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The unusual case of the Mexican subduction zone

Mexico is located on one of Earth's subduction zones, where the ocean floor of the Cocos tectonic plate is forcing its way down ("subducting") beneath the continental edge of the North American plate, forming a deep underwater trench parallel to the shoreline (dark blue line in figure). Subduction zones are prone to large earthquakes, volcanoes, and sometimes tsunamis.

There are many other subduction zones around the globe, such as Sumatra, Cascadia, and Japan. But through the work of Caltech's Tectonics Obsevatory (TO), scientists now know that the Mexican Subduction Zone is highly unusual.

• First, in most subduction zones, large earthquakes occur along the interface between the two plates, from the earth's surface down to depths of 600 km, and are used to locate the subducting plate. However, in southern Mexico, few earthquakes are seen below 100 km in depth, leaving the subducting plate's whereabouts unknown.
• Second, in most subduction zones, there is a volcanic arc running along the coast, parallel to the trench. But in southern Mexico, the volcanic arc bends inland away from the trench (red triangles in figure).
• Finally, "silent earthquakes" (or a slow slip events) have been discovered here. These occur so slowly that a single "earthquake" can last as long as a month, and the seismic waves they produce, called tremors, are so weak that we cannot feel them.

Scientists at the TO have been investigating the Mexican subduction zone to shed more light on these unusual features. New research results show that the subducting plate has flattened out to near horizontal and is hugging the crust above it. This has cut off the deep earthquakes and moved the volcanic arc inland. A thin layer has been discovered inbetween the two plates, and is shown to be associated with silent earthquakes and swarms of tremors.

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Jason and Zorka Saleeby appeared at the Kern Valley Spring Nature Festival sponsored by the Audubon Society. They presented the yearly banquet keynote address entitled "Kern Valley Backyard Geology," led a workshop on local geology, and led a field trip to the Kern Canyon fault.

Fingerprinting slow earthquakes and how they relate to the big one, ScienceDaily (April 30, 2009)
Team of researchers finds anomalous layer on top of a subducting slab. This layer, called the "ultra-slow velocity layer" (USL), slows seismic waves down by 30% to 50%, and is shown to be coincident with silent earthquakes. (see article in Science)

John Eiler - Recipient of the 2009 Epstein Medal, the European Association for Geochemistry (EAG)'s Science Innovation Award, "for particularly important and innovative breakthrough in geochemistry, considered to be of fundamental significance"

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