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Earthquake Report: 2018 Summary

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Here I summarize Earth’s significant seismicity for 2018. I limit this summary to earthquakes with magnitude greater than or equal to M 6.5. I am sure that there is a possibility that your favorite earthquake is not included in this review. Happy New Year.
However, our historic record is very short, so any thoughts about whether this year (or last, or next) has smaller (or larger) magnitude earthquakes than “normal” are limited by this small data set.
Here is a table of the earthquakes M ≥ 6.5.


Here is a plot showing the cumulative release of seismic energy. This summary is imperfect in several ways, but shows how only the largest earthquakes have a significant impact on the tally of energy release from earthquakes. I only include earthquakes M ≥ 6.5. Note how the M 7.5 Sulawesi earthquake and how little energy was released relative to the two M = 7.9 earthquakes.

Below is my summary poster for this earthquake year

  • I include moment tensors for the earthquakes included in the reports below.
  • Click on the map to see a larger version.


This is a video that shuffles through the earthquake report posters of the year

2018 Earthquake Report Pages

Other Annual Summaries

2018 Earthquake Reports

    General Overview of how to interact with these summaries

    • Click on the earthquake “magnitude and location” label (e.g. “M 6.9 Fiji”) to go to the Earthquake Report website for any given earthquake. Click on the map to open a high resolution pdf version of the interpretive poster. More information about the poster is found on the Earthquake Report website.
    • 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 ≥ 7.5 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.

    Background on the Earthquake Report posters

    • I placed a moment tensor / focal mechanism legend on the posters. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the maps. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

    Magnetic Anomalies

    • In the maps below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
    • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.

2018.01.10 M 7.6 Cayman Trough

Just a couple hours ago there was an earthquake along the Swan fault, which is the transform plate boundary between the North America and Caribbean plates. The Cayman trough (CT) is a region of oceanic crust, formed at the Mid-Cayman Rise (MCR) oceanic spreading center. To the west of the MCR the CT is bound by the left-lateral strike-slip Swan fault. To the east of the MCR, the CT is bound on the north by the Oriente fault.
Based upon our knowledge of the plate tectonics of this region, I can interpret the fault plane solution for this earthquake. The M 7.6 earthquake was most likely a left-lateral strike-slip earthquake associated with the Swan fault.

  • Plotted with a century’s earthquakes with magnitudes M ≥ 6.5


  • Plotted with a century’s earthquakes with magnitudes M ≥ 3.5


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  • There were two observations of a small amplitude (small wave height) tsunami recorded on tide gages in the region. Below are those observations.


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2018.01.14 M 7.1 Peru

We had a damaging and (sadly) deadly earthquake in southern Peru in the last 24 hours. This is an earthquake, with magnitude M 7.1, that is associated with the subduction zone forming the Peru-Chile trench (PCT). The Nazca plate (NP) is subducting beneath the South America plate (SAP). There are lots of geologic structures on the Nazca plate that tend to affect how the subduction zone responds during earthquakes (e.g. segmentation).
In the region of this M 7.1 earthquake, two large structures in the NP are the Nazca Ridge and the Nazca fracture zone. The Nazca fracture zone is a (probably inactive) strike-slip fault system. The Nazca Ridge is an over-thickened region of the NP, thickened as the NP moved over a hotspot located near Salas y Gomez in the Pacific Ocean east of Easter Island (Ray et al., 2012).
There are many papers that discuss how the ridge affects the shape of the megathrust fault here. The main take-away is that the NR is bull dozing into South America and the dip of the subduction zone is flat here. There is a figure below that shows the deviation of the subducting slab contours at the NR.


Well, I missed looking further into a key update paper and used figures from an older paper on my interpretive poster yesterday. Thanks to Stéphane Baize for pointing this out! Turns out, after their new analyses, the M 7.1 earthquake was in a region of higher seismogenic coupling, rather than low coupling (as was presented in my first poster).
Also, Dr. Robin Lacassin noticed (as did I) the paucity of aftershocks from yesterday’s M 7.1. This was also the case for the carbon copy 2013 M 7.1 earthquake (there was 1 M 4.6 aftershock in the weeks following the M 7.1 earthquake on 2013.09.25; there were a dozen M 1-2 earthquakes in Nov. and Dec. of 2013, but I am not sure how related they are to the M 7.1 then). I present a poster below with this in mind. I also include below a comparison of the MMI modeled estimates. The 2013 seems to have possibly generated more widespread intensities, even though that was a deeper earthquake.

2018.01.23 M 7.9 Gulf of Alaska

  • 2018.01.23 M 7.9 Gulf of Alaska UPDATE #1
  • 2018.01.24 M 7.9 Gulf of Alaska UPDATE #2

    • This earthquake appears to be located along a reactivated fracture zone in the GA. There have only been a couple earthquakes in this region in the past century, one an M 6.0 to the east (though this M 6.0 was a thrust earthquake). The Gulf of Alaska shear zone is even further to the east and has a more active historic fault history (a pair of earthquakes in 1987-1988). The magnetic anomalies (formed when the Earth’s magnetic polarity flips) reflect a ~north-south oriented spreading ridge (the anomalies are oriented north-south in the region of today’s earthquake). There is a right-lateral offset of these magnetic anomalies located near the M 7.9 epicenter. Interesting that this right-lateral strike-slip fault (?) is also located at the intersection of the Gulf of Alaska shear zone and the 1988 M 7.8 earthquake (probably just a coincidence?). However, the 1988 M 7.8 earthquake fault plane solution can be interpreted for both fault planes (it is probably on the GA shear zone, but I don’t think that we can really tell).
    This is strange because the USGS fault plane is oriented east-west, leading us to interpret the fault plane solution (moment tensor or focal mechanism) as a left-lateral strike-slip earthquake. So, maybe this earthquake is a little more complicated than first presumed. The USGS fault model is constrained by seismic waves, so this is probably the correct fault (east-west).
    I prepared an Earthquake Report for the 1964 Good Friday Earthquake here.

