Source: http://www.science.earthjay.com/?cat=17
Timestamp: 2019-04-21 02:41:58+00:00

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This morning (my time) there was a possibly shallow earthquake in western Iran with a magnitude of M = 6.3. This earthquake occurred in the aftershock zone of the 2017.11.12 M 7.3 earthquake. Here is my report for the M 7.3 earthquake. Here are the USGS webpagea for the M 6.3 and M 7.3 earthquakes.
The M 7.3 earthquake was a reverse/thrust earthquake associated with tectonics of the Zagros fold and thrust belt. This plate boundary fault system is a section of the Alpide belt, a convergent plate boundary that extends from the west of the Straits of Gibraltar, through Europe (causing uplift of the Alps and subduction offshore of Greece), the Middle East, India (causing the uplift forming the Himalayas), then to end in eastern Indonesia (forming the continental collision zone between Australia and Indonesia).
Some of the earthquakes (including this one) are strike-slip earthquakes (see explanation of different earthquake types below in the geologic fundamentals section). So, one might ask why there are strike-slip earthquakes associated with a compressional earthquake?
As pointed out by Baptiste Gombert, these strike-slip earthquakes are are evidence of strain partitioning. Basically, when relative plate motion (the direction that plates are moving relative to each other) is not perpendicular or parallel to a tectonic fault, this oblique motion is partitioned into these perpendicular and parallel directions.
A great example of this type of strain partitioning is the plate boundary offshore of Sumatra where the India-Australia plate subducts beneath the Sunda plate (part of Eurasia). The plate boundary is roughly N45W (oriented to the northwest with an azimuth of 325°) and the relative plate motion direction is oriented closer to a north-south orientation. The relative plate motion perpendicular to the plate boundary is accommodated by earthquakes on the subduction. These earthquakes are oriented showing compression in a northeast direction. Along the axis of Sumatra is a huge strike-slip fault called the Great Sumatra fault. This fault is parallel to the plate boundary and accommodates relative plate motion parallel to the plate boundary. The Great Sumatra fault is a fault called a forearc sliver fault.
There are other examples of this elsewhere, like here in western Iran/eastern Iraq. Relative plate motion between the Arabia and Eurasia plates is oriented north-south, but the plate boundary is oriented northwest-southeast (just like the Sumatra example). So this oblique relative plate motion is partitioned into fault normal compression (the M 7.3 earthquake) and fault parallel shear (today’s M 6.3 earthquake).
There is also a strike-slip fault in the region of today’s M 6.3, the Khanaqin fault. So, given what we know about the tectonics and historic seismicity, I interpret today’s M 6.3 earthquake to have been a strike-slip earthquake associated with the Khanaqin fault, triggered by changes in stress by the M 7.3 earthquake. I could be incorrect and this earthquake could be unrelated to the > 7.3 earthquake.
I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 5.0 in one version.
I include an inset map showing seismicity from 2016.11.22 through 2018.11.28 showing the aftershocks from the M 7.3 earthquake. Note the cluster of earthquakes to the south of the aftershock zone. This is a swarm with earthquakes in the lower to mid M 5 range. The earthquakes with mechanisms are compressional, oriented the same as the M 7.3.
In the upper left corner is a map showing the regional plate boundary faults and some information about relative plate motions (Stern and Johnson, 2010). As for other inset figures, I plate a transparent cyan star in the general location of today’s M 6.3 earthquake.
In the lower left corner is a similarly scaled tectonic map from Scharf et al. (2015) showing more information about the amount of plate motion in the Tertiary (post 66 Ma). Note the contrast of the extension (in red) associated with the rifting in east Africa and the convergence (in blue) associated with the Alpide belt in this area.
In the upper right corner is a structural cross section showing the folding of the crust and rocks associated with the convergence at this plate boundary (Verges et al., 2011). I show the general location for this cross section on the map as a cyan line with balls on the ends.
In the lower left center is a map from Emami et al. (2010). This map shows how this convergent plate boundary creates topography (uplift and mountains) with color. Lower elevations are shown as yellow and green and higher elevations are shown as red and brown. Note the location of the Khanaqin fault, a left-lateral strike slip fault..
In the upper left center is a map showing a kinematic interpretation of the faulting in this area (Hessami, 2002). While the focus of this PhD dissertation is for the faulting in the southern Zagros system, they show relative plate motions and how the Khanaqin fault may accommodate this plate motion (oblique to Zagros).
In the lower right corner is a map showing USGS seismicity from 2016.11.22 through 2018.11.25 for earthquakes M ≥ 3.0. The spatial extent of this area is shown in a dashed white rectangle on the main map.
In the lower right center is the USGS seismic hazard map for the region (Jenkins et al., 2014).
Here is the map with a century’s seismicity plotted for earthquakes M ≥ 5.0.
