It was a busy week (usual, right?). The previous week I was working on getting a house remodel done so someone could move in (they have been sleeping on couches for 6 months, so want to get them in asap). This week I spent lots of time putting final touches on a USGS National Earthquake Hazards Reduction Program external grant proposal together, proposing to conduct a paleoseismic investigation for a fault I discovered in late 2018 (see AGU poster here). So, I am catching up on my earthquake reporting for this earthquake offshore northern California.
On 18 May 2020 there was a magnitude M 5.5 extensional earthquake located near the Gorda Rise, an oceanic spreading ridge where oceanic crust is formed to create (love using the word create in science) the Gorda and Pacific plates.
https://earthquake.usgs.gov/earthquakes/eventpage/us70009jgy/executive
There are three types of plate boundaries and three types of earthquake faults (this is not a coincidence because plate boundaries are generally in the form of earthquake faults).
- Some plates move side-by-side to form transform plate boundaries (in the form of strike-slip faults, like the San Andreas fault).
- Some plates move towards each other to form convergent plate boundaries (in the form of subduction zone megathrust faults (like the Cascadia subduction zone), or collision zones(like the fault system that forms the uplift that created the Himalayas).
- Some plates move away from each other to form divergent plate boundaries (in the form of oceanic spreading ridges, or spreading centers, like the Mid Atlantic Ridge or the Gorda Rise; in these locations “normal” faults are formed).
More about different types of faults can be found here.
The northeast Pacific (aka Pacific Northwest as viewed by land lubbers) is dominated by the plate boundary formed between the Pacific (PP) and North America plates (NAP). In much of California, this plate boundary is realized in the form of the San Andreas fault (SAF), where the PP moves north relative to the NAP. Both plates are moving to the northwest, but the PP is moving faster, so it appears that the NAP is moving south. This southerly motion is relative not absolute. I present a background of the SAF in my review of the 1906 San Francisco earthquake here.
Near Cape Mendocino, in Humboldt County, California, the plate boundary gets more complicated and involves all three types of fault systems.
It appears that the San Andreas fault terminates in the King Range, causing some of the highest tectonic uplift rates in North America. There are sibling faults to the east of the San Andreas that continue further north (e.g. the Maacama fault turns into the Garberville fault and the Bartlett Springs fault (eventually) turns into the Bald Mountain/Big Lagoon fault. So, it looks like these San Andreas related faults extend offshore, possibly to at least the Oregon border. Geodetic evidence supports this, as first published by Williams et al. (2002).
The San Andreas ends near the beginning of the Cascadia subduction zone (CSZ), formed where the Gorda/Juan de Fuca/Explorer plates dive eastwards beneath the North America plate. More about the CSZ can be found here, where I describe the basis of our knowledge about prehistoric earthquakes and tsunami along the CSZ.
Far offshore of the CSZ are oceanic spreading ridges, the Gorda Rise and the Juan de Fuca Ridge. Because the plates are moving away from each other here (we think this is due to processes called slab pull and ridge push; slab pull describes the process that in the subduction zone, the downgoing oceanic plate is going deep into the mantle and pulling down the crust; ridge push is not really pushing from the ridge, but that there is additional mass added to the crust and this pushes down and then out, pushing the plate away from the ridge, towards the subduction zone). As these plates diverge, there is lowered pressure beneath this divergent zone. These lowered pressures cause the mantle to melt, leading to eruptions of mafic lava. When the lava cools, it becomes new oceanic crust.
Connecting the CSZ with these spreading ridges, and spreading ridges with other spreading ridges, are transform plate boundaries in the form of strike-slip faults. For example, the Mendocino fault and the Blanco fault. Here is a report that includes background information about the Mendocino fault. Here is a report with some background information about the Blamco fault.
The 18 May 2020 M 5.5 earthquake happened near the Gorda Rise and was an extensional earthquake. As the Gorda plate moves away from the spreading ridge, the normal faults formed at the ridge don’t disappear. The Gorda plate is a strange plate as it gets internally deformed, so as the plate moves towards the subduction zone, these normal faults get reactivated as strike-slip faults. These strike-slip faults have been responsible for some of the most damaging earthquakes to impact coastal northern California. More about these left-lateral strike-slip Gorda plate earthquakes can be found in a report here.
The M 5.5 earthquake happened along one of these normal faults, before that fault turns into a strike-slip fault. There is a good history of earthquakes just like this one. Here is a report for a similar event further to the north, also slightly east of the Gorda Rise.