    • The USGS updated their MMI contours to reflect their fault model. Below is my updated poster. I also added green dashed lines for the fracture zones related to today’s M 7.9 earthquake (on the magnetic anomaly inset map).


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    • These are the observations as reported by the NTWC this morning (at 4:15 AM my local time).


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    • Large Scale Interpretive Map (from update report)


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    As a reminder, if the M 7.9 earthquake fault is E-W oriented, it would be left-lateral. The offset magnetic anomalies show right-lateral offset across these fracture zones. This was perhaps the main reason why I thought that the main fault was not E-W, but N-S. After a day’s worth of aftershocks, the seismicity may reveal some north-south trends. But, as a drama student in 7th grade (1977), my drama teacher (Ms. Naichbor, rest in peace) asked our class to go stand up on stage. We all stood in a line and she mentioned that this is social behavior, that people tend to stand in lines (and to avoid doing this while on stage). Later, when in college, professors often commented about how people tend to seek linear trends in data (lines). I actually see 3-4 N-S trends and ~2 E-W trends in the seismicity data.
    So, that being said, here is the animation I put together. I used the USGS query tool to get earthquakes from 1/22 until now, M ≥ 1.5. I include a couple inset maps presented in my interpretive posters. The music is copyright free. The animations run through twice.
    Here is a screenshot of the 14 MB video embedded below. I encourage you to view it in full screen mode (or download it).


    2018.02.16 M 7.2 Oaxaca, Mexico

    There was just now an earthquake in Oaxaca, Mexico between the other large earthquakes from last 2017.09.08 (M 8.1) and 2017.09.08 (M 7.1). There has already been a M 5.8 aftershock.Here is the USGS website for today’s M 7.2 earthquake.
    The SSN has a reported depth of 12 km, further supporting evidence that this earthquake was in the North America plate.
    This region of the subduction zone dips at a very shallow angle (flat and almost horizontal).
    There was also a sequence of earthquakes offshore of Guatemala in June, which could possibly be related to the M 8.1 earthquake. Here is my earthquake report for the Guatemala earthquake.
    The poster also shows the seismicity associated with the M 7.6 earthquake along the Swan fault (southern boundary of the Cayman trough). Here is my earthquake report for the Guatemala earthquake.

    • Here is the same poster but with the magnetic anomalies included (transparent).


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    2018.02.25 M 7.5 Papua New Guinea

  • 2018.02.26 M 7.5 Papua New Guinea Update #1

    • This morning (local time in California) there was an earthquake in Papua New Guinea with, unfortunately, a high likelihood of having a good number of casualties. I was working on a project, so could not immediately begin work on this report.
    This M 7.5 earthquake (USGS website) occurred along the Papua Fold and Thrust Belt (PFTB), a (mostly) south vergent sequence of imbricate thrust faults and associated fold (anticlines). The history of this PFTB appears to be related to the collision of the Australia plate with the Caroline and Pacific plates, the delamination of the downgoing oceanic crust, and then associated magmatic effects (from decompression melting where the overriding slab (crust) was exposed to the mantle following the delamination). More about this can be found in Cloos et al. (2005).

  • The same map without historic seismicity.



    • The aftershocks are still coming in! We can use these aftershocks to define where the fault may have slipped during this M 7.5 earthquake. As I mentioned yesterday in the original report, it turns out the fault dimension matches pretty well with empirical relations between fault length and magnitude from Wells and Coppersmith (1994).
    The mapped faults in the region, as well as interpreted seismic lines, show an imbricate fold and thrust belt that dominates the geomorphology here (as well as some volcanoes, which are probably related to the slab gap produced by crust delamination; see Cloos et al., 2005 for more on this). I found a fault data set and include this in the aftershock update interpretive poster (from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia, CCOP).
    I initially thought that this M 7.5 earthquake was on a fault in the Papuan Fold and Thrust Belt (PFTB). Mark Allen pointed out on twitter that the ~35km hypocentral depth is probably too deep to be on one of these “thin skinned” faults (see Social Media below). Abers and McCaffrey (1988) used focal mechanism data to hypothesize that there are deeper crustal faults that are also capable of generating the earthquakes in this region. So, I now align myself with this hypothesis (that the M 7.5 slipped on a crustal fault, beneath the thin skin deformation associated with the PFTB. (thanks Mark! I had downloaded the Abers paper but had not digested it fully.

    • Here is the “update” map with aftershocks


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    2018.03.08 M 6.8 New Ireland

    We had an M 6.8 earthquake near a transform micro-plate boundary fault system north of New Ireland, Papua New Guinea today. Here is the USGS website for this earthquake.
    The main transform fault (Weitin fault) is ~40 km to the west of the USGS epicenter. There was a very similar earthquake on 1982.08.12 (USGS website).
    This earthquake is unrelated to the sequence occurring on the island of New Guinea.
    Something that I rediscovered is that there were two M 8 earthquakes in 1971 in this region. This testifies that it is possible to have a Great earthquake (M ≥ 8) close in space and time relative to another Great earthquake. These earthquakes do not have USGS fault plane solutions, but I suspect that these are subduction zone earthquakes (based upon their depth).
    This transform system is capable of producing Great earthquakes too, as evidenced by the 2000.11.16 M 8.0 earthquake (USGS website). This is another example of two Great earthquakes (or almost 2 Great earthquakes, as the M 7.8 is not quite a Great earthquake) are related. It appears that the M 8.0 earthquake may have triggered teh M 7.8 earthquake about 3 months later (however at first glance, it seemed to me like the strike-slip earthquake might not increase the static coulomb stress on the subduction zone, but I have not spent more than half a minute thinking about this).