The Alpide Belt, shown in this map, is a convergent plate boundary that extends from Australia to Portugal. This map shows the westernmost extent of this system. The convergence here drives uplift of the Himalayas and the European Alps. Subduction along the Makran and Sunda subduction zones are also part of this system.
Below is the tectonic map from Stern and Johnson (2010).
Simpli”ed map of the Arabian Plate, with plate boundaries, approximate plate convergence vectors, and principal geologic features. Note location of Central Arabian Magnetic Anomaly (CAMA).
Here is the Scharf et al., 2015 map.
This is the Enami et al., 2010 figure.
Tectonic map of the Zagros Fold Belt showing the position and geometry of the Mountain Front Flexure (MFF). Earthquakes of M ≥ 5 are indicated by small black diamonds. Focal mechanisms from Talebian & Jackson (2004) are also shown, in black (Mw ≥ 5.3) and grey (Mw ≥ 5.3). KH, Khavir anticline; SI, Siah Kuh anticline; ZDF, Zagros Deformation Front.
This is the tectonic map from Hessami, 2002.
a) Earthquakes with mb > 5.0 (Jackson and McKenzie, 1984) along seismogenic basement thrusts offset by major strike-slip faults. b) Schematic interpretative map of the main structural features in the Zagros basement. The overall north-south motion of Arabia increases along the belt from NW to SE (arrows with numbers). Central Iran acted as a rigid backstop and caused the strike-slip faults with N-S trends in the west to bulge increasingly eastward. Fault blocks in the north (elongated NW-SE) rotate anticlockwise; while fault blocks in the south (elongated NE-SW) rotate clockwise. c) Simple model involving parallel paper sheets illustrating the observed strike-slip faults in the Zagros. Opening between the sheets (i.e. faults) helped salt diapirs to extrude.
Below are a series of figures from Verges et al., 2011. First is a map that shows the tectonics and locations of the cross section.
Tectonic map of the Zagros showing the location of the previously published cross-sections with the calculated amount of shortening and the extent of major hydrocarbon fields. The balanced cross-section is marked by the thick black line. M – Mand anticline. Dark grey: Naien-Baft ophiolites (Stöklin, 1968).
Here are the cross sections from Verges et al. (2011).
Structural cross-sections showing the style of folding across the studied regional transect (see location in Fig. 3). (a) The front of the Zagros Fold Belt along the Anaran anticline above the Mountain Front Flexure (MFF in Emami et al. 2010); (b) the Kabir Kuh anticline, which represents a multi-detachment fold (Vergés et al. 2010); (c) folds developed in the Upper Cretaceous basinal stratigraphy showing much tighter and upright anticlines (modified from Casciello et al. 2009).
Here is a map that displays an estimate of seismic hazard for the region (Jenkins et al., 2010). This comes from Giardini et al. (1999).
The Global Seismic Hazard Map. Peak ground acceleration (pga) with a 10% chance of exceedance in 50 years is depicted in m/s2. The site classification is rock everywhere except Canada and the United States, which assume rock/firm soil site classifications. White and green correspond to low seismicity hazard (0%-8%g), yellow and orange correspond to moderate seismic hazard (8%-24%g), pink and dark pink correspond to high seismicity hazard (24%-40%g), and red and brown correspond to very high seismic hazard (greater than 40%g).
Just found this as it as posted to the Bertrand tweet (see social media below). This is a figure from Talebian and Jackson (2004) that uses Sumatra as an analogue to the oblique convergence along the Zagros thrust. Pretty cool.
(a) Summary sketch of the tectonic pattern in the Zagros. Overall Arabia–Eurasia motions are shown by the big white arrows, as before. In the NW Zagros (Borujerd-Dezful), oblique shortening is partitioned into right-lateral strike-slip on the Main Recent Fault (MRF) and orthogonal shortening (large gray arrows). In the SE Zagros (Bandar Abbas) no strike-slip is necessary, as the shortening is parallel to the overall convergence. The central Zagros (Shiraz) is where the transition between these two regimes occurs, with anticlockwise rotating strike-slip faults allowing an along-strike extension (black arrows) between Bandar Abbas and Dezful. (b) A similar sketch for the Himalaya (after McCaffrey & N´abˇelek 1998). In this case the overall Tibet-India motion is likely to be slightly west of north. (The India-Eurasia motion is about 020◦, but Tibet moves east relative to both India and Eurasia: Wang et al. 2001). Thrust faulting slip vectors are radially outward around the entire arc (gray arrows). This leads to partitioning of the oblique convergence in the west, where right-lateral strike-slip is prominent on the Karakoram Fault, but no strike-slip in the east, where the convergence and shortening are parallel. The region in between extends parallel to the arc, on normal faults in southern Tibet. (c) A similar sketch for the Java–Sumatra arc, based on McCaffrey (1991). Slip partitioning occurs in the NW, with strike-slip faulting through Sumatra, but not in the SE, near Java. This change along the zone requires the Java–Sumatra forearc to extend along strike.