One of the most common questions people have is, “does this earthquake change our chances for a CSZ earthquake?” The answer is no. The reason is because the stress changes from earthquakes extends for a limited distance from those earthquakes. I spend more time discussing this limitation for the Blanco fault here. Basically, this M 5.5 event was too small and too far away from the CSZ to change the chance that the CSZ will slip. Today is not different from a couple weeks ago: we always need to be ready for an earthquake when we live in earthquake country.
Below is my interpretive poster for this earthquake
- I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 5.0 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 right corner is a map and cross section for the Cascadia subduction zone. I spend more time describing these figures below.
- In the upper left corner is a map showing this entire region with historic seismicity plotted. I also include the plate boundaries (USGS) and include the magnetic anomalies too. Read more about magnetic anomalies here. Notice how the magnetic anomaly bands are parallel to the spreading ridges. Why do you think this might be?
- Yes, you are correct! The magnetic anomalies are parallel to the spreading ridges because they are formed when the crust cools along these spreading ridges.
- The Gorda plate is being crushed between all the other plates in the area. This causes the plate to deform internally. The figure in the lower right corner (Chaytor et al., 2004) shows some different models to explain the faults formed from this internal deformation. The map in the upper right center, also from Chaytor et al. (2004) shows how they interpret some of these normal faults to be reactivated as strike-slip faults.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- Here are two posters from the 2018 Gorda Rise earthquakes.
- This version includes earthquakes M ≥ 5.0 from the USGS. Note how the region where today’s earthquakes happened is a region of higher levels of seismicity. Perhaps this is because this region is the locus of the deformation within the Mendocino deformation zone?
Other Report Pages
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.
- Hemphill-Haley, E., 1995. Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington in GSA Bulletin, v. 107, p. 367-378.
- Nelson, A.R., Shennan, I., and Long, A.J., 1996. Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America in Journal of Geophysical Research, v. 101, p. 6115-6135.
- Atwater, B.F. and Hemphill-Haley, E., 1997. Recurrence Intervals for Great Earthquakes of the Past 3,500 Years at Northeastern Willapa Bay, Washington in U.S. Geological Survey Professional Paper 1576, Washington D.C., 119 pp.
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
- This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.
- Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.
- 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.
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.
- These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
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.
- 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).
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.
- 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 a link to the embedded video below, showing the week-long seismicity in April 1992.
- This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
- I include some inset maps.
- In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
- In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.
- Here is a link to the embedded video below, showing these earthquakes.
- 1700.09.26 M 9.0 Cascadia’s 315th Anniversary 2015.01.26
- 1700.09.26 M 9.0 Cascadia’s 316th Anniversary 2016.01.26 updated in 2017 and 2018
- 1992.04.25 M 7.1 Cape Mendocino 25 year remembrance
- 1992.04.25 M 7.1 Cape Mendocino 25 Year Remembrance Event Page
- Earthquake Information about the CSZ 2015.10.08
- 2020.05.18 M 5.5 Gorda Rise
- 2018.07.24 M 5.6 Gorda plate
- 2018.03.22 M 4.6/4.7 Gorda plate
- 2017.07.28 M 5.1 Gorda plate
- 2016.09.25 M 5.0 Gorda plate
- 2016.09.25 M 5.0 Gorda plate
- 2016.01.30 M 5.0 Gorda plate
- 2015.12.29 M 4.9 Gorda plate
- 2015.11.18 M 3.2 Gorda plate
- 2014.03.13 M 5.2 Gorda Rise
- 2014.03.09 M 6.8 Gorda plate p-1
- 2014.03.23 M 6.8 Gorda plate p-2
- 2010.01.10 M 6.5 Gorda plate
- 2019.08.29 M 6.3 Blanco transform fault
- 2018.08.22 M 6.2 Blanco transform fault
- 2018.07.29 M 5.3 Blanco transform fault
- 2015.06.01 M 5.8 Blanco transform fault p-1
- 2015.06.01 M 5.8 Blanco transform fault p-2 (animations)
- 2020.03.09 M 5.8 Mendocino fault
- 2018.01.25 M 5.8 Mendocino fault
- 2017.09.22 M 5.7 Mendocino fault
- 2016.12.08 M 6.5 Mendocino fault, CA
- 2016.12.08 M 6.5 Mendocino fault, CA Update #1
- 2016.12.05 M 4.3 Petrolia CA
- 2016.10.27 M 4.1 Mendocino fault
- 2016.09.03 M 5.6 Mendocino
- 2016.01.02 M 4.5 Mendocino fault
- 2015.11.01 M 4.3 Mendocino fault
- 2015.01.28 M 5.7 Mendocino fault
- 2020.03.18 M 5.2 Petrolia
- 2019.06.23 M 5.6 Petrolia
- 2017.03.06 M 4.0 Cape Mendocino
- 2016.11.02 M 3.6 Oregon
- 2016.01.07 M 4.2 NAP(?)