    Main Interpretive Poster with emag2


    Earthquakes M≥ 6.5 with emag2


    2018.03.26 M 6.6 New Britain

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

    2018.03.26 M 6.9 New Britain

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

    2018.04.02 M 6.8 Bolivia

    A couple days ago there was a deep focus earthquake in the downgoing Nazca plate deep beneath Bolivia. This earthquake has an hypocentral depth of 562 km (~350 miles).
    We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.
    As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.
    So, we don’t really know what causes earthquakes at the depth of this Bolivia M 6.8 earthquake. Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhotsk Earthquake.

    2018.05.04 M 6.9 Hawai’i

    There has been a swarm of earthquakes on the southeastern part of the big island, with USGS volcanologists hypothesizing about magma movement and suggesting that an eruption may be imminent. Here is a great place to find official USGS updates on the volcanism in Hawaii (including maps).
    Hawaii is an active volcanic island formed by hotspot volcanism. The Hawaii-Emperor Seamount Chain is a series of active and inactive volcanoes formed by this process and are in a line because the Pacific plate has been moving over the hotspot for many millions of years.
    Southeast of the main Kilauea vent, the Pu‘u ‘Ö‘ö crater saw an elevation of lava into the crater, leading to overtopping of the crater (on 4/30/2018). Seismicity migrated eastward along the ERZ. This morning, there was a M 5.0 earthquake in the region of the Hilina fault zone (HFZ). I was getting ready to write something up, but I had other work that I needed to complete. Then, this evening, there was a M 6.9 earthquake between the ERZ and the HFZ.
    There have been earthquakes this large in this region in the past (e.g. the 1975.1.29 M 7.1 earthquake along the HFZ). This earthquake was also most likely related to magma injection (Ando, 1979). The 1975 M 7.1 earthquake generated a small tsunami (Ando, 1979). These earthquakes are generally compressional in nature (including the earthquakes from today).
    Today’s earthquake also generated a tsunami as recorded on tide gages throughout Hawaii. There is probably no chance that a tsunami will travel across the Pacific to have a significant impact elsewhere.

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

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

    • Hilo, Hawaii


    • Kawaihae, Hawaii


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    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.05.05 Pele, the Hawai’i Goddess of Fire, Lightning, Wind, and Volcanoes
    2018.05.06 Pele, la Diosa Hawaiana del Fuego, los Relámpagos, el Viento y los Volcanes de Hawái

    2018.08.05 M 6.9 Lombok, Indonesia

    Yesterday morning, as I was recovering from working on stage crew for the 34th Reggae on the River (fundraiser for the non profit, the Mateel Community Center), I noticed on social media that there was an M 6.9 earthquake in Lombok, Indonesia. This is sad because of the likelihood for casualties and economic damage in this region.
    However, it is interesting because the earthquake sequence from last week (with a largest earthquake with a magnitude of M 6.4) were all foreshocks to this M 6.9. Now, technically, these were not really foreshocks. The M 6.4 has an hypocentral (3-D location) depth of ~6 km and the M 6.9 has an hypocentral depth of ~31 km. These earthquakes are not on the same fault, so I would interpret that the M 6.9 was triggered by the sequence from last week due to static coulomb changes in stress on the fault that ruptured. Given the large difference in depths, the uncertainty for these depths is probably not sufficient to state that they may be on the same fault (i.e. these depths are sufficiently different that this difference is larger than the uncertainty of their locations).
    I present a more comprehensive analysis of the tectonics of this region in my earthquake report for the M 6.4 earthquake here. I especially address the historic seismicity of the region there. This M 6.9 may have been on the Flores thrust system, while the earthquakes from last week were on the imbricate thrust faults overlying the Flores Thrust. See the map from Silver et al. (1986) below. I include the same maps as in my original report, but after those, I include the figures from Koulani et al. (2016) (the paper is available on researchgate).

    • Here is the map with a month’s seismicity plotted.


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    2018.08.15 M 6.6 Aleutians

    Well, yesterday while I was installing the final window in a reconstruction project, there was an earthquake along the Aleutian Island Arc (a subduction zone) in the region of the Andreanof Islands. Here is the USGS website for the M 6.6 earthquake. This earthquake is close to the depth of the megathrust fault, but maybe not close enough. So, this may be on the subduction zone, but may also be on an upper plate fault (I interpret this due to the compressive earthquake fault mechanism). The earthquake has a hypocentral depth of 20 km and the slab model (see Hayes et al., 2013 below and in the poster) is at 40 km at this location. There is uncertainty in both the slab model and the hypocentral depth.
    The Andreanof Islands is one of the most active parts of the Aleutian Arc. There have been many historic earthquakes here, some of which have been tsunamigenic (in fact, the email that notified me of this earthquake was from the ITIC Tsunami Bulletin Board).
    Possibly the most significant earthquake was the 1957 Andreanof Islands M 8.6 Great (M ≥ 8.0) earthquake, though the 1986 M 8.0 Great earthquake is also quite significant. As was the 1996 M 7.9 and 2003 M 7.8 earthquakes. Lest we forget smaller earthquakes, like the 2007 M 7.2. So many earthquakes, so little time.

    • Here is the map with a month’s seismicity plotted.


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    • Here is the map with a centuries seismicity plotted for earthquakes M ≥ 6.6.


