Chile earthquakes in the 2010 slip region

We had a small flurry of earthquake activity in Chile today, along the subduction zone there. Today’s activity is in the the region of the 2010 earthquake sequence. The largest magnitude earthquake in today’s sequence is a M = 6.2. Here is the USGS page for that earthquake.

Here is a map showing the seismicity for the past month or so. Below I list the USGS web pages for each of the earthquakes plotted on this map. I placed the location of the surface trace of the subduction zone fault in purple. This is based on the USGS location, which is approximate as it is based on the coarse resolution global topography data set. I have also placed the historic earthquake rupture limits in green. Note how the 3/9 M 5.1 earthquake plots west of the SZ fault, so it must be in the down going plate. This earthquake has an extensional moment tensor, consistent with either bending moment stresses, or slab pull extension stresses. Without more analyses, it would be difficult to distinguish between the two.

    Here are the USGS web pages for the earthquakes plotted in the above map.

  • 2/17 M 5.4
  • 3/2 M 5.3
  • 3/5 M 5.0
  • 3/9 M 5.1
  • 3/14 M 5.1
  • 3/18 M 6.2

Here is the map that I put together for some earthquakes in the seismic gap that I placed with an orange line in the above map. Here is my previous post about this series of M~5 earthquakes.

Here is a primer for the different types of earthquake faults and moment tensor/focal mechanisms. This comes from the USGS. This explains focal mechanisms. Moment tensors (which I use on my figures above) are determined differently, but their graphical solution/representation is the same as for focal mechanisms (for all practical purposes). Here is the USGS page on moment tensors if you want to learn more about them.

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

Molucca Earthquake

This region is a seismically active region. Today we had a M 6.2 earthquake. Here is the USGS web page for this earthquake.

    Last November, there were a series of large magnitude compression aearthquakes. Here are some posts about these earthquakes.

  • 11/15/14 M 7.1
  • 11/21/14 M 6.5
  • 11/26/14 M 6.8

Here is a map showing the earthquakes for the past month in the same region as the above earthquakes.

Here is a more detailed tectonic map with a cross section showing how these two opposing subduction zones are configured. This is from a Geological Society of America Special paper here.

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

Lake Almanor: Indian Valley fault activity

20150317_almanor_interpretation

There was an earthquake swarm in this region in 2013. The largest magnitude earthquake had a magnitude of M = 5.7. Here is the USGS page for that earthquake. “Today” we got a M = 3.8 earthquake in this same region. Here is the USGS web page for today’s earthquake. The Indian Valley fault is at the northern end of the Mohawk Valley fault system. We will be taking a look at this fault system (and the sedimentary/stratigraphic history) for this year’s Pacific Cell Friends of the Pleistocene field trip. The Mohawk Valley fault system is probably the northern extension of the Walker Lane. The Walker Lane is the northernmost extension of the east-of-the-Sierra-Nevada-mtns part of the plate boundary between the North America and Pacific plates (the most well known part of this plate boundary is the San Andreas fault). We looked at the Walker Lane for the 2010 Pacific Cell Friends of the Pleistocene field trip. We looked at faulting in the Lake Tahoe region for the 2012 Pacific Cell Friends of the Pleistocene field trip.

Here is a map showing the swarm from 2013, as well as the location of today’s M 3.8 earthquake. All orange dots represent earthquake epicenters from the year of 2013. On the map I have placed the moment tensors for the M 5.7 and M 3.8 earthquakes. The Indian Valley fault is shown in orange. I extended this fault (as a red dashed line) to where it may exist, based upon the recent seismicity. All the other lines are from the USGS fault and fold database. Anyone can use these fault data and they are downloadable here.

Here is the map that I made in 2013. Note the yellow fault lines that are near the town of Sierra. These represent the Mohwk Valley fault zone.

There was a swarm near Mt. Lassen in November 2014. I posted about this swarm at the time here. Here is a map showing that swarm.

Here is a primer for the different types of earthquake faults and moment tensor/focal mechanisms. This comes from the USGS. This explains focal mechanisms. Moment tensors (which I use on my figures above) are determined differently, but their graphical solution/representation is the same as for focal mechanisms (for all practical purposes). Here is the USGS page on moment tensors if you want to learn more about them.

    Here are three maps that have moment tensor data plotted. The first one shows the moment tensor for the M 3.8 earthquake. I have plotted the two moment tensor strike lines as light orange, each labeled. The mean strike line (geometrical construction) is plotted in red.

    This second one shows the moment tensor for the M 5.7 earthquake.