- 2015.10.29 M 3.4 Bayside
- 2018.10.22 M 6.8 Explorer plate
- 2017.01.07 M 5.7 Explorer plate
- 2016.03.19 M 5.2 Explorer plate
- 2017.06.11 M 3.5 Gorda or NAP?
- 2016.07.21 M 4.7 Gorda or NAP? p-1
- 2016.07.21 M 4.7 Gorda or NAP? p-2
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco transform fault
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
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>
- Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
- 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
- Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
- Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
- Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
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I was slowly waking up while looking at my social media feed. moments before (maybe minutes) Anthony Lomax had posted his first motion earthquake mechanism for a M 6.4 near the CA/NV border. I leaped out of bed and got off a map before i had the time to put any clothes on (one benefit of teleworking that i won’t go into too many details).
Simplified tectonic map of the western U.S. Cordillera showing the modern plate boundaries and tectonic provinces. Basin and Range Province is in medium gray; Central Nevada seismic belt (CNSB), eastern California shear zone (ECSZ), Intermountain seismic belt (ISB), and Walker Lane belt (WLB) are in light gray; Mina deflection (MD) is in dark gray.
Shaded relief map of the WLB and northern part of the ECSZ showing the major Quaternary faults. Solid ball is located on the hanging wall of normal faults; arrow pairs indicate relative motion across strike-slip faults; white dashed box outlines location of Figure 2; light gray shaded areas show the Mina deflection and the Carson domain. BSF—Benton Springs fault; CF—Coaldale fault; DSF—Deep Springs fault; DVFCFLVFZ—Death Valley–Furnace Creek–Fish Lake Valley fault zone; GHF—Gumdrop Hills fault; HLF—Honey Lake fault; HMF—Hunter Mountain fault; MVF—Mohawk Valley fault; OVF— Owens Valley fault; PLF—Pyramid Lake fault; PSF—Petrified Springs fault; QVF—Queen Valley fault; SLF—Stateline fault; SNFFZ—Sierra Nevada frontal fault zone; WMFZ—White Mountains fault zone; WRF—Wassuk Range fault; WSFZ—Warm Springs fault zone.
(Left) Model to visualize accommodation of strain and development of basins in northern Walker lane. The upper is a block of wax has been heated to become ductile and subjected to transtensional right-lateral shear. Ice has been applied to the surface of lower wax block to create brittle upon ductile layer, and then subjected to same shear. The transtensional shear results in a zone of deformation displaying rotation of ‘crustal blocks’, an en echelon arrangement of asymmetric ‘basins,’ observable extension along the axis of shear, and the ability to locally traverse the entire zone of shear without encountering a major fault structure. (Right) Oblique view of study area illustrates the en echelon arrangement and triangular shape of basins nested along the east edge of the Sierra Nevada. Black and colored lines are portions of Walker Lane faults shown in Fig. 1 (Wesnousky et al., 2012)
Physiographic and fault map of area of interest in northern Walker Lane shows major structural basins (numbered), active basin-bounding faults (thick black lines), and geodetic displacement field (red arrows). Shown in white boxes are geologically determined values of fault-normal extension (black-upper text), geodetic estimates of fault-normal extension (magenta-middle text) and geodetic estimates of fault-parallel strike-slip (blue-lower text) rates along each of the basin bounding faults. Two-headed arrows schematically show ranges of same values and correspond in arrangement and color to the values in boxes. The geologically determined extension rate arrows are placed adjacent to the sites of studies except for Lake Tahoe where the estimate is an average value across several submarine faults. Dotted (yellow) lines define paths AB, CD, and EF.
Regional map showing the block model boundaries (yellow lines) in relation to the topography and faults of the Central Walker Lane. The Central Walker Lane (region within the dashed black lines) lies between the northeast striking normal faults of the Basin and Range and the Sierra Nevada microplate. Black lines delin-eate major normal faults of the Central Walker Lane, and red lines mark the location of strike slip faults (arrows indicate slip direction). Paleomagnetic observations in-dicate that crustal blocks in the Carson Domain, Bodie Hills, and Mina Deflection accommodate dextral shear through clockwise vertical axis rotations (Cashman and Fontaine, 2000; Petronis et al., 2009; Rood et al., 2011b; Carlson et al., 2013). Orange lines mark the locations of surface rupture that resulted from historic earthquakes in the Central Nevada Seismic Belt. Faults traces are modified from the USGS Qua-ternary Fault and Fold database (U.S. Geological Survey, California Geological Survey, Nevada Bureau of Mines and Geology, 2006). Inset map shows the location of the study area in relation to other elements of the Pacific/North America Plate boundary zone.