Emami, H., Verges, J., nalpas, T., Gillespie, P., Sharp, I., Karpuz, R., Blanc, E.P., and Goodarzi, G.H., 2010. Structure of the Mountain Front Flexure along the Anaran anticline in the Pusht-e Kuh Arc (NW Zagros, Iran): insights from sand box models in LETURMY, P. & ROBIN, C. (eds) Tectonic and Stratigraphic Evolution of Zagros and Makran during the Mesozoic–Cenozoic. Geological Society, London, Special Publications, 330, 155–178.
Giardini, D., Grunthal, G., Shedlock, K., Zhang. P., and Global Seismic Hazards Program, 1999. Global seismic hazards map: Accessed on Jan. 9, 2007 at http://www.seismo.ethz.ch/GSHAP.
Jenkins, Jennifer, Turner, Bethan, Turner, Rebecca, Hayes, G.P., Sinclair, Alison, Davies, Sian, Parker, A.L., Dart, R.L., Tarr, A.C., Villaseñor, Antonio, and Benz, H.M., compilers, 2013, Seismicity of the Earth 1900–2010 Middle East and vicinity (ver 1.1, Jan. 28, 2014): U.S. Geological Survey Open-File Report 2010–1083-K, scale 1:7,000,000, https://pubs.usgs.gov/of/2010/1083/k/.
Stern, R.J. and Johnson, P., 2010. Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis in Earth-Science Reviews, v. 101, p. 29-67.
Woudloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt.
Last night I had completed preparing for class the next day. I was about to head to bed. I got an email from the Pacific Tsunami Warning Center notifying me that there was no risk of a tsunami due to an earthquake with a magnitude M 6.6. I noticed it was along the Sovanco fault, a transform fault (right-lateral strike-slip). Strike slip faults can produce tsunami, but they are smaller than tsunami generated along subduction zones. The recent M = 7.5 Donggala Earthquake in Sulawesi, Indonesia is an example of a tsunami generated in response to a strike-slip earthquake (tho coseismic landslides may be part of the story there too).
I thought I could put together a map in short time as I already had a knowledge base for this area (e.g. earthquake reports from 2017.01.07 and 2016.03.18). However, as I was creating base maps in Google Earth, before I completed making a set (the posters below each take 4 different basemaps displayed at different transparencies), there was the M 6.8 earthquake. Then there was the M 6.6 earthquake. I had to start all over. Twice. Heheh.
This region of the Pacific-North America plate boundary is at the northern end of the Cascadia subduction zone (CSZ). To the east, the Explorer and Juan de Fuca plates subduct beneath the North America plate to form the megathrust subduction zone fault capable of producing earthquakes in the magnitude M = 9 range. The last CSZ earthquake was in January of 1700, just almost 319 years ago.
The Juan de Fuca plate is created at an oceanic spreading center called the Juan de Fuca Ridge. This spreading ridge is offset by several transform (strike-slip) faults. At the southern terminus of the JDF Ridge is the Blanco fault, a transtensional transform fault connecting the JDF and Gorda ridges.
At the northern terminus of the JDF Ridge is the Sovanco transform fault that strikes to the northwest of the JDF Ridge. There are additional fracture zones parallel and south of the Sovanco fault, called the Heck, Heckle, and Springfield fracture zones.
The first earthquake (M = 6.6) appears to have slipped along the Sovanco fault as a right-lateral strike-slip earthquake. Then the M 6.8 earthquake happened and, given the uncertainty of the location for this event, occurred on a fault sub-parallel to the Sovanco fault. Then the M 6.5 earthquake hit, back on the Sovanco fault.
So, I would consider the M 6.6 to be a mainshock that triggered the M 6.8. The M 6.5 is an aftershock of the M 6.6.
Based upon our knowledge of how individual earthquakes can change the stress (or strain) in the surrounding earth, it is unlikely that this earthquake sequence changed the stress on the megathrust. Over time, hundreds of these earthquakes do affect the potential for earthquakes on the CSZ megathrust. But, individual earthquakes (or even a combination of these 3 earthquakes) do not change the chance that there will be an earthquake on the CSZ megathrust. The chance of an earthquake tomorrow is about the same as the chance of an earthquake today. Day to day the chances don’t change much. However, year to year, the chances of an earthquake get higher and higher. But of course, we cannot predict when an earthquake will happen.
So, if we live, work, or play in earthquake country, it is best to always be prepared for an earthquake, for tsunami, and for landslides.
I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.