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    2018.08.18 M 8.2 Fiji

    We just had a Great Earthquake in the region of the Fiji Islands, in the central-western Pacific. Great Earthquakes are earthquakes with magnitudes M ≥ 8.0.
    This earthquake is one of the largest earthquakes recorded historically in this region. I include the other Large and Great Earthquakes in the posters below for some comparisons.
    Today’s earthquake has a Moment Magnitude of M = 8.2. The depth is over 550 km, so is very very deep. This region has an historic record of having deep earthquakes here. Here is the USGS website for this M 8.2 earthquake. While I was writing this, there was an M 6.8 deep earthquake to the northeast of the M 8.2. The M 6.8 is much shallower (about 420 km deep) and also a compressional earthquake, in contrast to the extensional M 8.2.
    This M 8.2 earthquake occurred along the Tonga subduction zone, which is a convergent plate boundary where the Pacific plate on the east subducts to the west, beneath the Australia plate. This subduction zone forms the Tonga trench.

    • Here is the map with a centuries seismicity plotted with M ≥ 7.5.


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    2018.08.19 M 6.9 Lombok, Indonesia

    This ongoing sequence began in late July with a Mw 6.4 earthquake. Followed less than 2 weeks later with a Mw 6.9 earthquake.
    Today there was an M 6.3 soon followed by an M 6.9 earthquake (and a couple M 5.X quakes).
    These earthquakes have been occurring along a thrust fault system along the northern portion of Lombok, Indonesia, an island in the magamatic arc related to the Sunda subduction zone. The Flores thrust fault is a backthrust to the subduction zone. The tectonics are complicated in this region of the world and there are lots of varying views on the tectonic history. However, there has been several decades of work on the Flores thrust (e.g. Silver et al., 1986). The Flores thrust is an east-west striking (oriented) north vergent (dipping to the south) thrust fault that extends from eastern Java towards the Islands of Flores and Timor. Above the main thrust fault are a series of imbricate (overlapping) thrust faults. These imbricate thrust faults are shallower in depth than the main Flores thrust.
    The earthquakes that have been happening appear to be on these shallower thrust faults, but there is a possibility that they are activating the Flores thrust itself. Perhaps further research will illuminate the relations between these shallower faults and the main player, the Flores thrust.

    • Here is the map with a month’s seismicity plotted.


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    • Here is an updated local scale (large scale) map showing the earthquake fault mechanisms for the current sequence. I label them with yellow numbers according to the sequence timing. I outlined the general areas that have had earthquakes into two zones (phases). Phase I includes the earthquakes up until today and Phase II includes the earthquakes from today. There is some overlap, but only for a few earthquakes. In general, it appears that the earthquakes have slipped in two areas of the Flores fault (or maybe two shallower thrust faults).


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    • Here is the interpretive posted from the M 6.4 7/28 earthquake, with historic seismicity and earthquake mechanisms.


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    2018.08.21 M 7.3 Venezuela

    We just had a M 7.3 earthquake in northern Venezuela. Sadly, this large earthquake has the potential to be quite damaging to people and their belongings (buildings, infrastructure).
    The northeastern part of Venezuela lies a large strike-slip plate boundary fault, the El Pilar fault. This fault is rather complicated as it strikes through the region. There are thrust faults and normal faults forming ocean basins and mountains along strike.
    Many of the earthquakes along this fault system are strike-slip earthquakes (e.g. the 1997.07.09 M 7.0 earthquake which is just to the southwest of today’s temblor. However, today’s earthquake broke my immediate expectations for strike-slip tectonics. There is a south vergent (dipping to the north) thrust fault system that strikes (is oriented) east-west along the Península de Paria, just north of highway 9, east of Carupano, Venezuela. Audenard et al. (2000, 2006) compiled a Quaternary Fault database for Venezuela, which helps us interpret today’s earthquake. I suspect that this earthquake occurred on this thrust fault system. I bet those that work in this area even know the name of this fault. However, looking at the epicenter and the location of the thrust fault, this is probably not on this thrust fault. When I initially wrote this report, the depth was much shallower. Currently, the hypocentral (3-D location) depth is 123 km, so cannot be on that thrust fault.
    The best alternative might be the subduction zone associated with the Lesser Antilles.

    • Here is the map with a month’s seismicity plotted, along with USGS earthquakes M ≥ 6.0.


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    2018.08.24 M 7.1 Peru

    Well, this earthquake, while having a large magnitude, was quite deep. Because earthquake intensity decreases with distance from the earthquake source, the shaking intensity from this earthquake was so low that nobody submitted a single report to the USGS “Did You Feel It?” website for this earthquake.
    While doing my lit review, I found the Okal and Bina (1994) paper where they use various methods to determine focal mechanisms for the some deep earthquakes in northern Peru. More about focal mechanisms below. These authors created focal mechanisms for the 1921 and 1922 deep earthquakes so they could lean more about the 1970 deep earthquake. Their seminal work here forms an important record of deep earthquakes globally. These three earthquakes are all extensional earthquakes, similar to the other deep earthquakes in this region. I label the 1921 and 1922 earthquakes a couplet on the poster.
    There was also a pair of earthquakes that happened in November, 2015. These two earthquakes happened about 5 minutes apart. They have many similar characteristics, suggest that they slipped similar faults, if not the same fault. I label these as doublets also.
    So, there may be a doublet companion to today’s M 7.1 earthquake. However, there may be not. There are examples of both (single and doublet) and it might not really matter for 99.99% of the people on Earth since the seismic hazard from these deep earthquakes is very low.
    Other examples of doublets include the 2006 | 2007 Kuril Doublets (Ammon et al., 2008) and the 2011 Kermadec Doublets (Todd and Lay, 2013).

    • Here is the map with a century’s seismicity plotted, along with USGS earthquakes M ≥ 7.0.