Block motions, slip rates, and velocity residuals for the best fitting GPS model. (A) Rigid block rotation and translation exaggerated by a factor of 107(representing 10 million years of deformation). Color of block indicates vertical axis rotation rate. (B) Predicted fault slip rates represented by the thickness of black (red) line for dextral (sinistral) strike-slip motion and the length of blue (cyan) bar for fault normal extension (compression).
Schematic block diagrams illustrating two fault-slip transfer mechanisms between subparallel strike-slip faults proposed for the eastern California shear zone and Walker Lane belt. (A) Displacement transfer model whereby the magnitude of extension along the connecting normal faults is proportional to the amount of strike-slip motion transferred (modified from Oldow et al., 1994). (B) Block rotation model in which clockwise rotation of blocks, bounded by dextral faults, is accommodated by sinistral faults (model of McKenzie and Jackson, 1983, 1986).
Block diagrams illustrating models proposed to explain fault slip transfer across the Mina deflection. (A) Displacement transfer model in which normal slip along connecting faults transfers fault slip (modified from Oldow, 1992; Oldow et al., 1994). (B) Transtensional model showing a combination of sinistral and normal slip along connecting faults. (C) Clockwise block rotation model in which sinistral slip along connecting faults, combined with vertical axis rotation of intervening fault blocks, transfers fault slip (modified from McKenzie and Jackson, 1983, 1986). Single-barbed arrows show dextral fault motion across faults of the Eastern California shear zone (ECSZ) and Walker Lane belt (WLB) and sinistral motion along faults in the Mina deflection; half-circle double-barbed arrows indicate clockwise rotating fault blocks; solid ball is located on the hanging wall of normal slip faults; thin short lines indicate slip direction on fault surfaces.
Shaded relief map of the southern part of the Mina deflection and northern part of the eastern California shear zone showing the major Quaternary faults. Solid black ball is located on the hanging wall of normal faults; arrow pairs indicate relative motion across strike-slip faults. Heavy arrow in northwest corner of map shows the present-day motion of the Sierra Nevada (SN) with respect to North America (NA) (Dixon et al., 2000). Location of the Adobe Hills geologic map shown in Figure 4A is outlined with a dashed line and location of this map is shown in Figure 1. PS—Pizona Springs; CF—Coaldale fault; CSF— Coyote Springs fault; DSF—Deep Springs fault; FLVFZ—Fish Lake Valley fault zone; HCF—Hilton Creek fault; OVF—Owens Valley fault; QVF—Queen Valley fault; RVF— Round Valley fault; WMFZ—White Mountains fault zone.
First-motion mechanism: Mw6.4 #earthquake California – Nevada Border Region — Anthony Lomax 🌍🇪🇺 (@ALomaxNet) May 15, 2020 Mw=6.5, NEVADA (Depth: 10 km), 2020/05/15 11:03:26 UTC – Full details here: https://t.co/Q4iICPhE1H pic.twitter.com/ZpxNMOu8gO — Earthquakes (@geoscope_ipgp) May 15, 2020 #earthquake This mornings earthquake outside Tonopah has damaged a portion of US95 at Mile Marker 98 in Esmerelda County. US 95 is closed in both directions, seek alternate routes. DOT is on scene making repairs, no timeframe given. #nhpsocomm pic.twitter.com/IF2Dp1s3Zc — NHP Southern Command (@NHPSouthernComm) May 15, 2020 GSN and other global surface & body wave record sections for the Mww 6.5 NEVADA https://t.co/ek0b6iw69i pic.twitter.com/6HmqiEzlC0 — IRIS Earthquake Sci (@IRIS_EPO) May 15, 2020 Aftershocks from the M6.5 #NevadaEarthquake cluster in proximity to the mainshock, as expected but, interestingly, the sequence includes a halo of more distance aftershocks… pic.twitter.com/UnAImI9j30 — Susan Hough (@SeismoSue) May 15, 2020 .