I include the earthquake mechanisms for 2 special earthquakes that happened in the past two decades along this plate boundary system. In 2001 the M 6.8 Nisqually earthquake struck the Puget Sound region of Washington causing extensive damage. This earthquake was an extensional earthquake in the downgoing JDF plate. The damage was extensive because the earthquake was close to an urban center, where there was lots of infrastructure to be damaged (the closer to an earthquake, the higher the shaking intensity).
In 2012 was a M = 7.8 earthquake along the northern extension of the CSZ. The northern part of the CSZ is a very interesting region, often called the Queen Charlotte triple junction. There are some differences than the Mendocino triple junction to the south, in northern California. There continues to be some debate about how the plate boundary faults are configured here. The Queen Charlotte is a right lateral strike slip fault that extends from south of Haida Gwaii (the large island northwest of Vancouver Island) up northwards, where it is called the Fairweather fault. There are several large strike-slip earthquakes on the Queen Charlotte/Fairweather fault system in the 20th century. However, the 2012 earthquake was a subduction zone fault, evidence that the CSZ megathrust (or some semblance of this subduction zone) extends beneath Haida Gwaii (so the CSZ and QCF appear to over lap).
We can see the roughly northeast-southwest trends of these red and blue stripes in the JDF and Pacific plates. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying North America plate (and accretionary prism) mask the evidence for the JDF plate.
In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I place a blue star in the general location of today’s seismicity.
In the upper left corner is a map showing the plate boundary faults associated with the northern CSZ and to the north (including the Queen Charlotte fault; Braunmiller and Nabalek, 2002). I place a red star in the general location of today’s seismicity. These earthquakes occurred in the region east of the Explorer rift. This region of the world still contains some major tectonic mysteries and this is quite exciting. This shows the Winona Block as a microplate between the Pacific and North America plates, north of the Explorer plate. The Winona Block is labeled “WIN BLOCK” on the map. Note that there are two spreading ridges on the western and central part of this block. It is possible that the Explorer ridge-rift system extends into the Winona Block to form a third spreading ridge in the Winona Block.
In the lower left corner is a map from Dziak (2006). Dziak (2006) used bathymetric and seismologic data to evaluate the faulting in the region and discussed how the Explorer plate is accommodating a reorganization of the plate boundary.
Here is a video showing the earthquake epicenters for the period of 1900-2017 for USGS earthquakes with magnitudes M ≥ 5.0. Here is a link to the embedded video below (2.5 MB mp4). Note how the earthquakes that happen between the northern terminus of the JDF Ridge and the southern terminus of the Queen Charlotte fault form a wide band (not a stepwise patter that might reflect steps in ridges and spreading centers). This pattern is key to unravelling the mysteries of the western Explorer plate.
Here is the map with the seismicity from 1900-2017 plotted. These are USGS earthquakes with magnitudes M ≥ 7.0 for this time period. I include the moment tensors from the 2012 and 2013 earthquakes (the only earthquakes for this time period that have USGS moment tensors). The 2012 earthquake generated a tsunami. I discuss the 2012 “Haida Gwaii” earthquake here.
This map shows the shape of the seafloor in this region and there is an inset map that shows the major fault systems here.
This map shows the line work Dziak (2006) used to delineate the structures shown in the bathymetric map.
This map shows the seismicity patterns (this matches the patterns in the animation above).
Here Dziak (2006) shows how they interpret that this plate boundary is being reconfigured with time. Like the rest of the adjacent plate boundary (Queen Charlotte/Fairweather, Cascadia, San Andreas), there is an overall dextral (right-lateral) shear couple between the North America and Pacific plates. Some of the existing structures represent the orientation of faults from an earlier strain field. Eventually through going faults will align with the band of seismicity in the above map and above animation. At least, that is one hypothesis. Seems reasonable to me, given the very short record of earthquakes.
From Audet et al. (2008), here is another view of the fault system in this part of the plate boundary.
Speaking of the Queen Charlotte/Fairweather fault system, here is another map that shows the tectonics of this region. Hyndman (2015) shows the region where the 2012 Haida Gwaii earthquake ruptured. I include two more figures below. This figure Below I include the text from the original figure caption in blockquote.
Small #earthquake near Yosemite NP California riding on the surface waves of the M6.5+ #earthquakes W of Vancouver earthquakes.
I put these together earlier this week for me classes and finally have a moment to write about these earthquakes. The Philippines region has been quite active lately, as it frequently is. I show below a series of earthquakes from the past ~30 days. These earthquakes occurred in 4 different regions and 3 different tectonic settings. These are probably unrelated to each other, but it is difficult to really know without further analyses. After I made these posters, there was an earthquake with a magnitude of M 5.8 on the island of Mindanao (I include the USGS link below), possibly associated with the Davao River fault (the closest fault mapped in this region).