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    2018.09.05 M 6.6 Hokkaido, Japan

    Following the largest typhoon to strike Japan in a very long time, there was an earthquake on the island of Hokkaido, Japan today. There is lots on social media, including some spectacular views of disastrous and deadly landslides triggered by this earthquake (earthquakes are the number 1 source for triggering of landslides). These landslides may have been precipitated (sorry for the pun) by the saturation of hillslopes from the typhoon. Based upon the USGS PAGER estimate, this earthquake has the potential to cause significant economic damages, but hopefully a small number of casualties. As far as I know, this does not incorporate potential losses from earthquake triggered landslides [yet].
    This earthquake is in an interesting location. to the east of Hokkaido, there is a subduction zone trench formed by the subduction of the Pacific plate beneath the Okhotsk plate (on the north) and the Eurasia plate (to the south). This trench is called the Kuril Trench offshore and north of Hokkaido and the Japan Trench offshore of Honshu.
    One of the interesting things about this region is that there is a collision zone (a convergent plate boundary where two continental plates are colliding) that exists along the southern part of the island of Hokkaido. The Hidaka collision zone is oriented (strikes) in a northwest orientation as a result of northeast-southwest compression. Some suggest that this collision zone is no longer very active, however, there are an abundance of active crustal faults that are spatially coincident with the collision zone.
    Today’s M 6.6 earthquake is a thrust or reverse earthquake that responded to northeast-southwest compression, just like the Hidaka collision zone. However, the hypocentral (3-D) depth was about 33 km. This would place this earthquake deeper than what most of the active crustal faults might reach. The depth is also much shallower than where we think that the subduction zone megathrust fault is located at this location (the fault formed between the Pacific and the Okhotsk or Eurasia plates). Based upon the USGS Slab 1.0 model (Hayes et al., 2012), the slab (roughly the top of the Pacific plate) is between 80 and 100 km. So, the depth is too shallow for this hypothesis (Kuril Trench earthquake) and the orientation seems incorrect. Subduction zone earthquakes along the trench are oriented from northwest-southweast compression, a different orientation than today’s M 6.6.
    So today’s M 6.6 earthquake appears to have been on a fault deeper than the crustal faults, possibly along a deep fault associated with the collision zone. Though I am not really certain. This region is complicated (e.g. Kita et al., 2010), but there are some interpretations of the crust at this depth range (Iwasaki et al., 2004) shown in an interpreted cross section below.

    • Here is the map with a centuries seismicity plotted.


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    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.09.06 Violent shaking triggers massive landslides in Sapporo Japan earthquake

    2018.09.09 M 6.9 Kermadec

    Today, there was a large earthquake associated with the subduction zone that forms the Kermadec Trench.
    This earthquake was quite deep, so was not expected to generate a significant tsunami (if one at all).
    There are several analogies to today’s earthquake. There was a M 7.4 earthquake in a similar location, but much deeper. These are an interesting comparison because the M 7.4 was compressional and the M 6.9 was extensional. There is some debate about what causes ultra deep earthquakes. The earthquakes that are deeper than about 40-50 km are not along subduction zone faults, but within the downgoing plate. This M 6.9 appears to be in a part of the plate that is bending (based on the Benz et al., 2011 cross section). As plates bend downwards, the upper part of the plate gets extended and the lower part of the plate experiences compression.

    • Here is the map with a month’s seismicity plotted.


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    • Here is the map with a centuries seismicity plotted.


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    2018.09.28 M 7.5 Sulawesi

  • 2018.10.16 M 7.5 Sulawesi UPDATE #1

    • Well, around 3 AM my time (northeastern Pacific, northern CA) there was a sequence of earthquakes including a mainshock with a magnitude M = 7.5. This earthquake happened in a highly populated region of Indonesia.
    This area of Indonesia is dominated by a left-lateral (sinistral) strike-slip plate boundary fault system. Sulawesi is bisected by the Palu-Kola / Matano fault system. These faults appear to be an extension of the Sorong fault, the sinistral strike-slip fault that cuts across the northern part of New Guinea.
    There have been a few earthquakes along the Palu-Kola fault system that help inform us about the sense of motion across this fault, but most have maximum magnitudes mid M 6.
    GPS and block modeling data suggest that the fault in this area has a slip rate of about 40 mm/yr (Socquet et al., 2006). However, analysis of offset stream channels provides evidence of a lower slip rate for the Holocene (last 12,000 years), a rate of about 35 mm/yr (Bellier et al., 2001). Given the short time period for GPS observations, the GPS rate may include postseismic motion earlier earthquakes, though these numbers are very close.
    Using empirical relations for historic earthquakes compiled by Wells and Coppersmith (1994), Socquet et al. (2016) suggest that the Palu-Koro fault system could produce a magnitude M 7 earthquake once per century. However, studies of prehistoric earthquakes along this fault system suggest that, over the past 2000 years, this fault produces a magnitude M 7-8 earthquake every 700 years (Bellier et al., 2006). So, it appears that this is the characteristic earthquake we might expect along this fault.
    Most commonly, we associate tsunamigenic earthquakes with subduction zones and thrust faults because these are the types of earthquakes most likely to deform the seafloor, causing the entire water column to be lifted up. Strike-slip earthquakes can generate tsunami if there is sufficient submarine topography that gets offset during the earthquake. Also, if a strike-slip earthquake triggers a landslide, this could cause a tsunami. We will need to wait until people take a deeper look into this before we can make any conclusions about the tsunami and what may have caused it.

    • There have been tsunami waves recorded on a tide gage over 300 km to the south of the epicenter, at a site called Mumuju. Below is a map and a plot of water surface elevations from this source.




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    • Here is the map with a month’s seismicity plotted.


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    • Here is the map with a centuries worth of seismicity plotted.


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    Here is a map that shows the updated USGS model of ground shaking. The USGS prepared an updated earthquake fault slip model that was additionally informed by post-earthquake analysis of ground deformation. The original fault model extended from north of the epicenter to the northernmost extent of Palu City. Soon after the earthquake, Dr. Sotiris Valkaniotis prepared a map that showed large horizontal offsets across the ruptured fault along the entire length of the western margin on Palu Valley. This horizontal offset had an estimated ~8 meters of relative displacement. InSAR analyses confirmed that the coseismic ground deformation extended through Palu Valley and into the mountains to the south of the valley.

