Andaman Sea Earthquake!


Today we had a small earthquake in the Andaman Sea, west of Thailand and east of the Andaman Isles (India).

The Andaman Sea is a region of backarc spreading and connects the Sumatra fault (a (strike-slip) sliver fault that accommodates oblique motion on the Sunda subduction zone offshore of Sumatra and the Andaman/Nicobar Islands), with the strike-slip Sagaing fault in Myanmar/Burma. I have two pages that discuss the historic subduction zone earthquakes along the Sunda subduction zone. The first page covers the 2004 earthquake in general. The second page discusses various measures of energy released during the 2004 earthquake.

    Here are the USGS web pages for the earthquakes displayed on the map below.

  • 1971.03.28 M 6.3 (not labeled on map, but very near to the M 5.8 from 2015)
  • 2004.12.26 M 7.2
  • 2005.07.24 M 7.2
  • 2008.06.27 M 6.6
  • 2009.08.10 M 7.5
  • 2010.06.12 M 7.5
  • 2015.07.03 M 5.8

Below is a map of the region. The M 5.8 did not have a moment tensor nor focal mechanism calculated, but I placed a generic strike-slip focal mechanism in an orientation that aligns with transform plate boundary that likely ruptured during this earthquake. The M 5.8 epicenter is depicted by a red and black star. I pose that this was a right-lateral strike slip earthquake along a transform plate boundary (shown in blue). Note that I have also added updated fault locations in this area, based upon seismicity (the USGS located plate boundaries are not quite correct; mine are imperfect too, but are consistent with the seismicity). The USGS fault lines have a stepped appearance and the ones that I drew look more smooth.

I placed moment tensors for some of the largest earthquakes in this region. The 2009 and 2008 earthquakes in the northwest are extensional, so are probably in the downgoing India plate (extension from bending of the plate or slab pull). The 2010 and 2005 earthquakes in the southwest are strike-slip and may be due to the oblique subduction (strain partitioning). There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

Here is a graphic that depicts how a sliver fault accommodates the strain partitioning from oblique subduction.

This is a general plate tectonic map of the region from Subarya et al. (2006). The Sumatra fault is the right-lateral strike-slip fault that rips through the center of the Island of Sumatra. The Sagaing fault is the right-lateral strike-slip fault extending into Myanmar/Burma north of the Andaman Sea. The purple arrows show the plate motion related to the oceanic spreading ridges in the Andaman Sea.

This map shows how the Sunda subduction zone and Sagaing fault extend into Asia (Wang et al., 2012). This figure was later modified and included in Wang et al. (2014). Click on the map to see it at a larger scale.

Here is a figure from Wang et al. (2014) that incorporates both the preceding maps. Below is the text that goes along with this figure (from the original Wang document). Click on the map to see it at a larger scale.

Major tectonic elements of the Myanmar region and the extreme variation in rainfall that influence the preservation of tectonic landforms. (a) Crustal thickness associated with the major plates and tectonic blocks of the region. The blue arrow shows the direction of Indian plate motion relative to Sunda plate [e.g., Socquet et al., 2006]. The black arrow shows the opening direction of the Central Andaman spreading center. The velocity is in mm/yr. (b) Two major fault systems accommodate the northward translation of the Indian plate into Eurasia. These are the northern extension of the Sunda megathrust and the Sagaing fault system, which form the western and eastern margin of the Burma Plate. Conjugate right- and left-lateral faults of the Shan plateau and southern China accommodate southwestward extrusion of the Sichuan-Yunnan block around the eastern Himalayan syntaxis. (c) Extreme variations in annual rainfall across the region result in extreme variations in preservation of tectonic landforms. The precipitation data are from GPCC global data [Rudolf and Schneider, 2005].

Here is an animation showing seismicity in this region from 1970 through July 3, 2015. The largest earthquakes (magnitude greater than 6.0) are plotted as grey circles. Note how there is a large increase in seismicity associated with the 2004 Sumatra Andaman subduction zone earthquake. There is also a swarm of activity associated with the outer rise earthquakes (the largest strike-slip earthquakes ever recorded on a seismograph).