Here are the USGS websites for these earthquakes.
I took a look at the seismicity from the past century. Here are Google Earth kml files from the USGS website for earthquakes from 1917-2017 with magnitudes M ≥ 7.0 and M ≥ 7.5.
I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 7.5.
In the upper left corner is a figure from Hall, 2011. This shows the plate tectonic configuration in the equatorial Pacific. Note how the upper panel shows a west dipping slab on the east side of the Philippines. Note the contrast in the center panel (Halmahera), where the eastern fault is dipping to the east (westward vergent) and the western fault is dipping to the west (eastward vergent). This region near Halmahera forms the Molucca Strait, one of the most tectonically active areas in this region.
In the lower left corner I include a map showing the seismicity and tectonic plate boundary faults for this region (Smoczyk et al., 2013). Earthquakes are plotted with color representing depth and diameter representing magnitude (see legend).
I choose 4 earthquakes for which I plot the MMI intensity.
The easternmost two are earthquakes related to the subduction of the Philippine Sea plate.
The northwesternmost earthquake series (largest M = 5.9) is interesting and mysterious. There is a left-lateral forearc sliver fault (Philippine fault) that parallels the Philippine trench. Internal deformation in the upper plate is accommodated by a complicated series of other strike-slip faults. In the region near Manila, there is a north-south striking right-lateral Mirikina Valley fault system. The MV fault trends south from Manila into the region of the M 5.9 earthquake. This fault seems to terminate in a northeast-southwest trending zone of “extension and young volcanism” (Nelson et al., 2000). Further to the south is an east-west striking left-lateral Lubang fault. This may be an extension of the Sibuyan Sea fault (Noda, 2013), a splay of the Philippine fault. If we look at the moment tensor for the M 5.5 and 5.9 earthquakes, the nodal planes suggest either NE striking left-lateral or NW striking right-lateral motion. This does not fit the orientation nor sense of motion for any of the mapped faults in the region. It is possible that these earthquakes are related to the extensional rifting instead. The moment tensor for the M 5.1 earthquake here shows an extensional beach ball. The orientation is not completely correct based upon the Nelson et al. (2000) figure, but there are no faults shown on their map.
The southwestern earthquakes from March (both M 5.6) are also interesting. The epicenters show locations on north Sulawesi, part of Indonesia. There is a NW striking left-lateral strike-slip faults mapped to the west and east of these earthquakes. However, the moment tensor shows that this would be a right-lateral fault in that orientation. The NW striking reverse fault is also equally challenging to interpret. Needless to say, this is a tectonically complicated region.
Below is my interpretive poster for the M 5.9 earthquake.
Here is a video shot by some divers who were diving at Anilao Camper, just a few kilometers from the epicenter. This was posted on facebook here by Jan Paul Rodriguez.
This is the low-angle oblique view of the region (Hall, 2011).
3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.
Here is the fault map for the region on the Island of Luzon (where Manila is located) from Nelson et al. (2000). The panel on the right (B) shows the Marikina Valley fault system (MV) and the Lubang fault. The MV is a right-lateral strike slip fault and the Lubang fault is a left-lateral strike-slip fault.
Tectonic setting of the Marikina Valley fault system (MV) in central Luzon, the Philippines. Diagram A shows subduction zone trenches by barbed lines, other faults with high rates of Quaternary activity by heavy black lines. White dots show locations of recent earthquakes on the Philippine fault in Luzon (M 7.8; 1990) and the Aglubang River fault in Mindoro (M 7.1; 1994). Diagram B shows how the Marikina Valley pull-apart basin (MV) may have been formed through extension caused by clockwise rotation (dashed circle) and shearing of central Luzon, which is caught between two active left-lateral strike-slip faults—the Philippine fault (Nakata et al., 1977; Barrier et al., 1991; Ringenbach et al., 1993; Aurelio et al., 1993) and the Lubang fault. A zone of extension and young volcanism south of the fault system has also influenced the structural development of the valley (Fo¨rster et al., 1990; Defant et al., 1988).
Here is the map from Noda (2013) that shows various strike slip faults associated with subduction zones.
Modern examples of trench-linked strike-slip faults. (A) The Median Tectonic Line (MTL) active fault system in southwestern Japan, related to oblique subduction of the Philippine Sea Plate (PS) along the Nankai Trough (NT). (B) The Great Sumatra Fault system (GSF) along the Java–Sumatra Trench (JST). (C) Strike-slip faults in Alaska. Fault names: DF, Denali; BRF, Boarder Ranges; CSEF, Chugach St. Elias; FF, Fairweather; TF, Transition. (D) The Philippine Fault system (PF). Abbreviations: SSF, Sibuyan Sea Fault; MT, Manila Trench; PT, Philippine Trench; ELT, East Luzon Trough. Plate names: AM, Amur; OK, Okhotsk; PS, Philippine Sea; AU, Australian; SU, Sundaland; NA, North American; PA, Pacific; YMC, Yukutat microcontinent. Black and purple lines are subduction zones and trench-linked strike-slip faults, respectively. All maps were drawn using SRTM and GEBCO with plate boundary data . Blue arrows indicate the direction and velocity of relative plate motion (mm yr-1) based on .
