Earthquake swarm in western Iran

Here we find an earthquake swarm in western Iran. The convergence between the Afica and Indo-Australia plates with the Eurasia plates is evidenced by the subduction zone along Sumatra and Java, the uplift of the Himalayas, the fold and thrust belt in Iran (where this swarm is located), and subduction in the Mediterranean (and uplift of the Alps). Here is the USGS page for the largest magnitude earthquake of this swarm.

Here is the Mww moment tensor from the largest magnitude (M 6.2), supporting the interpretation of a northwest striking compressional earthquake. This is consistent with the northwest trending fold and thrust belt shown in the aerial imagery below.

This is a global scale map showing the plate boundaries in red and the epicenters in orange:

This is a larger scale map showing the regional fold and thrust belt in western Iran.

The USGS “pager” is an estimate of damage to people and their infrastructure. This estimate is generated by overlaying population information and building inventories with an estimate of ground shaking generated by a ground motion prediction model. This is only an estimate that is useful for emergency managers to use to determine the scope of any aid that might be offered or sent to the region.

Here is the USGS map of the plate boundaries and historic seismicity.

This image is a primer for us who want to learn more about focal mechanisms and moment tensors. Moment tensors and focal mechanisms are calculated in different ways, but their graphical depiction is largely the same. We may use this graphic to help us interpret the moment tensor of the M 6.2 earthquake shown above>

Posted in College Redwoods, earthquake, education, geology, plate tectonics

M 7.9 Rat Island / Aleutian Islands: aftershocks reveal more

There have been a nice suite of aftershocks to help us interpret these data. There were several aftershocks less than or equal to M = 6. These occurred within hours of the mainshock. They also occurred northwest of the mainshock. This suggested that the fault that ruptured strikes west-northwest, based on the moment tensors. I mentioned this spatial relation on the first page about this earthquake swarm here. There is more information about the mainshock and tsunami measurements on that first page.

Here is the mainshock moment tensor:

We now have some more aftershocks, including a slightly larger magnitude earthquake M = 6.3 further northwest from the main swarm. This earthquake occurred ~7 hrs after the main shock. At first I thought I might consider this a triggered earthquake (not an aftershock). Following are a couple figures I put together to explain how I interpret this swarm.

Here is a map that shows the epicenters, the magnitudes, and the time the earthquakes were recorded. The USGS moment tensor for the M = 7.9 earthquake is in green. The USGS moment tensor for the M = 6.3 earthquake is in orange.

In this map I placed lines that are oriented for my preferred fault plane solution from each of the moment tensors. I center the red strike line on the epicenter for each earthquake. There is some spread to the strike (neither of the two largest earthquakes align perfectly with their partner’s strike).

This map shows a strike line that is the average between the strike lines for the two largest earthquakes. This is far from being quantitative, but it shows a little about how if we consider the different sources of error for each measurement (aleatory uncertainty) and what we understand about how these moment tensors represent the real earthquake(s) (epistemic uncertainty), it is reasonable to interpret these earthquakes to be along the same fault (or at least a synthetic or subparallel fault).

And here is a primer on focal mechanisms. The moment tensor plots are made differently than are focal mechanisms (they use tensors instead of first motions). However, these moment tensor plots use the same graphical representation as focal mechanism plots (i.e. beach balls). Basically, each focal mechanism results in a graphic that shows two possible fault plane solutions (the criss-cross separating the colors, orange or green in this case). The regions of color represent earth that has “bulged out” (compression) and the white represents earth that has contracted (tension) or “bulged in.” This graphic explains this further and click on the graphic to see it full size. This graphic is from here at the USGS. If you want to see a primer on moment tensors, here is the USGS page on them.

These are deep earthquakes and are clearly not on the subduction zone fault. Also, the moment tensors show mostly strike slip (the M 7.9 slightly extensional-oblique). If we start thinking about forearc slivers (“strain partitioning,” generally in the upper plate, where a subduction zone fault is not perpendicular to the convergence motion, a strike slip fault system will form to accommodate the margin parallel strain). This earthquake swarm appears to be rupturing a forearc sliver in the lower plate. We saw another potential analogy of this in the southwest Pacific earlier this year (the earthquake swarm in the Solomon Islands. I will need to see if anyone has documented (philosophically or otherwise) such a phenomenon. Most forearc slivers that have been documented are in the upper plate (the great Sumatra fault is a great example of a forearc sliver). The Aleutians form blocks that may rotate in response to oblique convergence.

This map shows historic earthquakes in pink. Today’s earthquake is on the eastern boundary of the 1965 M 8.7 subduction zone earthquake. Note the plate motion vectors in white. Note how the plate motion vector nearest the Rat Islands is not perpendicular to the margin (the subduction zone fault).