    This final one shows a pure strike slip focal mechanism for an earthquake with the same strike as the mapped Indian Valley fault, immediately to the south east of Lake Almanor.

Posted in Chemeketa Community College, education, geology, HSU, San Andreas

Earthquakes in Chile: Between the 2010 and 2014 Slip Patches

20150314_chile_interpretation

There have been a few earthquakes in the region between the 2010 and 2014 Chile subduction zone earthquakes. This is a seismic gap that has not had a Great earthquake since 1977, a ~M = 8.5 earthquake that spanned the distance between the ’10 and ’14 earthquakes.

Here is a map that shows the recent swarm of ~M = 5 earthquakes. There are moment tensors for the earthquakes listed below, some recent historic subduction zone earthquakes. I placed the general along-strike distance for older historic earthquakes in green (and labeled their years). The largest earthquake ever recorded, the Mw = 9.5 Chile earthquake, had a slip patch that extends from the south of the map to just south of the 2010 earthquake swarm. The 2010 and 2014 earthquake swarm epicenters are plotted as colored circles, while most other historic earthquake epicenters are plotted as gray circles. Note how this March 2015 swarm is at the northern end of the 1922/11/11 M 8.3 earthquake. At the bottom of this page, I put a USGS graphic about what these moment tensor plots (beach balls) tell us about the earthquakes.

Hundreds of people died as a result of the 1922 earthquake. The USGS has more news reports about the 1922 earthquake here. There were also reports of a tsunami over 9 meters. So we know that this segment of the fault can produce large earthquakes and tsunami. However, it has been about a century since the last Great subduction zone earthquake in this region of the fault.

Here is a map that shows the 2014 EQ swarm as it relates to historic slip patches in the region (Hayes et al., 2014). Note how there are large portions of the 1877 segment that have not gone off since 1877. This swarm of M~5 earthquakes is to the south of the 1995 earthquake that is plotted on the southern portion of this figure.

I remember discussing the 2014 earthquake at the 2014 SSA meeting. It was just after the swarm and it was fresh in my mind. Especially since I put together that animation of the time series. I mentioned to someone whose name I wont mention how interesting it was that there appeared to be two distinct slaong-strike slip patches to the swarm. They claimed that it could not be the case. He looked at the deformation data (inSAR) and claimed that it was clearly a single patch. Later I realized that inSAR could not distinguish these patches due to the time limitations of the overflight. Basically, there would need to be an acquisition before the swarm, in the middle of the swarm (between the two large slips), and one after the swarm was over. Since this is not possible (due to the short time of the swarm), it would be impossible to say there could not be two distinct patches. I wish I had this in my mind at the time. Here is a figure showing how these two patches have resolvable slip. Ironically, the person I spoke with about this at the meeting reviewed the Hayes manuscript. From Hayes et al. (2014).

Here is the figure I was looking for (Hayes et al., 2014). This shows the moment/slip deficit based on seismicity since 1900. The slip deficit is an estimate of the amount of “slip” or moment that has been accumulated as strain upon the fault over a given amount of time. This is determined with several assumptions:

  1. The geometry of the fault, plate configuration, and convergence rates
  2. The coupling ratio, or how much of the plate convergence is actually contributing to accumulated strain and how much is aseismic, or not contributing to accumulated strain.

The region between 300 and 550 km is the region of the 1877 slip that has not ruptured since then, except for the 1995 and 2007 earthquakes (which contribute to the moment/slip deficit estimate in this figure). This is the region immediately to the north of this March 2015 swarm. This region (300-550 km) is the same region in the seismic gap in the next map (Fig. 1 Hayes et al., 2014).

Here is figure 1 from Hayes et al. (2014). This shows this seismic gap for the region of the subduction zone that ruptured in 1977. The southern boundary of this figure stops north of the 2015 EQ swarm.

Going to the north, we can see this slip deficit map modeled by Chlieh et al. (2011). They also placed the historic and prehistoric along-strike slip extents as part of this plot. The 2001 M 8.4 earthquake in Peru is shown in this figure, as well as the Hayes et al. (2014) figure above. The southernmost part of this figure (red slip deficit) is the region that slipped in 2014. There remains a patch with a large slip deficit offshore of Tacna, Peru.

Going to the south we can take a look at the 1835, 1960, and 2010 slip patches and how they might relate to each other. Here is a map from Melnick et al. (2014). Below I paste the figure caption from this Melnick et al. (2014) figure.


