (shallow earthquakes with undetermined depth have been omitted.
Six 15 km deep seismic sections acquired by BGR from west to east traversing oceanic crust, deep sea trench, accretionary prism, outer arc high and fore-arc basin, derived from Kirchoff prestack depth migration (PreSDM) with a frequency range of 4-60 Hz. Profile BGR06-313 shows exemplarily a velocity-depth model according to refraction/wide-angle
Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: 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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
Mw=7.0, SOUTHWEST OF SUMATRA, INDONESIA (Depth: 43 km), 2019/08/02 12:03:26 UTC – Full details here: https://t.co/TN6Lbeycmc pic.twitter.com/LuyXLEYFql — Earthquakes (@geoscope_ipgp) August 2, 2019
Earthquake Report: Blanco fault
This will also save me some time and make writing these reports simpler.
The tectonics of the northeast Pacific is dominated by the Cascadia subduction zone, a convergent plate boundary, where the Explorer, Juan de Fuca, and Gorda oceanic plates dive eastward beneath the North America plate.
These oceanic plates are created (formed, though I love writing “created” in science writing) at oceanic spreading ridges/centers.
When oceanic spreading centers are offset laterally, a strike-slip fault forms called a transform fault. The Blanco transform fault is a right-lateral strike-slip fault (like the San Andreas fault). Thanks to Dr. Harold Tobin for pointing out why this is not a fracture zone.
This plate boundary fault system (BF) is quite active with ten magnitude M ≥ 6.0 earthquakes in the past 50 years (one every 5 years) and about 150 M ≥ 5 earthquakes in the same time range.
https://earthquake.usgs.gov/earthquakes/eventpage/us700059qh/executive
When there are quakes on the BF, people always wonder if the Cascadia megathrust is affected by this… “are we at greater risk because of those BF earthquakes?”
The main take away is that we are not at a greater risk because of these earthquakes. More on this below the interpretive poster.Below is my interpretive poster for this earthquake
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Stress Triggering
Typically the maximum distance from an earthquake that these stress changes can trigger an earthquake is about twice the length of the source earthquake.
If we use data from historic earthquakes to correlate earthquake fault slip length to magnitude, we can estimate the length of the BF that slipped during the M 6.3 temblor (Wells and Coppersmith, 1994).
Below is a figure from Wells and Coppersmith (1994) that shows the empirical relations between surface rupture length (SRL, the length of the fault that ruptures to the ground surface) and magnitude. If one knows the SRL (horizontal axis), they can estimate the magnitude (vertical axis). The left plot shows the earthquake data. The right plot shows how their formulas “predict” these data.
(a) Regression of surface rupture length on magnitude (M). Regression line shown for all-slip-type relations. Short dashed line indicates 95% confidence interval. (b) Regression lines for strike-slip, reverse, and normal-slip relations. See Table 2 for regression coefficients. Length of regression lines shows the range of data for each relation.
To give us an idea about this stress triggering stuff, below is a figure from Rollins and Stein (2010). This figure shows the results from their model. This model shows the change in stress imparted upon the megathrust from a strike-slip fault in the Gorda plate (a 1980 M 7.3 earthquake, which was very close to the megathrust).
The red areas show areas of increased stress, blue areas show decreased stress. This is based on a left-lateral strike-slip fault (so a right-lateral quake would produce changes in stress the opposite as this, red regions would be blue and blue regions would be red, generally).
The M 7.3 SRL may have been between 86-104 km. Compare this with the 12-14 km SRL for a M 6.3. The changes in coulomb stress for the M 6.3 is much much less than for the > 7.3.
2018.08.22 M 6.2 Blanco transform fault
Other Report Pages
Some Relevant Discussion and Figures
Cascadia subduction zone
Blanco transform fault
BF Historic Seismicity
Here are links to the video files (it might be easier to download them and view them remotely as the files are large).