Here is a graphic showing what a forearc sliver might look like (from GSA).

Here is a map from Krutikov et al. 2008 (Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179 Copyright 2008 by the American Geophysical Union. 10.1029/179GM07)
Note that there are blocks that are rotating to accomodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

Here is something cool too… This animation was created by IRIS here. The seismologic record of the M 7.9 earthquake as recorded at many operating seismometers is compiled to show the vertical motion across the United States. Red is up and blue is down. Click on the map to view the animation.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction, tsunami

M 7.9 Aleutians Earthquake!

This one is large and fairly deep. Here is the USGS web page for this earthquake. There are several aftershocks, the largest so far is M 6. Today’s earthquakes are located along the Aleutian subduction zone, nearby the Bolan Ridge (the horseshoe shaped ridge to the north of the earthquake epicenters).

Here is the moment tensor, which shows this to be an oblique earthquake mechanism. This earthquake is possibly related to the internal deformation in the downgoing slab, which appears to be extensional in this location.

Here is a regional map showing the shaking intensity (Modified Mercalli Scale).

Here is a local map showing the shaking intensity and some of the island names.

This map shows historic earthquakes in pink. Today’s earthquake is on the eastern boundary of the 1965 M 8.7 subduction zone earthquake.

This is a different map of the historic earthquakes (a better resolution than the one above, but with fewer earthquake slip patches plotted).

Here is the list of recorded wave heights reported by:
435 PM AKDT MON JUN 23 2014

Here are some aftershocks plotted. They align with the northwest striking moment tensor.

Here are some more aftershocks plotted. I left the earlier map since it shows how the largest aftershocks are aligned with each other.

Major triggered earthquake along strike with the west-northwest trend of the main shock and largest aftershocks. This swarm looks like it has reactivated a ridge-parallel fault. The magnetic anomalies are sub-parallel to this trend (look at the historic earthquake slip map above to see the magnetic anomalies).

According to the pager report, there is low exposure (not many people, nor their infrastructure, were in region of strong shaking determined from the computer simulations). Note the large probability for <1 fatality and <1 million USD loss.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction, tsunami

Panama Aftershocks Reveal Likely Fault Solution

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All rightee… We have a number of aftershocks that are lighting up the potential fault that ruptured a couple days ago. Here is my page for this earthquake. Here is the USGS page.

The aftershocks are aligned with the north-south transform fault system (named the Colba Ridge) that separates the Cocos plate to the West and the Nazca plate to the East. Here is the local map with historic epicenters in gray and the mainshock and aftershock epicenters plotted as orange circles. Note that many of the historic epicenters also align along this fracture zone.

Here is a map with only the recent epicenters in orange.

Here is an updated moment tensor (Mwb). With these aftershocks, we can better interpret this as rupture on a N-S striking right-lateral fault.

There are 5 moment tensors on the USGS page. For those looking for a refresher on focal mechanisms, here is the USGS page that describes them.

Here is the USGS poster that describes the historic seismicity and plate tectonic setting.

Here is a more comprehensive view of the local and regional tectonics in this region. This is a compilation by Harley M. Benz and others.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, research
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I have placed some photos into a basic photo gallery. These are from the 2014 Seismological Society of America Meeting in Anchorage, Alaska.

1964 Earthquake Animation

Here is a simulation of the 1964 earthquake. I found this posted online on livescience (they do not typically provide a link to the original content, but repost without attribution) but here is the source. Carl is  an assistant professor of geophysics at the University of Alaska Fairbanks  at the Geophysical Institute (GI)  and the Department of Geology and Geophysics 

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

Earthquake offshore Panama

Good sized earthquake offshore Panama! Here is the USGS page.

This is probably an earthquake on an oblique high angle reverse fault, based on this early moment tensor. The hypocentral depth is pretty shallow too.

This looks related to the seismicity that plots along the North-South transform fault system that separates the Cocos and Nazca plates. However, it is near a triple junction and could be related to the East-West transform fault, or the subduction zone that strikes to the northwest. Very interesting.

Here is a regional map showing historic epicenters in grey. Today’[s earthquake is plotted in orange.

Here is a local map showing the same historic quakes, just zoomed in more.

This shows that the shaking was probably felt throughout coastal Panama.

Here is the USGS regional tectonic map:

Here is the PAGER report, that indeed shows there were many people who probably felt this earthquake.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, tsunami

Update Commentary on the OSO Slide: Scott Burns

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Scott Burns, emeritus at Portland State and a student of Peter Birkeland, wrote an excellent article about the OSO Slide. I made a few animations and armchair interpretations about this slide shortly after it happened (here).