@EsmeraldaCounty Sheriff showing the damage down after the 6.5 magnitude earthquake near Mina, NV. Road damage and hazards on Hwy 95 and US 6 this morning. pic.twitter.com/K998rhVOvK — Steve Bender (@SteveBenderWx) May 15, 2020 Watch the waves from the M6.5 #NevadaEarthquake roll across seismic stations in North America! (THREAD) pic.twitter.com/4SHwiMWtRK — IRIS Earthquake Sci (@IRIS_EPO) May 15, 2020 Eyewitnesses are RT sensors of M6.4 Nevada quake! Look at the propagation of the propagation of seismic waves and how fast users launch LastQuake app (orange dots) pic.twitter.com/9QAi5GBJ2k — EMSC (@LastQuake) May 15, 2020 Auto solution FMNEAR (Géoazur/OCA) with regional records for the 2020/05/15 11:03:28 M6.5 NEVADA, USA(Loc EMSC used to trigger inversion).https://t.co/UHDsc1hVXA (not on mobile version) — Bertrand Delouis (@BertrandDelouis) May 15, 2020 Real-time displacements for the Tonopah EQ this morning at P132. This site is roughly 65 km north of the event, so the signal is transverse to the propagation direction. @UNAVCO pic.twitter.com/CkdmhOV9uy — Brendan Crowell (@bwcphd) May 15, 2020 Aftershocks to Mina/Tonopah M6.5 #earthquake as of about 1800hrs. Definitely looks like sinistral E-W plane, which would fit with possible surface rupture on US95 pic.twitter.com/xdsyvFaVPR — Michael Bunds (@cataclasite) May 15, 2020 And some velocities at P133, 81 km north of the Tonopah EQ. Similar strong east motions, would be roughly MMI4. pic.twitter.com/hVzZlp84YR — Brendan Crowell (@bwcphd) May 15, 2020 Auto slipmap with regional records (SLIPNEAR method, single fault plane) for the 2020/05/15 11:03:28 M6.5 NEVADA(Loc EMSC), FM solution FMNEAR. Plane NS (strike 355) selected as rupt plane, possible directivity towards the North. Géoazur/OCA – EU project SERA pic.twitter.com/mTXgrDFZJL — Bertrand Delouis (@BertrandDelouis) May 15, 2020 Interesting! Looks like similar strong E-W displacements at Little Huntoon Valley station (56 km west of epicenter). Almost 2 cm, not too shabby. pic.twitter.com/JItG0H8Kl0 — Brian OLSON-19 (@mrbrianolson) May 15, 2020 Strong Nevada earthquake felt in San Francisco Bay Area, Bakersfield a https://t.co/xWjXtgo1Nl via @temblor — temblor (@temblor) May 15, 2020 Finally, it's worth noting how today's sequence sits with respect to a line of ruptures extending from 2019 Ridgecrest to 1915 Pleasant Valley. Does this portend anything larger? Dunno, there are so many active faults in here you can almost choose your own adventure. But: Yikes. pic.twitter.com/Go7fzQlMMW — Rich Briggs (@rangefront) May 16, 2020 @patton_cascadia The stress modeling caused by the Mw 6.4 on optimally oriented strike slip faults show a causative relation between stress and aftershocks distributions. The regional stress used in this second modeling are from Frizzell and Zoback ( https://t.co/ueLHOWVihn) pic.twitter.com/BwEO83QmgP — Jugurtha Kariche (@JkaricheKariche) May 16, 2020 Final tweet of this thread (my first thread!) – circling back to #MinaDeflection! Here are source focal mechs aka beach balls for ~330 #earthquakes and a preliminary stress inversion I did 6 years ago as part of my #dissertation #proposal Green=LL, Yellow=RL, Red=Normal pic.twitter.com/qgdlpIaqHx — Dr. Christine J. Ruhl (@christineruhl) May 16, 2020 It also might be "ground oscillation" as described by Youd (1995). This is common on very flat ground surfaces (<<2%). When you saw it on the ground, did you observe any sand boils along the margins or within the disturbed ground? pic.twitter.com/Xas2qnmHsv — Brian OLSON-19 (@mrbrianolson) May 16, 2020 — Cynthia L. Pridmore (@earthquakemom) June 11, 2020
Well, last weekend I was working on a house, so did not have the time to write this up until now. (2023 update: the magnitude is now M 6.5) https://earthquake.usgs.gov/earthquakes/eventpage/us700098qd/executive The eastern Mediterranean Sea region is dominated by plate tectonics (no surprise, right?). The plate boundary fault system that is responsible for this earthquake near Crete is a convergent plate boundary called a subduction zone. Convergent means that one plate is moving towards another plate. One of the largest plate boundary systems in the world is a convergent plate boundary that extends from between the north side of Australia and Indonesia, through southern Asia forming the Himalayan Mountains, through the Middle East, into Europe and west past the Mediterranean. Near Crete the Africa plate is diving (northwards) beneath the Anatolia plate (a sliver of the Eurasia plate). The 2 May magnitude M 6.6 earthquake appears to have been an earthquake on the subduction zone megathrust fault interface (a subduction zone earthquake). The earthquake even caused a tsunami that was recorded at teh Lerapetra tide gage in Crete, Greece. The wave was small at about 40 cm peak to trough (measured vertically from the highest part of the wave, the peak, to the lowest part of the wave, the trough).
