Thursday, August 16, 2007

Earthquake location and fault movement

There have already been informative posts in the geoblogosphere about the recent very large earthquake in Peru (see Highly Allocthonous). I thought I'd put up a brief post relating to earthquakes, but not one specific one. When I have posted on various earthquakes, one thing I have always tried to do is put up a map that includes that focal mechanism for the earthquake. The USGS actually maintains a large historic database of these focal mechanisms, which are really interesting to browse through. I will not explain how to read them here, mainly because Professor Vince Cronin at Baylor University has already put together a very well written and easy to read guide on them (download the pdf by clicking here). If you are curious, I highly recommend checking it out. For educators in the crowd, the hand out also includes some exercises, it is something he created with teaching in mind.

In short, earthquake focal mechanism solutions are the beach balls that appear on maps like those below. They are produced by compiling and analyzing the waves that are produced by an earthquake and recorded by seismographs. As the handout explains, they provide a wealth of information, including information on the type of fault motion that occurred, that is was it a strike-slip, reverse, or normal fault.

Some of the compilations of historic focal mechanisms show exceptionally well the modern state of deformation in different regions of the world. For example, the most recent Peruvian earthquake occurred on a thrust fault (dipping at ~27° according to the USGS). Most of the beach balls shown from historic earthquakes are the same character.

From the USGS website here.

Which are different from strike slip faults, which dominated in the two images below.

From the USGS website located here.

From the USGS web site located here.

Learning to qualitatively read these focal mechanisms exponentially increases the amount of information you can read from earthquake reports. The USGS also has a good figure showing the basics of the beachball diagrams. It is by the analysis of these waves, by the way, that geologists have determined that the recent coal mine collapse in Utah was not caused by an earthquake (also here) but instead produced the earthquake sensed by regional seismographs.


BrianR said...

nice post, Thermochronic is back...I was wondering what happened.

Miguel Vera said...

Very nice, the "beach ball" guide is an excellent read. Thanks for posting about this event.


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