Here is the map from Smocyk et al., 2013, followed by the legend. The entire poster is here (92 MB pdf).
Below are two cross sections that show the subduction zone seismicity, followed by the legend. The location of these cross sections are labeled on the map above.
This map shows the seismic hazard for this region. The color represents the likelihood of any region experiencing ground shaking of a particular magnitude. The scale is “Peak Ground Acceleration.” Units are m/s^2. Purple represents gravitational acceleration of 1 g, gravity at Earth’s surface. Note how most of the earthquakes were in the region of higher likely ground shaking, except for the Sulawesi earthquakes.
In January of this year, there was an M 7.3 earthquake in the Celebes Sea south of the Philippines. Below is my interpretive map for that earthquake. I also present the same poster with 1917-2017 seismicity for earthquakes M ≥ 6.5. Here is my earthquake report for this M 7.3 earthquake. I include more background information for the Molucca Strait region on this page.
Hall, R., 2011. Australia–SE Asia collision: plate tectonics and crustal flow in Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 75–109.
McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
Nelson, A.R., Personius, S.F., Rimando, R.E>, Punongbayan, R.S., Tungol, N, Mirabueno, H., and Rasdas, A., 2000. Multiple Large Earthquakes in the Past 1500 Years on a Fault in Metropolitan Manila, the Philippines in BSSA vol. 90, p. 73-85.
Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.
This is a very interesting M 6.5 earthquake, which was preceded by a probably unrelated M 5.2 earthquake.
I plot the seismicity from the past century, with color representing depth and diameter representing magnitude (see legend). I include the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 2.5. The M 5.2 earthquake happened in a region that is seismically active and this preceded the M 6.5 earthquake. They are at a large distance and are unlikely related to each other.
I also include the generalized location of the East Africa Rift (EAR) in this region as yellow bands with white dashed lines. These are the Eastern Branch and Southwestern Branch of the EAR.
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. This earthquake is a normal fault event that strikes to the northwest and dips to the northeast or southwest. Due to the paucity of seismic data and mapped faults here, it is difficult to tell which is the principal fault plane. Someone online suggested that this may not be an earthquake, but an alien invasion force. Just joking. However, it is possible that this might have been an underground explosion. But, at 25 km, this seems highly unlikely.
In the lower right corner I include a geologic map for southern Africa and Botswana (Leseane et al., 2015). Note that the Southwestern Branch of the EAR extends into northwestern Botswana (the white fault lines in the red colored geologic unit). I place a red star in the general location of the M 6.5 earthquake. This earthquake happened in the Kaapvaal Craton, a region of low historical seismicity.
In the upper right corner I include a map that shows the seismic hazard for the EAR region of Africa (Hayes et al., 2014). Today’s M 6.5 earthquake happened in the lower left corner of the map. I place a red star in the general location of the M 6.5 earthquake. Note that this earthquake happened in a region of low seismic hazard.
In the upper left corner I include two maps and their associated plots. The map on the left is the shaking intensity map as modeled by the USGS, which uses the MMI scale. The plot below shows the results from the Ground Motion Prediction Equation (GMPE) used to generate the map. This plot shows how ground shaking (MMI) attenuates (diminishes as the seismic waves are absorbed by the earth) with distance from the earthquake. The map on the right is the “Did You Feel It?” map generated by the online responses from people who observed the ground shaking. This also uses the MMI scale. The plot below that shows how the reported observations match the GMPE relations for this earthquake. The GMPE relations are plotted as orange and green lines, which represent GMPE models developed for different geologic settings (e.g. hard rock like granite vs. soft rock like accreted terrane).
In the lower left corner I include the Rapid Assessment of an Earthquake’s Impact (PAGER) report. More on the PAGER program can be found here. An explanation of a PAGER report can be found here. PAGER reports are modeled estimates of damage. On the top left is a histogram showing estimated casualties and on the top right is an estimate of possible economic losses. There is a list of cities in the lower right corner which shows their populations and the MMI that they were likely exposed to.
This is the geologic map from the poster (Leseane et al., 2015). I include their caption below in blockquote.
Precambrian tectonic map of (a) southern Africa and (b) Botswana outlining the spatial extent of Archean cratons and Proterozoic orogenic belts. White lines represent the fault system of the Okavango Rift Zone. Modified after Singletary et al.  and Begg et al. .