Tectonic map of the Aegean and eastern Mediterranean region showing the main plate boundaries, major suture zones, fault systems and tectonic units. Thick, white arrows depict the direction and magnitude (mm a21) of plate convergence; grey arrows mark the direction of extension (Miocene–Recent). Orange and purple delineate Eurasian and African plate affinities, respectively. Key to lettering: BF, Burdur fault; CACC, Central Anatolian Crystalline Complex; DKF, Datc¸a–Kale fault (part of the SW Anatolian Shear Zone); EAFZ, East Anatolian fault zone; EF, Ecemis fault; EKP, Erzurum–Kars Plateau; IASZ, Izmir–Ankara suture zone; IPS, Intra–Pontide suture zone; ITS, Inner–Tauride suture; KF, Kefalonia fault; KOTJ, Karliova triple junction; MM, Menderes massif; MS, Marmara Sea; MTR, Maras triple junction; NAFZ, North Anatolian fault zone; OF, Ovacik fault; PSF, Pampak–Sevan fault; TF, Tutak fault; TGF, Tuzgo¨lu¨ fault; TIP, Turkish–Iranian plateau (modified from Dilek 2006).
Summary sketch map of the faulting and bathymetry in the Eastern Mediterranean region, compiled from our observations and those of Le Pichon & Angelier (1981), Taymaz (1990), Taymaz et al. (1990, 1991a, b); S¸arogˇlu et al. (1992), Papazachos et al. (1998), McClusky et al. (2000) and Tan & Taymaz (2006). Large black arrows show relative motions of plates with respect to Eurasia (McClusky et al. 2003). Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). Shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; PTF, Paphos Transform Fault; CTF, Cephalonia Transform Fault; PSF, Pampak–Sevan Fault; AS, Apsheron Sill; GF, Garni Fault; OF, Ovacık Fault; MT, Mus¸ Thrust Zone; TuF, Tutak Fault; TF, Tebriz Fault; KBF, Kavakbas¸ı Fault; MRF, Main Recent Fault; KF, Kagˇızman Fault; IF, Igˇdır Fault; BF, Bozova Fault; EF, Elbistan Fault; SaF, Salmas Fault; SuF, Su¨rgu¨ Fault; G, Go¨kova; BMG, Bu¨yu¨k Menderes Graben; Ge, Gediz Graben; Si, Simav Graben; BuF, Burdur Fault; BGF, Beys¸ehir Go¨lu¨ Fault; TF, Tatarlı Fault; SuF, Sultandagˇ Fault; TGF, Tuz Go¨lu¨ Fault; EcF, Ecemis¸ Fau; ErF, Erciyes Fault; DF, Deliler Fault; MF, Malatya Fault; KFZ, Karatas¸–Osmaniye Fault Zone.
GPS horizontal velocities and their 95% confidence ellipses in a Eurasia-fixed reference frame for the period 1988–1997 superimposed on a shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). Large arrows designate generalized relative motions of plates with respect to Eurasia (in mm a21) (recompiled after McClusky et al. 2000). NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; CTF, Cephalonia Transform Fault; PTF, Paphos Transform Fault; CMT, Caucasus Main Thrust; MRF, Main Recent Fault.
Schematic map of the principal tectonic settings in the Eastern Mediterranean. Hatching shows areas of coherent motion and zones of distributed deformation. Large arrows designate generalized regional motion (in mm a21) and errors (recompiled after McClusky et al. (2000, 2003). NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; CTF, Cephalonia Transform Fault; PTF, Paphos Transform Fault.