  • Rudolf, B., and U. Schneider, 2005. Calculation of gridded precipitation data for the global land-surface using in-situ gauge observations,
    paper presented at Proceedings of the 2nd Workshop of the International Precipitation Working Group IPWG.
  • Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., McCaffrey, R., 2006. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake: Nature, v. 440, p. 46-51.
  • Socquet, A., C. Vigny, N. Chamot-Rooke, W. Simons, C. Rangin, and B. Ambrosius (2006), India and Sunda plates motion and deformation along their boundary in Myanmar determined by GPS, J. Geophys. Res., 111, B05406, doi:10.1029/2005JB003877.
  • Wang, Y., K. Sieh, S. T. Tun, K.-Y. Lai, and T. Myint, 2014. Active tectonics and earthquake potential of the Myanmar region, J. Geophys. Res. Solid Earth, 119, 3767–3822, doi:10.1002/2013JB010762.

Posted in asia, education, Extension, geology, HSU, Indonesia, plate tectonics, strike-slip, subduction, sumatra, Transform

Mid Atlantic Ridge Earthquake!

Dale Kramer liked this post

Yesterday (I was busy preparing revisions to a paper that was due, the 5th 18 hr day in a row) there was a large earthquake along a fracture zone (transform plate boundary) near the Mid Atlantic ridge (MAR). Here is the USGS web site for this earthquake.

We recently had a couple other MAR earthquakes on ~2015.05.24. This Mw = 6.3 earthquake occurred northwest of St. Helena, where Napoleon spent his last years of his life (and experienced a large earthquake which is known as Napoleon’s Earthquake on 1796.10.22).

Here is a map that shows yesterday’s Mw 7.0 earthquake, as well as the 2015.05.24 M = 6.3 earthquake (which has a compressional moment tensor). This occurred along an unnamed fracture zone.

Here is a map that shows the fracture zones and recent (2015) seismicity to the north of the M 7.0 earthquake.

Here is an animation from IRIS that shows the M 7.0 seismic waves propagating through the USArray seismic network.

Posted in education, geology, HSU, plate tectonics, strike-slip, Transform

Redding Earthquakes: 1945-2015


There have not been many earthquakes since 1945 in this region. Below is a map that shows the largest earthquake as a M = 5.4 earthquake from 1998.11.26. Here is the USGS query that I used to create this map. This is the first page that I posted about the M 2.9 earthquake. I also threw in the 1992 Petrolia earthquake in for good measure. Faults and fault zones are labeled. There are also some ghosted moment tensors that show the general relative plate motion across some fault zones.

In 1998, there was a M 4.5 earthquake that was followed in November by three earthquakes.

Posted in cascadia, Chemeketa Community College, education, geology, HSU, plate tectonics, San Andreas, strike-slip

Earthquake in Redding, CA!


We just had a small earthquake in Redding, California. The shaking was felt as far as 40 km from the epicenter, which was not deep (so probably in the North America plate). The focal mechanism shows that this was a reverse (compressional) earthquake. This is the USGS web page for this earthquake.

Here is a map of the region, showing the focal mechanism and USGS epicenter.

Here is a map for those unfamiliar with the region of northern California where Redding is located. The epicenter is plotted as a red dot.

This is the USGS “Did You Feel It?” map, showing the responses from real people who actually made observations about this earthquake. The color represents the magnitude of ground shaking (intensity) and uses the Modified Mercalli Intensity scale. If anyone felt this earthquake, go to this web page to report your observations. These data are valuable to the scientific community.

Posted in cascadia, Chemeketa Community College, education, geology, HSU, humboldt, plate tectonics

Movies: Seismic Record of a Deep Izu-Bonin Earthquake

Tashina Taylor liked this post

Scientists at the National Research Institute for Earth Science and Disaster Prevention have collected and processed seismic data from the High Sensitivity Seismograph Network Japan (Hi-net). They created movies of these data, in map view. Here is the page where they posted these movies. I have translated the text below to make it easier to read, but this is completely their work. I include links to the videos. The first movie shows long period seismic data and the second movie shows short period seismic data.

Here is a link to the USGS web page for this M 7.8 earthquake.

Here is a map that I put together that shows the mainshock, some aftershocks, and some triggered earthquakes.

I place the translated text in block quotes. The original text all comes from here.