Here is the second animation that uses a one-year moving window. This way, one year after an earthquake is plotted, it is removed from the plot. This animation is good to see the spatiotemporal variation of seismicity along the BFZ.
Here is a map with all the fore- and after-shocks plotted to date.
Gorda Plate Seismicity
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.
Cascadia subduction zone Earthquake Reports
General Overview
Earthquake Reports
Gorda plate
Blanco transform fault
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
Earthquake Report: Sunda Strait, Indonesia
https://earthquake.usgs.gov/earthquakes/eventpage/us60004zhq/executive
There was also an interesting earthquake in Chile, but I can’t do it all. (If I get a chance, I will write that one up too.)
The tectonics are both simple and complicated in this part of the world. The islands of Sumatra and Java (and more) are rows of volcanoes (called an island arc) formed by the partial melt of mantle material associated with the subduction of the oceanic India-Australia plate beneath the Sunda plate (part of Eurasia).
The downgoing plate has lots of water embedded in the rocks and sediments, when this plate is subducted, those fluids make their way into the overlying mantle. This changes the conditions so that the mantle partially melts, which results in the material being less dense, so it rises and erupts as volcanoes.
We can see some of these volcanoes in the interpretive poster below (look at the eastern part of the Island of Java).
So, the subduction zone is the main player on the scene. But the orientation (strike and changes in strike) of the subduction zone megathrust fault, in comparison to the relative motion between these plates, and in comparison to pre-existing structures in the India-Australia plate, leads to a number of additional faults.
The major fault system that accommodates the different relative plate motions is the Great Sumatra fault. The relative plate motion is oblique (not perpendicular to) the orientation of the subduction zone fault. Therefore, while the megathrust accommodates the fault perpendicular motion, the Sumatra accommodates the fault parallel motion (as a strike slip-fault). There are other strike slip faults too. These faults are called “forearc sliver faults.”
Some of the historic faults in the interpretive poster below are subduction zone earthquakes. The 2007 M 8.4 quake is a great example of this.
There are a couple good examples of “outer rise” earthquakes, temblors that occur in the downgoing plate, where there is flexure of the plate, causing the plate to bend and cause earthquakes along these bends. These are extensional earthquakes (the 2011 & 2013 quakes near Christmas Island).
There are two quakes that appear related to the Sumatra fault (19994 and 1995 quakes).
The 2 Aug 2019 M 6.9 quake is interesting because it does not appear to be a megathrust quake, an outer rise quake, or a Sumatra fault quake. The M 6.9 is (1) too deep for those types of quakes and (2) has an orientation that is not consistent with those types of quakes. This quake is in the India-Australia plate and could be along a reactivated fracture zone. The inset maps shows several of these north-south trending fracture zones (e.g. the Investigator fracture zone).
Thus, I interpret this as a north-south oriented left-lateral strike-slip earthquake. It is pretty deep, and could also be related to some other processes going on within the slab or uppermost mantle. The slab depth at this location is 20 km, so the quake is possibly about 35 km beneath the top of the India Australia plate. Oceanic crust is, on average, 7km. So, this M 6.9 is probably within the mantle beneath the slab.
There is an analogous M 7.0 earthquake on 2009.09.02 to the east, just south of the label “Java” on the interpretive poster. This earthquake shows trench parallel compression (perpendicular to the compression from the subduction zone). This quake is almost 40 km deep, so is also probably beneath the slab, within the uppermost “lithospheric” mantle.
So, these 2019 M 6.9 and 2009 M 7.0 earthquakes are really cool.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), in addition to some relevant historic earthquakes.
Magnetic Anomalies
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Landslide, Liquefaction, and Shaking Intensity
Other Report Pages
Some Relevant Discussion and Figures
seismic tomography on coincident profile P31 (modified after Lüschen et al, 2011).
Geologic Fundamentals
Compressional:
Extensional:
Indonesia | Sumatra
General Overview
Earthquake Reports
Social Media
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.