Simplified map showing the main structural features along the Hellenic arc and trench system, as well as the main active structures in the Aegean area. The mean GPS horizontal velocities in the Aegean plate, with respect to a Eurasia-fixed reference frame, are shown (after Kahle et al., 1998; McClusky et al., 2000). The lengths of vectors are
Simplified tectonic map of the Mediterranean region showing the plate boundaries, collisional zones, and directions of extension and tectonic transport. Red lines A through G show the approximate profile lines for the geological traverses depicted in Figure 2. MHSZ—mid-Hungarian shear zone; MP—Moesian platform; RM—Rhodope massif; IAESZ— Izmir-Ankara-Erzincan suture zone; IPS—Intra-Pontide suture zone; ITS—inner Tauride suture zone; NAFZ—north Anatolian fault zone; KB—Kirsehir block; EKP—Erzurum-Kars plateau; TIP—Turkish-Iranian plateau.
Simplified tectonic cross-sections across various segments of the broader Alpine orogenic belt.
Late Mesozoic–Cenozoic geodynamic evolution of the western Anatolian orogenic belt as a result of collisional
Focal mechanisms of earthquakes over the Aegean Anatolian region.
Early morning (my time) there was an intermediate depth earthquake in the Banda Sea.
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).
Reconstructions of eastern Indonesia, adapted from Hall (2012), depict collision of Australia with Southeast Asia and slab rollback into Banda Embayment. Yellow star indicates Seram. Oceanic crust is shown in purple (older than 120 Ma) and blue (younger than 120 Ma); submarine arcs and oceanic plateaus are shown in cyan; volcanic island arcs, ophiolites, and material accreted along plate margins are shown in green. A: Reconstruction at 15 Ma. B: Reconstruction at 7 Ma. C: Reconstruction at 2 Ma. D: Visualization of present-day slab morphology of proto–Banda Sea based on earthquake hypocenter distribution and tomographic models
The Banda arc and surrounding region. 200 m and 4,000 m bathymetric contours are indicated. The numbered black lines are Benioff zone contours in kilometres. The red triangles are Holocene volcanoes (http://www.volcano.si.edu/world/). Ar=Aru, Ar Tr=Aru trough, Ba=Banggai Islands, Bu=Buru, SBS=South Banda Sea, Se=Seram, Sm=Sumba, Su=Sula Islands, Ta=Tanimbar, Ta Tr=Tanimbar trough, Ti=Timor, W=Weber Deep.
Tomographic images of the Banda slab. Vertical sections through the tomography model along the lines shown in Fig. 1. Colours: P-wave anomalies with reference to velocity model ak135 (ref. 30). Dots: earthquake hypocentres within 12 km of the section. The dashed lines are phase changes at ~410 km and ~660 km. The sections are plotted without vertical exaggeration; the horizontal axis is in degrees. The labelled positive anomalies are the Sunda (Su) and Banda (Ba) slabs: BuDdetached slab under Buru, FlDslab under Flores, SDslab under Seram, TDslab under Timor. a, The Sunda slab enters the lower mantle whereas the Banda embayment slab is entirely in the upper mantle with the change under Sulawesi. b–e, Banda slab morphology in sections parallel to Australia plate motion shows a transition from a steep slab with a flat section (fs) (b) to a spoon shape shallowing eastward (c–e).
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.
Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.
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.
Plate boundary segments in the Banda Arc region from Nugroho et al (2009). Numbers inside rectangles show possible micro-plate blocks near the Sumba Triple Junction (colored) based on GPS velocities (black arrows) with in a stable Eurasian reference frame.
Schematic map views of kinematic relations between major crustal elements in the Sumba Triple Junction region. CTZ= collisional tectonic zone. Red arrow size designates schematic plate motion relations based on geological data relative to a fixed Sunda shelf reference frame (pin).
Earthquake Report: Mina Deflection in Nevada
https://earthquake.usgs.gov/earthquakes/eventpage/nn00725272/executive
There was a swarm of earthquakes east of Mono Lake in April along the Huntoon Valley fault zone. These events are aligned with faults that trend east-northeast. The plate boundary relative plate motion is generally aligned with the San Andreas fault system (north-northwest trending right-lateral strike-slip faulting). To the east, tectonics are dominated by ~east-west extension in the Basin and Range geomorphic province.
As the plate boundary organizes itself along the east side of the Sierra Nevada, it has some disruptions, with blocks that are rotating in ways (within this right-lateral shear) that lead to these northeast trending faults.
As the blocks rotate, the faults that bound these blocks (the east-northeast faults) are left-lateral strike-slip faults. More on this below.
In April these earthquakes were along some of these east-northeast left-lateral faults, with the largest magnitude = M 5.2. Today’s ongoing sequence is to the east along the same trend of faulting, along faults mapped by Tom Sawyer of Piedmont Geosciences called “unnamed faults of the Candelaria Hills.”