Scott discusses the geologic history and how heavy rainfall was a likely co-conspirator that led to the triggering of this horribly destructive landslide. Here is the article.

This is a general location map:

Here is an animation from David George and Dick Iverson at the USGS:

As a reminder, here is a map showing the historic landslides in the area:

Here is the geologic mapping done at the 1:24,000 scale published by the Washington State Department of Natural Resources. Click on the map below to see the entire map. Below is a clip of the map in the region of the OSO Landslide of 2014. Click here to see this inset map in a new browser window. Note the location of a dotted strike slip fault in the region of the OSO slide.

Posted in College Redwoods, earthquake, education, geology, landslides

1964 Great Alaska Earthquake: Before and After Photos

I just got back from a great Seismological Society Meeting in Anchorage, Alaska. The meeting was held there in part to celebrate the 50th anniversary of the 1964 Great Alaska Earthquake. I will post more online in the coming week. I present here a couple maps below, as well as a link to the USGS open file report that shows before and after photos in Anchorage.

Here is a map that shows the regional extent of the 1964 earthquake. Regions of coseismic uplift/subsidence are delineated by blue/red polygons.

Here is a map that shows the extent of historic earthquakes in southern Alaska. This is from a USGS open file report.

Here is a link to the USGS open file report. Click on the image to get the 12 MB pdf.

Posted in College Redwoods, earthquake, education, geology, plate tectonics, subduction, tsunami

Vancouver Isle M 6.6 Earthquake

Interesting little swarm here, on the coast of northwestern Vancouver Island. The convergence of these plates results in the Cascadia subduction zone (CSZ), that extends from Cape Mendocino in the South to at least Haida Gwaii in the North (there was a small subduction zone earthquake swarm there in 2013).

Here is a map that shows the CSZ (plotted as yellow), along with the MMI USGS ShakeMap for the M 6.6 earthquake.

This map shows some historic earthquake epicenters. Note how they align with some plate boundaries (e.g. the Blanco fracture zone, the transform plate boundary plotted as a west-northwest oriented blue line) and in some deformation zones (e.g. the Mendocino deformation zone that extends into the Gorda plate in the south).

The depth of the magnitude M 6.6 earthquake seems to align roughly with where we would expect the subduction zone fault. Today’s magnitude M 6.6 earthquake has a strike slip moment tensor.

Here is a regional view of the shaking intensity from the M 6.6 earthquake.

This map shows the results of the USGS Did You Feel It Program (that was initiated because of the work of Bob McPherson and Lori Dengler).

This shows the attenuation of shaking intensity with distance from the earthquake. Ground Motion Prediction Equations (attenuation relations) have been developed using thousands of ground motion measurements from earthquakes. As time passes, we get more earthquakes and we get more records. With the larger sized databases, the relations between shaking intensity and distance can be perfected for different fault settings, different site settings, and different earthquake magnitudes. Each of these factors can contribute to how the shaking from any given earthquakes is felt.

The estimated losses for this M 6.6 Vancouver Island earthquake are fairly low. Note the Green Designation.

This regional map shows today’s M 6.6 swarm as it relates to the Haida Gwaii earthquake along the Queen Charlotte plate boundary system to the northwest of Vancouver Island.

Here is the moment tensor from the M 7.7 subduction zone earthquake in Haida Gwaii on 10/28/2012

Here is a map that shows the M 7.7 earthquake swarm using a global scale.

This is a map of the Haida Gwaii swarm. Note how the main shock is plotted east of the mapped fault. One may also observe an accretionary prism that is probably related to subduction along this plate boundary. The fault line should really be mapped to the west of where it is drawn here. This fault system has alternately been interpreted as a strike-slip (or transform) plate boundary. If we consider the obliquity of subduction, the strike-slip faulting (aka Queen Charlotte – Fairweather fault system) is possibly a sliver fault (like the Sumatra fault). Also, note that some of the smaller aftershocks appear to align on north-northeast oriented patterns. Perhaps these are northeast striking strike slip faults (or normal faults formed at the spreading ridge, reactivated with strike slip motion) triggered by the subduction zone earthquake.

    The M 7.7 earthquake generated a tsunami. Here is a map showing some model estimates of trans Pacific tsunami heights.

  • Here are two plots of the tsunami record as measured in Crescent City.
  • This first plot shows the modeled wave height compared to the measured wave height.

  • This plot shows the measurement of the tsunami plotted on top of the tidal water surface elevation.

The USGS put together this poster to help explain their interpretations of the M 7.7 Haida Gwaii earthquake swarm.

Posted in cascadia, College Redwoods, earthquake, education, geology, plate tectonics, research, subduction, tsunami