    My 2018.10.01 BC Newshour Interview

    InSAR Analysis

    Synthetic Aperture Radar (SAR) is a remote sensing method that uses Radar to make observations of Earth. These observations include the position of the ground surface, along with other information about the material properties of the Earth’s surface.
    Interferometric SAR (InSAR) utilizes two separate SAR data sets to determine if the ground surface has changed over time, the time between when these 2 data sets were collected. More about InSAR can be found here and here. Explaining the details about how these data are analyzed is beyond the scope of this report. I rely heavily on the expertise of those who do this type of analysis, for example Dr. Eric Fielding.

    • I prepared a map using the NASA-JPL InSAR data. They post all their data online here. I used the tiff image as it is georeferenced. However, some may prefer to use the kmz file in Google Earth.
    • I include the faults mapped by Wilkinson and Hall (2017), the PGA contours from the USGS model results. More on Peak Ground Acceleration (PGA) can be found here. I also include the spatial extent of the largest landslides that I mapped using post-earthquake satellite imagery provided by Digital Globe using their open source imagery program.


    M 7.5 Landslide Model vs. Observation Comparison

    Landslides during and following the M=7.5 earthquake in central Sulawesi, Indonesia possibly caused the majority of casualties from this catastrophic natural disaster. Volunteers (citizen scientists) have used satellite aerial imagery collected after the earthquake to document the spatial extent and magnitude of damage caused by the earthquake, landslides, and tsunami.
    Until these landslides are analyzed and compared with regions that did not fail in slope failure, we will not be able to reconstruct what happened… why some areas failed and some did not.
    There are landslide slope stability and liquefaction susceptibility models based on empirical data from past earthquakes. The USGS has recently incorporated these types of analyses into their earthquake event pages. More about these USGS models can be found on this page.
    I prepared some maps that compare the USGS landslide and liquefaction probability maps. Below I present these results along with the MMI contours. I also include the faults mapped by Wilkinson and Hall (2017). Shown are the cities of Donggala and Palu. Also shown are the 2 tide gage locations (Pantoloan Port – PP and Mumuju – M). I also used post-earthquake satellite imagery to outline the largest landslides in Palu Valley, ones that appear to be lateral spreads.

    • Here is the landslide probability map (Jessee et al., 2018). Below the poster I include the text from the USGS website that describes how this model is prepared.


    Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.

    • Here is the liquefaction probability (susceptibility) map (Zhu et al., 2017). Note that the regions of low slopes in the valleys and coastal plains are the areas with a high chance of experiencing liquefaction. Areas of slopes >5° are excluded from this analysis.
    • Note that the large landslides (yellow polygons) are not in regions of high probability for liquefaction.


    Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications proposed by Baise and Rashidian (2017), including a peak ground acceleration (PGA) threshold of 0.1 g and linear interpolation of the input layers. We also exclude areas with slopes >5°. We linearly interpolate the original input layers of ~1 km resolution to 500 m resolution. The model inputs are described below. More details about the model can be found in the original publication.

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.09.28 The Palu-Koro fault ruptures in a M=7.5 quake in Sulawesi, Indonesia, triggering a tsunami and likely more shocks
    2018.10.03 Tsunami in Sulawesi, Indonesia, triggered by earthquake, landslide, or both
    2018.10.16 Coseismic Landslides in Sulawesi, Indonesia

    2018.10.10 M 7.0 New Britain, PNG

    In this region of the world, the Solomon Sea plate and the South Bismarck plate converge to form a subduction zone, where the Solomon Sea plate is the oceanic crust diving beneath the S.Bismarck plate.
    The subduction zone forms the New Britain Trench with an axis that trends east-northeast. To the east of New Britain, the subduction zone bends to the southeast to form the San Cristobal and South Solomon trenches. Between these two subduction zones is a series of oceanic spreading ridges sequentially offset by transform (strike slip) faults.
    Earthquakes along the megathrust at the New Britain trench are oriented with the maximum compressive stress oriented north-northwest (perpendicular to the trench). Likewise, the subduction zone megathrust earthquakes along the S. Solomon trench compress in a northeasterly direction (perpendicular to that trench).
    There is also a great strike slip earthquake that shows that the transform faults are active.
    This earthquake was too small and too deep to generate a tsunami.

    • Here is the map with a century’s seismicity plotted.


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    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.10.10 M 7.5 Earthquake in New Britain, Papua New Guinea

    2018.10.22 M 6.8 Explorer plate

    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.

    • Here is the map with a century’s seismicity plotted.


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    2018.10.25 M 6.8 Greece

    Before I looked more closely, I thought this sequence might be related to the Kefallonia fault. I prepared some earthquake reports for earthquakes here in the past, in 2015 and in 2016.
    Both of those earthquakes were right-lateral strike-slip earthquakes associated with the Kefallonia fault.
    However, today’s earthquake sequence was further to the south and east of the strike-slip fault, in a region experiencing compression from the Ionian Trench subduction zone. But there is some overlap of these different plate boundaries, so the M 6.8 mainshock is an oblique earthquake (compressional and strike-slip). Based upon the sequence, I interpret this earthquake to be right-lateral oblique. I could be wrong.

    • Here is the map with a century’s seismicity plotted.


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    • Here is the tide gage data from Katakolo, which is only 65 km from the M 6.8 epicenter.


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    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.10.26 Greek earthquake in a region of high seismic hazard

    2018.11.08 M 6.8 Mid Atlantic Ridge (Jan Mayen fracture zone)

    There was a M = 6.8 earthquake along a transform fault connecting segments of the Mid Atlantic Ridge recently.
    North of Iceland, the MAR is offset by many small and several large transform faults. The largest transform fault north of Iceland is called the Jan Mayen fracture zone, which is the location for the 2018.11.08 M = 6.8 earthquake.