Here is the figure text from their article (you will need to read their article to get the references they list in this caption): FIG. 1. Modeled plate-boundary slip during the 2010 Maule earthquake and slip deficit of Darwin seismic gap. a. Slip from Lorito et al. (2011) with computed (grey arrows) and GPS (orange arrows) displacements (Vigny et al., 2011; Moreno et al., 2012). White squares show GPS sites used by Lorito et al. (2011). Inset shows histogram of residuals between measured and modeled displacements. SE-standard error. Note that Lorito et al. ‘s model underestimates GPS displacements between 36-37.5°S where they forecasted a Mw 7.5-8 earthquake; b. Alternative slip distribution (Moreno et al., 2012) with modeled and GPS displacements. Inset shows histogram of residuals; c. Slip deficit after the 2010 earthquake including plate coupling over 175 years at an heterogeneous rate (Moreno et al., 2010; Moreno et al., 2011), slip release by the 1960, 1928, and 1985 events (Moreno et al., 2009; Moreno et al., 2012), and the slip distribution of Moreno et al. (2012). Note that the deficit is negative or null over most of the rupture zone, suggesting the 2010 earthquake closed the gap opened in 1835. Extent of 1835 rupture inferred from a compilation of historical sources (see text). The Santa María splay fault system, which slipped during the Maule earthquake (Melnick et al., 2012b), may be associated with the positive slip deficit near Concepción; d. Slip deficit using the same constraints as in (c) but with the slip model of Lorito et al. Note the large positive region northwest of Concepción. The extent of the 1835 rupture assumed by Lorito et al. (2011) is shown.

Here is a primer for the different types of earthquake faults and moment tensor/focal mechanisms. This comes from the USGS. This explains focal mechanisms. Moment tensors (which I use on my figures above) are determined differently, but their graphical solution/representation is the same as for focal mechanisms (for all practical purposes). Here is the USGS page on moment tensors if you want to learn more about them.

    References:

  • Chlieh et al., 2011. Interseismic coupling and seismic potential along the Central Andes subduction zone, Journal of Geophysical Research, v. 116, B12405, 21 p.
  • Hayes, G.P., Herman, M.W., Barnhart, W.D., Furlong, K.P., Riquelme, S., Benz., H.M., Bergman, E., Barrientos, S., Earle, P.S., and Samsonov, S., 2014. Continuing megathrust earthquake potential in Chile after the 2014 Iquique earthquake: Nature, v. 512, p. 295-299.

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

Moderately Deep Focus Earthquake in Colombia

There was a moderately deep focus extensional earthquake beneath Colombia today. Here is the USGS web page for this earthquake.

Here is a map that shows the epicenter as a green dot. Historic earthquake epicenters are plotted in grey. The USGS slab contours (the depth to the subduction zone fault, as estimated by Hayes et al. (2012) based on seismicity) show the slab to possibly be at about 200-300 km at the location of today’s earthquake. Unfortunately, their slab model does not extend to this region of Colombia. It is likely that today’s earthquake is in the downgoing slab (the Nazca plate) and is extensional due to bending of the slab, or due to slab pull (the plate that is deeper is going downwards due to its density, so it is pulling the slab down with it). Alternately, these may be due to the volcanism in this region.

This map is from the USGS here. I have placed the moment tensor on the map. There have been numerous earthquakes in this region (green epicenters) in the recent past.

This map shows the recent seismic activity in this region. These earthquakes, from the past year or so, align with the seismicity shown as green epicenters in the USGS map above.

Here is a map that shows the location of the cross sections below. These are from Moore and Twiss (1995) and were presented on this web site.


Cross Sections (Moore and Twiss, 1995):

Here is a paper that discusses the thermal control on subduction. They show how the slab is flat in this subduction zone.
Here is their tectonic setting map:

Here is a cross section in northern Columbia:

Here is their cross section where they present their models of different types of subduction zones. Their figure text: “Effect of subduction style on thermal regime and deformation in the upper plate. Note the increased viscous interplate coupling for fiat subduction and transfer of deformation inboard (to the backarc).”


References:

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

Earthquake near Siberut, Indonesia: Sumatra Megathrust

Yesterday (I was busy getting a Mt. St. Helens field trip together, here is the link for that) we had an earthquake with a magnitude M = 6.1. While this is not a large earthquake, it is notable because this part of the subduction zone fault is thought to have a large slip deficit. In other words, this part of the fault has been accumulating sufficient strain that we think it is ready to go. In today’s post, I will discuss the seismicity of the past decade in this region and I will follow up with a post where I present more on what we think about the current state of stress on the fault, based on the work of my colleagues.