Here is the USGS “Seismicity of the Earth” poster for this region (Hayes et al., 2014).
This is the latest geologic maps of Africa (Thieblemont, D., 2016). Click on the map for a 67 MB pdf version.
Leseane, K., Atekwana, E.A., Mickus, K.L., Abdelsalam, M.G., Shemanq, E.M., and Atekwana, E.A., 2015. Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana in JGR: Solid Earth, v. 120, doi:10.1002/2014JB011029.
There was an earthquake yesterday in the Gulf of California nearby a series of earthquakes that happened in 2015 and earlier in 2013. The 2017 and 2013 earthquakes are happening along a fault that forms the Carmen Basin and the 2015 earthquakes are rupturing a fault that appears to be in the middle of the Farallon Basin. Here is my Earthquake Report for the 2013 earthquake (an early report, so it is rather basic). Here is my Earthquake Report for the 2015 earthquake sequence. This is an update to the initial 2015 report.
I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 6.5.
In the upper left corner I include a map that shows the tectonic setting of this region, with the geological units colored relative to their age and type (marine or continental).This is from a paper that discusses the interaction between spreading ridges and subduction trenches (Fletcher et al., 2007).
In the upper right corner I include a larger saled version of this map with the same seismicity plotted, but without the MMI contours. One can see the shape of the seafloor and how this is formed due to plate tectonics. Note the spreading ridge in the lower right corner and the parallel ridges formed as the plates extend from the ridge. The magnitude scale is slightly different than the main map.
In the lower left corner I include a map from the 2015 earthquake series. I include this because I labeled the Basins formed by the enechelon steps in this plate boundary. In my 2015 report, I provide more maps that include the names of some of these fracture zones (the transform plate boundary strike-slip faults).
Here is a great diagram showing the major faults in the region. I include their figure caption below.
(A) Tectonic map of the southern Baja California microplate (BCM) and Gulf of California extensional province (GEP). The Magdalena fan is deposited on oceanic crust of the Farallon-derived Magdalena microplate located west of Baja California. Deep Sea Drilling Project Site 471 is shown as black dot on the Magdalena fan. Abbreviations: BCT—Baja California trench, BM—Bahia Magdalena, LC—Los Cabos block, T—Trinidad block, LP—La Paz, PV—Puerto Vallarta, SMSLF—Santa Margarita–San Lazaro fault, TAF—Tosco-Abreojos fault, TS—Todos Santos, V—Vizcaino peninsula. Geology is simplifi ed from Muehlberger (1996). Interpretation of marine magnetic anomalies, with numbers denoting the chron of positively magnetized stripes, is from Severinghaus and Atwater (1989) and Lonsdale (1991).
Map-view time slices showing the widely accepted model for the two-phase kinematic evolution of plate margin shearing around the Baja California microplate. (A) Configuration of active ridge segments (pink) west of Baja California just before they became largely abandoned ca. 12.3 Ma. (B) It is thought that plate motion from 12.3 to 6 Ma was kinematically partitioned into dextral strike slip (325 km) on faults west of Baja California and orthogonal rifting in the Gulf of California (90 km). This is known as the protogulf phase of rifting. (C) From 6 to 0 Ma faults west of Baja California are thought to have died and all plate motion was localized in the Gulf of California, which accommodated ~345 km of integrated transtensional shearing. Despite its wide acceptance, our data preclude this kinematic model. In all frames, the modern coastline is blue. Continental crust that accommodated post–12.3 Ma shearing is dark brown. Unfaulted microplates of continental crust are light tan. Farallon-derived microplates are light green. Middle Miocene trench-filling deposits like the Magdalena fan are colored dark green. Deep Sea Drilling Project Site 471 is the black dot on the southern Magdalena microplate. Yellow line (296 km) in the northern Gulf of California connects correlated terranes of Oskin and Stock (2003). Maps have Universal Transverse Mercator zone 12 projection with mainland Mexico fixed in present position.
Here is the Earthquake Report Poster from the 2015 sequence.
This is a nice simple figure, from the University of Sydney here, showing the terminology of strike slip faulting. It may help with the following figures.
Here is a fault block diagram showing how strike-slip step overs can create localized compression (positive flower) or extension (negative flower). More on strike-slip tectonics (and the source of this image) here.
Here is another great figure showing how sedimentary basins can be developed as a result of step overs in strike slip fault systems (source: Becky Dorsey, University of Oregon, Dept. of Geological Sciences).
I also put together an animation of seismicity from 1065 – 2015. First, here is a map that shows the spatial extent of this animation.
Here is the animation link (2 MB mp4 file) if you cannot view the embedded video below. Note how the animation begins in 1965, but has the recent seismicity plotted for reference.
There have been two large magnitude earthquakes in this region over the past 50 years.