Around 24 pm on May 30, 2015, 20:24, an earthquake of M 8.1 (by the Japan Meteorological Agency) has occurred in the Ogasawara Islands off the west coast. The earthquake, which occurred at a very deep place of about 680km, because scale is large, with observing the strong shaking of magnitude 5 [V] in Tokyo and Ogasawara Village and Kanagawa Prefecture Ninomiya Town, seismic intensity of one or more sway in all prefectures me was observed. In addition, long-period ground motion “around the Kanto region was also observed. Whether by this earthquake Japan archipelago is how the shaking, it was visualized using the observation data of NIED of high sensitivity seismograph network (Hi-net). Try compared the differences transmitted the way of waves of long period (wobbling and then shook) and short period waves (shaking was rattled).

Here is the link of the mp4 file (7 MB).

The Hi-net using seismic appropriate to be observed oscillation of period shorter than one second, but by correcting the characteristics of the seismometer and data acquisition system, of the wave that vibrates with a period several tens of seconds general features it is possible to also be considered . Here, by applying a band-pass (band-pass) filter to the waveform after the characteristic correction, we visualized how the transmitted seismic waves 25 to 50 seconds range. From around 20:25, you can confirm that wave that has spread from the epicenter to the longitudinal Japan to north-northwest. Initially, it is striped red and blue will continue to spread in concentric circles around the epicenter, from around 20:27, the shift has begun to occur in a stripe pattern at the center of the Japanese archipelago. From around 20:28, reflected by the underground, also joined the wave, which is thought to have through the refraction, etc., wave field shows the complex aspects.

Here is the link of the mp4 file (7 MB).

Through the Izu Islands from around 20:25 Kanto, amplitude is increased from the Tokai region coast, you can see that P wave (initial) has arrived. This wave, over a period of about 1 minute half, will continue to longitudinal the Japanese archipelago to Hokkaido northern tip. Over time and towards the West is seen how the amplitude ahead to decay than the East , but this is considered to reflect the region of the attenuation structure of the underground ( * )

Posted in asia, Chemeketa Community College, education, geology, HSU, plate tectonics

Los Angeles: Newport Inglewood Earthquake!


Last night, we had another earthquake on the Newport Inglewood fault (NIF) system. This fault ruptured in 1933, which led to Long Beach adopting the most strict building codes in the nation. Later, these codes were adopted by the state, the nation, and the world. (Of course, they have been updated since then.) The 2015 Seismological Society of America (SSA) meeting was held in Pasadena. I attended a field trip where we reviewed the latest research on some of the faults in the region. The guidebook is available on the SSA 2015 Annual Meeting website here.

Here is a map showing the location of today’s M 3.4 strike-slip earthquake (slightly oblique, as evidenced by the focal mechanism). I also plot the moment tensor for the 1994 Northridge earthquake (a compressional earthquake). Focal mechanisms and moment tensors are two ways to use seismologic observations to learn about two possible fault plane solutions for an earthquake. These two calculations are performed differently, but their graphical depictions are the same. Take a looksie at the USGS websites to learn more about focal mechanisms and moment tensors.

In the above map, I also include the “Did You Feel It” map showing how broadly this earthquake was felt across the southland. I plot the Modified Mercalli Intensity (MMI) contours, along with the MMI legend. The MMI is a scale that relates ground shaking observations to a qualitative shaking intensity scale (ranging from I to XII). There is more on the MMI scale here.

The NIF system has been active lately. In April and May of this year (2015) we had two swarms of seismic activity. I discuss the 1994 Northridge earthquake more on the April NIF page. I summarize the seismic activity from April and May on the May NIF page. Don’t forget about the March 2014 La Habra earthquake. That was larger in magnitude (M 5.1) and had a more widespread effect.

Here is a great illustration from UNAVCO showing an interpretation of the fault configuration in the LA basin.

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

Borneo Earthquake!


We had an extensional earthquake in northwest Borneo today. The region is an old accretionary prism, probably still active. Here is the USGS web page for this M 6.0 earthquake.

Here is a map that shows where today’s earthquake is, along with the USGS moment tensor. The earthquake is plotted as an orange circle (represents depth) with a diameter related to the magnitude of the earthquake. Moment tensors (and focal mechanisms) are graphical depictions that show the 3-D orientation of the stresses associated with the earthquake. These graphical solutions are determined differently for moment tensors and for focal mechanisms, however, their graphical depiction of their solutions are the same. There is also a small legend explaining the implications for the different moment tensors. I also plot the moment tensor for an earthquake that happened in November of 2014.