    • Here is the map with a century’s seismicity plotted.


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    • Here is the large scale map showing earthquake mechanisms for historic earthquakes in the region. Note how they mostly behave well (are almost perfectly aligned with the Jan Mayen fracture zone). There are a few exceptions, including an extensional earthquake possibly associated with extension on the MAR (2010.06.03 M = 5.6). Also, 2 earthquakes (2003.06.19 and 2005.07.25) are show oblique slip (not pure strike-slip as they have an amount of compressional motion) near the intersection of the fracture zone and the MAR.


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    2018.11.30 M 7.0 Alaska

    Today’s earthquake occurred along the convergent plate boundary in southern Alaska. This subduction zone fault is famous for the 1964 March 27 M = 9.2 megathrust earthquake. I describe this earthquake in more detail here.
    During the 1964 earthquake, the downgoing Pacific plate slipped past the North America plate, including slip on “splay faults” (like the Patton fault, no relation, heheh). There was deformation along the seafloor that caused a transoceanic tsunami.
    The Pacific plate has pre-existing zones of weakness related to fracture zones and spreading ridges where the plate formed and are offset. There was an earthquake in January 2016 that may have reactivated one of these fracture zones. This earthquake (M = 7.1) was very deep (~130 km), but still caused widespread damage.
    The earthquake appears to have a depth of ~40 km and the USGS model for the megathrust fault (slab 2.0) shows the megathrust to be shallower than this earthquake. There are generally 2 ways that may explain the extensional earthquake: slab tension (the downgoing plate is pulling down on the slab, causing extension) or “bending moment” extension (as the plate bends downward, the top of the plate stretches out.

  • Temblor Report
    • Here is the map with a century’s seismicity plotted.


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    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.11.30 Exotic M=7.0 earthquake strikes beneath Anchorage, Alaska
    2018.12.11 What the Anchorage earthquake means for the Bay Area, Southern California, Seattle, and Salt Lake City

    2018.12.05 M 7.5 New Caledonia

    There was a sequence of earthquakes along the subduction zone near New Caledonia and the Loyalty Islands.
    This part of the plate boundary is quite active and I have a number of earthquake reports from the past few years (see below, a list of earthquake reports for this region).
    But the cool thing from a plate tectonics perspective is that there was a series of different types of earthquakes. At first view, it appears that there was a mainshock with a magnitude of M = 7.5. There was a preceding M 6.0 earthquake which may have been a foreshock.
    The M 7.5 earthquake was an extensional earthquake. This may be due to either extension from slab pull or due to extension from bending of the plate. More on this later.
    Following the M 7.5, there was an M 6.6 earthquake, however, this was a thrust or reverse (compressional) earthquake. The M 6.6 may have been in the upper plate or along the subduction zone megathrust fault, but we won’t know until the earthquake locations are better determined.
    A similar sequence happened in October/November 2017. I prepared two reports for this sequence here and here. Albeit, in 2017, the thrust earthquake was first (2017.10.31 vs. 2017.11.19).
    There have been some observations of tsunami. Below is from the Pacific Tsunami Warning Center.

    • Here is the map with a month’s seismicity plotted.


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    • Here is the map with a century’s seismicity plotted.


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    2018.12.20 M 7.4 Bering Kresla

  • 2018.12.20 M 7.3 Bering Kresla UPDATE #1

    • A large earthquake in the region of the Bering Kresla fracture zone, a strike-slip fault system that coincides with the westernmost portion of the Aleutian trench (which is a subduction zone further to the east).
    This earthquake happened in an interesting region of the world where there is a junction between two plate boundaries, the Kamchatka subduction zone with the Aleutian subduction zone / Bering-Kresla Shear Zone. The Kamchatka Trench (KT) is formed by the subduction (a convergent plate boundary) beneath the Okhotsk plate (part of North America). The Aleutian Trench (AT) and Bering-Kresla Shear Zone (BKSZ) are formed by the oblique subduction of the Pacific plate beneath the Pacific plate. There is a deflection in the Kamchatka subduction zone north of the BKSZ, where the subduction trench is offset to the west. Some papers suggest the subduction zone to the north is a fossil (inactive) plate boundary fault system. There are also several strike-slip faults subparallel to the BKSZ to the north of the BKSZ.

    • Here is the map with a month’s seismicity plotted, including the age of the crust.


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    • Here is the map with a century’s seismicity plotted, with earthquakes M ≥ 6.0, including the age of the crust.


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    UPDATE #1

    • Here is the map with a month’s seismicity plotted.


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    • Here is the map with a century’s seismicity plotted, with earthquakes M ≥ 6.0.


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    2018.12.29 M 7.0 Philippines

    This magnitude M = 7.0 earthquake is related to the subduction zone that forms the Philippine trench (where the Philippine Sea plate subducts beneath the Sunda plate). Here is the USGS website for this earthquake.
    The earthquake was quite deep, which makes it less likely to cause damage to people and their belongings (e.g. houses and roads) and also less likely that the earthquake will trigger a trans-oceanic tsunami.
    Here are the tidal data:

    • Here is the map with a century’s seismicity plotted.


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    Geologic Fundamentals

    • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

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


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    • Here is another way to look at these beach balls.

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

    Return to the Earthquake Reports page.

    Earthquake Report: Madagascar!