This is a link to a tectonic summary map from the USGS. Click on the map below to download the 20 MB pdf file.
Fletcher, J.M., Grove, M., Kimbrough, D., Lovera, O., and Gehrels, G.E., 2007. Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon U-Pb ages from the Magdalena fan and adjacent areas in GSA Bulletin, v. 119, no. 11/12, p. 1313-1336.
Umhoefer, P.J., Mayer, L., and Dorse, R.J., 2002. Evolution of the margin of the Gulf of California near Loreto, Baja California Peninsula, Mexico in GSA Bulletin, v. 114, no. 7, p. 849-868.
In the past couple of weeks, there were some earthquakes in the equatorial western Pacific.
Here are the USGS websites for the two largest earthquakes in the two regions.
The M 6.1 was a well behaved subduction zone earthquake associated with subduction of the Solomon Sea plate beneath the Solomon Islands, an island arc formed between opposing subduction zones. The M 6.0 earthquake and related earthquakes are more interesting. Baldwin et al. (2012) and Holm et al. (2016) both consider the North Solomon Trench (NST) to be inactive subduction zone, while the South Solomon Trench (SST) is an active subduction zone fault. There were 9 earthquakes larger than magnitude 2.5. The depths ranged between 4.2 and 53 km. There does not appear to be any direction that favors a deepening trend (i.e. getting deeper in the direction of the downgoing subduction zone fault). It is possible that these depths are not well constrained (the M 6.0 has a 4.2 km depth). These NNT earthquakes are either related to the NST, related to crustal faults, or reactivation of NST fault structures. Regardless, these are some interesting and mysterious earthquakes.
I plot the seismicity from the past year, with color representing depth and diameter representing magnitude (see legend).
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The moment tensor shows northeast-southwest compression, perpendicular to the convergence at these plate boundaries.
In the upper right corner is a map from Holm et al. (2016) that shows various key elements of the plate configuration in this region. Colors representing the age of the crust shows how the Woodlark Basin has an active spreading ridge system with ridges striking East-West. Large Igneous Provinces are shown here in dark gray. A relevant cross section is C-D, with the location highlighted in magenta on the map. The cross section shows how the North Solomon Trench is configured.
In the lower left corner is a generalized tectonic map of the region from Holm et al., 2016. This map shows the major plate boundary faults. Active subduction zones have shaded triangle fault symbols, while inactive subduction zones have un-shaded triangle fault line symbols.
In the upper left corner a figure from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” This is from Johnson, 1976.
In the lower right corner is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross sections ‘A’ and ‘B’ are the most representative for the earthquakes today. I placed the general location of these cross sections on the main map as dashed orange lines. I present the map and this figure again below, with their original captions.
The M 6.1 is probably related to a series of earthquakes, including two M 7.9 megathrust events, further north along the San Crostobal Trench. Below are two earthquake interpretive posters for these earthquakes. This first poster is from my earthquake report on 2017.01.22.
Here is my interpretive poster from the 12/17 M 7.9 Bougainville Earthquake, possibly (probably) related to today’s M 7.9 earthquake. This is my Earthquake Report for the 12/17 earthquake.
This figure shows an interpretation of the regional tectonics (Holm et al., 2016). I include the figure caption below as a blockquote.
Tectonic setting of Papua New Guinea and Solomon Islands. a) Regional plate boundaries and tectonic elements. Light grey shading illustrates bathymetry b 2000 m below sea level indicative of continental or arc crust, and oceanic plateaus; 1000 m depth contour is also shown. Adelbert Terrane (AT); Bismarck Sea fault (BSF); Bundi fault zone (BFZ); Feni Deep (FD); Finisterre Terrane (FT); Gazelle Peninsula (GP); Kia-Kaipito-Korigole fault zone (KKKF); Lagaip fault zone (LFZ); Mamberamo thrust belt (MTB); Manus Island (MI); New Britain (NB); New Ireland (NI); North Sepik arc (NSA); Ramu-Markham fault (RMF); Weitin Fault (WF);West Bismarck fault (WBF); Willaumez-Manus Rise (WMR).
This figure shows details of the regional tectonics (Holm et al., 2016). I include the figure caption below as a blockquote.
(LP); Manus Basin (MB); New Britain trench (NBT); North Bismarck microplate (NBP); North Solomon trench (NST); Ontong Java Plateau (OJP); Ramu-Markham fault (RMF); San Cristobal trench (SCT); Solomon Sea plate (SSP); South Bismarck microplate (SBP); Trobriand trough (TT); projected Vanuatu slab (VS); West Bismarck fault (WBF); West Torres Plateau (WTP); Woodlark Basin (WB).
Holm, R.J., Rosenbaum, G, and Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Earth-Science Reviews, v. 156, pp. 66-81.

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