Also, I plot a cross section from Tingay et al. (2005). This cross section (marked A-A’ ) shows the Tertiary-Quaternary accretionary prism structure, as well as the seismic stratigraphic and geotechnical interpretation of the sedimentary deposits. Note the predominance of thrust faults in this cross section, the result of convergence.

Here is a map that shows the earthquake at a more global scale, to help orient us for the above map. Today’s M 6.0 earthquake is plotted as an orange dot.

Here is a map from Cullen (2011). This shows how the region has both thrust/reverse and normal faults. The cross section is located near the Baram Delta shown in this map.

This is the moment tensor legend at a larger scale.

Posted in Chemeketa Community College, earthquake, education, geology, HSU, Indonesia, plate tectonics, subduction

Errata for Phantom Earthquakes: USGS


There have been a couple of reported earthquakes that have since been removed from the database. This happens when seismic waves are incorrectly interpreted by the computers, typically from the results of larger magnitude earthquakes. These large magnitude earthquakes can produce seismic waves that are easily interpreted to be local earthquakes in other locations. When people review the data, these errors are omitted. However, the phantom earthquakes are often submitted to the earthquake notification system prior to this removal. These deletions raise angst in the conspiracy theorists’ minds (we can only imagine what actually is in their minds, if much at all), suggesting that there is some global conspiracy to hide seismic data, for some nefarious reason. I cannot think of a single reason why someone might want to hide seismic data (well, maybe for nuclear testing). It would be difficult to really hide seismic data because there are so many unique and independent seismic networks, which publish their data online.

Sign up for the Earthquake Notification System here.

The USGS has posted information about these recent Phantom Earthquakes and the reasons behind their deletions. I paste the relevant information below, just in case there is a conspiracy that will later remove these words from the internets [sarcasm].

Here was the page for the M 5.1 Lewiston earthquake, which was the result of seismic waves travelling from the M 6.7 earthquake along the Alaska Peninsula. This is my first post about the M 6.7 earthquake and here is a post that I wrote that includes animations of historic seismicity in the region.

Here was my post about this earthquake. This happened at a time when I needed to go to sleep, so I was putting off from posting a more detailed accounting until the next day. Of course, when I awoke the next day, it was deleted… So I had little to post about. This is the link to the USGS for this earthquake.

This is the pager alert (which is also an automated product):

Here is the “Did You Feel It?” map, with no reports (should be no surprise, since there was no earthquake).

Here is the Modified Mercalli Intensity shake map, showing that people would have felt it if it had happened (and probably would have reported it!).

Here is what the USGS states about these earthquakes:

Commentary for Multiple “Phantom Events” in California – posted June 2, 2015

Automated notification systems are a convenient and often essential component of modern life. The USGS has invested heavily in developing automated systems that provide the public with timely and accurate earthquake information. On rare occasions the Earth throws a curveball and on May 29th and 30th, the USGS issued multiple alerts for false earthquakes in Northern California. The first, a M5.1 near Lewiston, CA, was distributed on Friday. More false alerts were distributed on Saturday, including a M5.5 near Ukiah and M4.7 near San Simeon.

These erroneous earthquake notifications were created by the seismic waves from large, distant earthquakes. On Friday, a M6.7 earthquake occurred at a depth of approximately 60 km, 111 km off of Chirikof Island, Alaska. It was this earthquake that fooled the automatic processing of the Northern California Seismic System to issue the first false alert. Just 28 hours later, a M7.8 earthquake off of Japan with a depth of more than 660 km – the deepest earthquake of its size to have occurred during our history of recording – spawned two more phantom events in Northern California.

Large earthquakes have created challenges for regional seismic monitoring in the past. This problem is particularly acute for deep earthquakes as they generate very impulsive seismic waves which may be misinterpreted as a local earthquake. The USGS and its partners have developed a number of methods to stop or screen these events from being distributed on the Web and through such mechanisms as the Earthquake Notification Service. The USGS will be implementing changes to improve the system and minimize the chances of this occurring in the future.

The erroneous events were deleted quickly by a duty seismologist. Unfortunately, a problem with the distribution software prevented the delete messages from being transmitted to recipients of the Earthquake Notification Service. The inability to transmit the information about the false events to the users of the Earthquake Notification Service caused significant confusion and the USGS regrets the problems caused by this failure. USGS staff have identified the problem in the distribution software and fixed it.