Here are the USGS web pages for the earthquakes I will discuss in this post:

Here are a couple posts I put together regarding the initial instigator to this entire series of earthquakes (Mw = 9.15) and then the two largest strike slip earthquakes ever recorded (Mw = 82 and 8.6).

Here is a map showing moment tensors for the largest earthquakes since the 26 December 2004 Mw = 9.15 Megathrust Great Sumatra-Andaman subduction zone (SASZ) earthquake. Below is a map showing the earthquake slip contours. The beginning of this series started with the Mw 9.15 and Mw = 8.7 Nias earthquakes. There were some other earthquakes along the Mentawaii patch to the south (Mw = 8.5, 7.9m and 7.0). These were also subduction zone earthquakes, but failed to release the strain that had accumulated since the last large magnitude earthquakes to have slipped in this region in 1797 and 1833 (more on this in my next post). In 2012 we had two strike slip earthquakes in the outer rise, where the India-Australia plate flexes in response to the subduction. At first I interpreted these to be earthquakes on northeast striking faults since those the orientation of the predominant faulting in the region. The I-A plate has many of these N-S striking fracture zones, most notably the Investigator fracture zone (the most easterly faults shown in this map as a pair of strike slip faults that head directly for the epicenter of yesterday’s earthquake). However, considering the aftershocks and a large number of different analyses, these two earthquakes (the two largest strike slip earthquakes EVER recorded!) were deemed to have ruptured northwest striking faults. We called these off fault earthquakes, since the main structural grain is those N-S striking fracture zones. Also of note is the focal depth of these two large earthquakes (Mw 8.2 & 8.6). These earthquakes ruptured well into the mantle. Before the 2004 SASZ earthquake and the 2011 Tohoku-Oki earthquake (which also probably ruptured into the mantle), we would not have expected earthquakes in the mantle.

As a primer for those who want it, here is an info graphic that explains what the graphical representation of a focal mechanism is. Moment Tensors are determined differently, but their graphical representation is similar (so the info graphic can still help understand them). The text that goes along with this infographic is from the USGS here.

This map shows the Modified Mercalli [Shaking] Intensity contours modeled for this Mw 6.1 earthquake. These are just modeled values and not based on felt reports. The MMI scale is based on a qualitative measure of shaking intensity. This method of determining shaking intensity is very useful for regions of the world that do not have seismometers to record ground motions.

Here is one example of the MMI scale from the wiki site.

This shows how the model used in the above MMI map attenuates the ground motion with distance from the earthquake. Basically, seismic energy radiates outward from the earthquake focus and, therefore, diminishes with distance. The reported data are the small green dots which are averaged into the orange dots. There are only three reports for this earthquake, so the model is poorly constrained. Note, once again, that this location attenuates seismic energy much like the region modeled in California (which is largely accreted terrain, much like this region of Sumatra).

This is the pager file that shows an estimate of exposure to the hazards posed by this Mw 6.1 earthquake. This is based on the modeling in the above map (also shown in this map) and there are histograms depicting the estimated losses to people’s lives and their belongings (roads, buildings, etc.).

This map shows the slab contours for the subduction zone (Hayes et al., 2012). These contours are based largely upon historical seismicity. Since there is considerable depth variation for these earthquakes, the fault interface is not too well constrained in this way. If we had dozens of drill holes to constrain the fault location, that would be better (though orders of magnitude more expensive). The two shallowest depth contours are 20 and 40 km. So, the Mw 6.1 earthquake, with a hypocentral depth of ~28 km, fits nicely with this slab model.

References:

Posted in Chemeketa Community College, College Redwoods, education, geology, humboldt, Indonesia, plate tectonics, subduction, sumatra

another earthquake in the east Java / Timor region of Indonesia

Joe Magner liked this post

We had another earthquake in this region today. This is an interesting area of the world since this is a place where there is a transition between subduction to the west and collision to the east. The Island of Timor is interpreted to be part of Australia and has continental rocks that are found in Australia. While there is a large compressional plate boundary fault south of Timor, Timor was actually more part of Australia than it is part of the volcanic arc of Indonesia to the west (e.g. Java, Bali, etc.). Here is my post about the Mw 7.0 earthquake from a couple of days ago.

Here is a map showing the M = 5.0 earthquake as it relates to the Mw = 7.0 earthquake from a couple days ago. In the map, there is a large thrust fault that strikes east-west, north of the archipelago in this region. This is a back thrust (the Wetar Backthrust) as interpreted by Audley (2011) below.

Hall et al. (2012) provides a tectonic reconstruction for our enjoyment. Note how the Island of Timor is part of Australia back to 150 Ma.

Snyder et al. (1996) uses gravity data to interpret the plate boundary fault in this region.

