As oceanic plates subducts down into the mantle at convergent plate boundaries, friction on the interface with the overriding plate causes a stick-slip behavior. The overriding plate is pulled down by the subducting plate in areas of strong coupling, accumulating strain on the megathrust fault until slip occurs and the overriding plate pops back up. This process generates major earthquakes such as the 2004 tsunami-generating Sumatra-Andaman event. Recent observations have also revealed slow slip episodes on the deeper plate interface with the motion indicating the release of accumulated strain, but these episodes typically last much longer than an earthquake. Corresponding seismic tremor composed of relatively small, monotonic signals also appear to occur near the deeper plate interface. Together the correlated strain and seismic observations characterize episodic tremor and slip (ETS) events, which recur with regular intervals that range from months to years. The processes that govern ETS and the potential relationships to major earthquakes remain unknown. Nevertheless, these events have been proposed to have significant impact on the likelihood of megathrust earthquakes since they appear to occur at different depths along the same plate interface. This project seeks to examine whether there are observable characteristics of the plate interface that could influence the frictional behavior and hence what type of seismic activity occurs. 

For this internship, I am looking for a student to assist with the investigation of seismic velocity structure of a subduction zone and how it relates to the seismic behavior on the plate interface. There are two geographic areas where the student will have an opportunity to contribute. The first is the Cascadia subduction zone in the Pacific Northwest, where the student will be able to join a two-week field workshop that incorporates teaching about subduction zone behavior, field service of an EarthScope Flexible Array deployment of seismometers, and attendance at the IRIS meeting in June. The second area is the Mexican section of the Middle America subduction zone, where data from another deployment needs to be processed to characterize the seismic velocity structure where the two plates collide. In this case, preliminary analysis has shown that small earthquakes are concentrating near the Oaxaca coast, separating a zone of ETS further inland from an aseismic gap offshore that we anticipate will eventually be released in the form of a large, damaging earthquake. The hypothesis this student will help test is whether these three types of seismic behavior (gap, small earthquakes, ETS) have representative seismic velocity signatures that can be illuminated with seismic tomography.

Return to 2008 Intern List