It seems hard to believe that these earthquakes are unrelated. They are within about a month of each other. They are along the same fault system.
Taking this thought experiment through, it seems that an earthquake could happen between these earthquakes along this fault system.
Using Wells and Coppersmith (1994) empirical relations between surface rupture length (the length of the fault that would break through the ground surface) and earthquake magnitude can help us estimate what magnitude of an earthquake may be given a rupture length.
If we use these relations, and the distance between these earthquakes, we measure a distance of 50 km and can calculate a magnitude of M 7.0. Thus, it would be prudent for people in the region to stay safe and ensure that they are prepared for a potentially larger earthquake.
There is an alternate explanation for today’s sequence. The M 6.4 was on a north-south oriented fault and was right-lateral strike-slip, while many of the aftershocks are actually instead triggered earthquakes on the east-west trending left-lateral strike-slip earthquake fault system. Which recent earthquake sequence had two almost perpendicularly relative orientations? Yes, the Ridgecrest Earthquake Sequence. Now, this is a more complicated explanation, but it is possible (tho at this point, i deem it unlikely).
UPDATE 16 MAY
Reports from the field are that the surface rupture is north-south just east of Rock Hill (feature shown on USGS earthquake event page web map). Rock Hill is west of the M 6.5 epicenter, and just east of HWY 95 38.149 N 117.941 W. Observations from Jamie Shutmut.
perhaps the alternate hypothesis mentioned last night was the correct hypothesis?
UPDATE later
There have been many different observations in the field to suggest that the surface deformation from this event sequence is broadly distributed and the offsets at the surface are on the order of centimeters, maybe a half a decimeter.
It appears that the main fault was ~east-west, but that there may be some north-south structure involved too.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.
USGS Shaking Intensity
Other Report Pages
Some Relevant Discussion and Figures
Basin and Range
General Overview
Earthquake Reports
Utah
Idaho
Nevada
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Shows strike-slip faulting https://t.co/6dsdgupJzY pic.twitter.com/zMm4VSdVTM
Thanks to the seismic records provided by NCEDC, SCEC, and IRIS pic.twitter.com/LKGvEIRUtK
UPDATE 2020.06.11
References:
Basic & General References
Specific References
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Earthquake Report: M 6.5 in Crete, Greece
The earthquake was felt across the region with intensity as high as MMI 6 in Crete, to around MMI 4 in Cairo, Egypt.
Here are the tide gage data downloaded from the IOC website here. The tsunami starts at around 13:00 hours.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.
Other Report Pages
Some Relevant Discussion and Figures
proportional to the amount of movement. The thick black arrows indicate the mean motion vectors of the plates. The polygonal areas on the map (dashed lines) define the approximate borders of the five different structural regions discussed in the text. The borders between structural regions are not straightforward, and wide transitional zones probably exist between them. The inset shows a schematic map with the geodynamic framework in the eastern Mediterranean area (modified from McClusky et al., 2000). DSF—Dead Sea fault; EAF—East Anatolia fault; HT—Hellenic trench; KFZ— Kefallonia fault zone; MRAC—Mediterranean Ridge accretionary complex; NAF—North Anatolia fault; NAT—North Aegean trough.
necking and asthenospheric upwelling have produced locally well-developed alkaline volcanism (e.g., Sardinia). Slab tear or detachment in the Calabria segment of Adria, as imaged through seismic tomography (Spakman and Wortel, 2004), is probably responsible for asthenospheric upwelling and alkaline volcanism in southern Calabria and eastern Sicily (e.g., Mount Etna). Modified from Séranne (1999), with additional data from Spakman et al. (1993); Doglioni et al. (1999); Spakman and Wortel (2004); Lentini et al. (this volume).
and extensional processes in the upper plate of north-dipping subduction zone(s) within the Tethyan realm. See text
for discussion.
Seismic Hazard and Seismic Risk
Europe
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References:
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Earthquake Report: Banda Sea
https://earthquake.usgs.gov/earthquakes/eventpage/us70009b14/executive
This earthquake was a strike-slip earthquake in the Australia plate. There are analogical earthquakes in the same area in 1963, 1987, 2005, and 2012 that appear to have occurred on the same fault.
In June 2019 there was an earthquake nearby with a similar mechanism.Below is my interpretive poster for this earthquake
Global Strain
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
Seismic Hazard and Seismic Risk
Indonesia | Sumatra
General Overview
Earthquake Reports
Social Media
References:
Basic & General References
Specific References
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