Errata for “Phantom Events” in Central and Northern California resulting from the M6.7 Alaska Earthquake on 2015-05-29 07:00:29 UTC

Strong earthquakes generate seismic waves that spread across the entire globe. When the earthquakes are deep, the distant recordings are quite impulsive and are often mistakenly identified by automated systems as local earthquakes. On 2015/05/29 07:00 UTC, a 60 km-deep M6.7 earthquake occurred offshore of Chirikof Island, Alaska and swept across the seismic networks in northern California. The automatic earthquake detection systems recognized the arrival of seismic energy but misinterpreted it as several earthquakes, including an M 5.1 event occurring near Lewiston, rather than one large distant event. These “phantom events” were automatically released for public distribution on the Web and through the Earthquake Notification Service. All “phantom events” were cancelled by the duty seismologist within 15 minutes.

Posted in Chemeketa Community College, earthquake, education, geology, HSU, plate tectonics, research

Blanco Fracture Zone: 2000 – 2015 Seismicity Animation


I put together an animation that includes the seismicity from 1/1/2000 until 6/1/2015 for the region near the Blanco fracture zone, with earthquake magnitudes greater than or equal to M = 5.0. The map here shows all these epicenters, with the moment tensors for earthquakes of M = 6 or more (plus the two largest earthquakes from today’s swarm). This is the search that I used for the earthquakes plotted in the map and animations below. Here is the page that I posted regarding the beginning of this swarm. Here is a post from some earthquakes last year along the BFZ.

Earthquake epicenters are plotted with the depth designated by color and the magnitude depicted by the size of the circle. These are all fairly shallow earthquakes at depths suitable for oceanic lithosphere.

    Here is the list of the earthquakes with moment tensors plotted in the above maps (with links to the USGS websites for those earthquakes):

  • 2000/06/02 M 6.0
  • 2003/01/16 M 6.3
  • 2008/01/10 M 6.3
  • 2012/04/12 M 6.0
  • 2015/06/01 M 5.8
  • 2015/06/01 M 5.9
    Here are some files that are outputs from that USGS search above.

  • csv file
  • kml file (not animated)
  • kml file (animated)


    Here are links to the video files (it might be easier to download them and view them remotely as the files are large).

  • First Animation (20 mb mp4 file)
  • Second Animation (10 mb mp4 file)

Here is the first animation that first adds the epicenters through time (beginning with the oldest earthquakes), then removes them through time (beginning with the oldest earthquakes).

Here is the second animation that uses a one-year moving window. This way, one year after an earthquake is plotted, it is removed from the plot. This animation is good to see the spatiotemporal variation of seismicity along the BFZ.

Here is a map with all the fore- and after-shocks plotted to date.

Late addition:
Here is a seismogram from Gold Mountain Washington. Thanks for posting this Pacific Northwest Seismic Network fb page. Here is the page that it came from.

Posted in cascadia, Chemeketa Community College, earthquake, education, Extension, geology, HSU, humboldt, oregon, plate tectonics, strike-slip, subduction, Transform, tsunami

Blanco Fracture Zone Earthquakes!


This morning we had three earthquakes related to the Blanco fracture zone, a transform (shear) plate boundary. These earthquakes occurred quite a bit north of the bathymetric expression of the BFZ (though the M 5.8 is actually plotting closer to the BFZ this morning), similar to the seismicity earlier this year to the southeast of today’s swarm. Here is a post regarding the seismicity along the BFZ in April 2015.

Here is a map showing these earthquakes, with moment tensors plotted for the M 5.8 and M 5.5 earthquakes. I include an inset map showing the plate configuration based upon the Nelson et al. (2004) and Chaytor et al. (2004) papers (I modified it). I also include a cross section of the subduction zone, as it is configured in-between earthquakes (interseismic) and during earthquakes (coseismic), modified from Plafker (1972).

Here is a version of the CSZ map alone (Chaytor et al., 2004; Nelson et al., 2004).

Here is a version of the CSZ cross section alone (Plafker, 1972).

Here is a map showing the BFZ seismicity from April 2015.

Posted in cascadia, Chemeketa Community College, education, geology, HSU, humboldt, plate tectonics, strike-slip, subduction, Transform