Here is Snyder et als. (1996) interpretation of the structure of the plate boundary in this region.

Finally, here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor.

This shows the Audley (1986) interpretation of the faulting in this region. One may see that the ISland of Timor is being popped up like an onion skin (compare with the cross section above^^^).

Here is a paper by my colleague Beau Whitney regarding the tectonics immediately to the south of Timor (Hengesh and Whitney, 2014). Here is their map showing the region of their work:

References:

  • Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
  • Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics
    of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
  • Hall, R., Audley-Charles, M.G., Banner, F.T., Hidayat, S., and Tobing, S.L., 1988. Basement rocks of the Halmahera region, eastern Indonesia: a Late Cretaceous-early Tertiary arc and fore-arc in: Journal of the Geological Society, London, Vol. 145, 1988, pp. 65-84.
  • Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate
    Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
  • Snyder, D.B., Milsom, J., and Prasetyo, H., 1996. Geophysical evidence for local indentor tectonics in the Banda arc east of Timor in Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 61-73.

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

Indonesia Strikes Again

A deep earthquake has occurred probably within the downgoing Indo-Australia plate. The moment tensor shows oblique extension (part strike-slip, part normal). The depth aligns with the depth of the plate given the Hayes et al. (2012) slab models (which ends immediately adjacent to this earthquake, probably due to the more complicated double collision/subduction zone here).

Here is the same map with historic earthquakes plotted as well.

Randy Web has a blog about the tectonics of this region here.
Here is his map showing where he interprets the continental crust to be (which matches what I know of it, based on the literature I have read).

Here is another view of the local tectonics from Darman (2012).

References

  • Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
  • Hayes, G.P., and Wald, D.J., 2009. Developing framework to constrain the geometry of the seismic rupture plane of subduction interface a priori – a probabilistic approach, Geophys. Jour. Int., 176, 951-964
  • Hayes, G.P., Wald, D.J., and Keranen, K., 2009. Advancing techniques to constrain the geometry of the seismic rupture plane on subduction interfaces a priori – higher order functional fits, Geochem. Geophys. Geosyst., 10, Q09006, doi:10.1029/2009GC002633.

Posted in Chemeketa Community College, College Redwoods, education, Extension, geology, Indonesia, plate tectonics, subduction

another northeast Japan earthquake in the Tohoku-Oki earthquake region

Joe Magner liked this post

We have had another small aftershock. Here is the USGS web page for this earthquake. This time it is in a different region than the swarm of the past couple of weeks.

Earlier posts:

Here is a map showing today’s earthquake with a couple of the larger earthquakes also plotted. See my prior posts about these.
The USGS websites for these earlier earthquakes are here:


This map shows the USGS (Hayes et al., 2012) slab depth contours. These contours represent the depth to the top of the downgoing plate (i.e. the subduction zone fault interface location). The depth is currently set at ~56 km. This is a little deep for the slab model, but the slab model is constrained by seismicity (which has considerable depth variation, so cannot be considered precise).

Posted in Chemeketa Community College, College Redwoods, earthquake, education, geology, Japan, plate tectonics, subduction

Gorda plate earthquake 2015.02.24

Where else would we get a focal mechanism or moment tensor for an earthquake of such a small magnitude! We are so lucky. This is the USGS web page for the earthquake. We get Gorda plate earthquakes frequently and most of them (if not all) are probably aligned with the northeast striking strike-slip faults.

Here is a map showing the epicenter with the USGS focal mechanism.

The Gorda plate is deforming due to north-south compression between the Pacific and Juan de Fuca plates. There have been many papers written about this. The most recent and comprehensive review is from Jason Chaytor (Chaytor et al., 2004). Here is a map of the Cascadia subduction zone, as modified from Nelson et al. (2006) and Chaytor et al. (2004).

Here is the Chaytor et al. (2004) map that shows their interpretation of the structural relations in the Gorda plate.

This is also from Chaytor et al. (2004) and shows moment tensor solutions for earthquakes in the Gorda plate. Note how they could predominantly be interpreted as northeast striking strike-slip faults.

Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

This map shows an earthquake swarm from 2014, which appears to align along another northeast striking strike-slip earthquake fault in the Gorda plate. This swarm is related to a Mw 6.8 earthquake. Check out my pages about the mainshock and the aftershocks. I made soem animations of these earthquakes here.

Here is a primer for those who want to learn more about focal mechanisms. This is from the USGS, where you can read more about them. Moment tensors are calculated differently, but their graphical representation is very similar to that of focal mechanisms.

References:

  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.

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