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    Today there was an earthquake in a region that we don’t hear about that often. Madagascar is off the coast of southeastern Africa and the oceanic basin to the west is likely formed as part of the East Africa Rift system, but also due to the post Gondwana plate tectonics. Madagascar was once part of India, back when India was part of Gondwana.
    Today’s (and of the last few days) earthquakes are located along the Comoros Archipelago, volcanic islands formed from hotspot volcanism.
    There exist fracture zones in this region. Below, we see that these fracture zones have been interpreted as right-lateral strike-slip faults. However, the relative offsets of magnetic anomalies (and spreading ridges) show that these faults are instead left-lateral. So, that is my interpretation for this M 5.8 earthquake, a left-lateral strike-slip earthquake. I placed white dashed lines in the poster below to show where some of these fracture zones may be located, based upon the magnetic anomaly data (EMAG2).

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5.
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.8 earthquake.

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

    • I include some inset figures.

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


    USGS Earthquake Pages

    Some Relevant Discussion and Figures

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


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

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


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

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


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

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


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

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


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

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


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

    Geologic Fundamentals

    • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

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


      •

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

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      References:

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

    Earthquake Report: Malawi & Mozambique

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    Busy day today. This is my second earthquake report today.
    This report is about a M 5.6 earthquake along the Malawi Rift (MR) system, part of the larger East Africa Rift (EAR) extensional plate boundary. The EAR is currently the locus of extension between the Nubia and Somalia plates. The orientation of extension in this region has changed over time (for more on this, see Castaing, 1991). There are many normal faults that accommodate this extension (forming the rift valleys where so much paleoanthopologic evidence has been archived by rift volcanic deposits, and later exposed due to the extension). As the faults change strike (compass orientation), the type of faulting also changes (there are lots of strike-slip faults that have formed to accommodate the mismatch between fault strike and extension direction).
    Topday’s M 5.6 earthquake is extensional, showing extension in the northeast-southwest direction. At first, we might think that this is strange, since the predominant direction of extension is east-west. However, upon further investigation, we learn that the normal faults in the region of today’s earthquake have northwest strike (they are oriented northwest-southeast). So, northeast extension makes sense here.
    There have been a number of earthquakes along the EAR and I include these in the poster. Links to the USGS websites are listed below.

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include USGS earthquake epicenters from 1918-2018 with magnitudes M ≥ 5.5.
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.6 earthquakes, in addition to some relevant historic earthquakes.
    I include the magnetic anomaly data (emag 2).

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

    • I include some inset figures.

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


    USGS Earthquake Pages

      These are from this current sequence

    • 2018.03.08 M 5.6 Malawi

    Some Relevant Discussion and Figures

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


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

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


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

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


      •

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


      • Stereograms showing successive stress fields in South Malawi

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


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

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


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

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


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

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


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

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


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

    • Regional tectonic strain, Chorowicz (2005).


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

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


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

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


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


      •

    Geologic Fundamentals

    • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

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


      •

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

      • Strike Slip:

      Compressional:

      Extensional:

    Social Media

      References:

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

    Earthquake Report: Botswana!

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    This is a very interesting M 6.5 earthquake, which was preceded by a probably unrelated M 5.2 earthquake. Last September, there was an M 5.7 earthquake in Tanzania along the western shores of Lake Victoria. Here is my report for that earthquake.

      Here are the USGS web pages for these earthquakes

    • 2017.04.03 M 5.2
    • 2017.04.03 M 6.5

    Below is my interpretive poster for this 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.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

      I include some inset figures in the poster.

    • 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. [2003] and Begg et al. [2009].

    • 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.


      •

    References:

    • Hayes, G.P., Jones, E.S., Stadler, T.J., Barnhart, W.D., McNamara, D.E., Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2014, Seismicity of the Earth 1900–2013 East African Rift: U.S. Geological Survey Open-File Report 2010–1083-P, 1 sheet, scale 1:8,500,000 http://dx.doi.org/10.3133/of20101083P
    • Leseane, K., Atekwana, E.A., Mickus, K.L., Abdelsalam, M.G., Shemanq, E.M., and Atekwana, E.A., 2015. Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana in JGR: Solid Earth, v. 120, doi:10.1002/2014JB011029.
    • Thieblemont, D. (ed.), 2016. Geological Map of Africa et 1:10M scale, CGMW-BRGM 2016

    Earthquake Report: Tanzania

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    Well, I am a little late preparing this report. I actually put it together over the weekend, but am only now uploading this.
    There was an interesting earthquake along the southwestern edge of Lake Victoria, in the East-Africa Rift (EAR) system. The EAR is one of the few continental rift systems on Earth. We think that continental rift systems are early in the development of an oceanic spreading ridge system. This may be what the Mid Atlantic Ridge system looked like long ago. There are no mapped faults in this region (at least in the papers that I found on the subject). On 2016.09.10, there was a M 5.9 earthquake and here is the USGS website for this earthquake. My poster below shows it as a M 5.7, but the magnitude has been adjusted to M 5.9.
    Below is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    I 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. Because there are no mapped faults to give me a clue as to which slip plane this earthquake may have occurred on, it is equivocal. However, the faults to the west, which are mapped as normal (extensional) faults, may have some strike-slip faults associated with them. If this earthquake is related in that way, the right-lateral shear across the plate boundary here would suggest that this M 5.9 earthquake was along a north-south striking fault, with a right-lateral sense of motion.


      Here are some of the figures on their own. I include the original figure captions below them as blockquotes.

    • Regions of extension (Saemundsson, 2010).


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

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


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

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


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

    • Regional tectonic strain, Chorowicz (2005).


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

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


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

      References

    • Baker, B.H., Mohr P.A., and Williams, L.A.J., 1972: Geology of the Eastern Rift System of Africa in The Geol. Soc. of America. Special Paper, 136, 67 pp.
    • Chorowicz, J., 2005. The East African rift system in Journal of African Earth Sciences, v. 43., p. 379-410.
    • Saemundsson, K., 2010. East African Rift System – an Overview presented at Short Course V on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Oct. 29 – Nov. 19, 2010, 10 pp.