AGU Abstract

Spatiotemporal Distribution of Earthquakes During Slow Slip in the Nicoya Peninsula, Costa Rica 

Ada R. Dominguez¹, Susan Y. Schwartz², Noel M. Bartlow³, Andrew V. Newman⁴

¹ Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States, ardomin (at) umich.edu

² Department of Earth and Planetary Science, University of California Santa Cruz , Santa Cruz, CA 95064, United States, sschwartz (at) pmc.ucsc.edu

³ Department of Geophysics, Stanford University, Stanford, CA 94305, United States, noelb (at) stanford.edu

⁴ School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, United States, anewman (at) gatech.edu

Slow slip events have been documented at several different subduction zones, with the best examples occurring in Cascadia and Southwest Japan. These two areas have several features in common, such as being relatively hot, with locked and shallow seismogenic zones with little, if any, interplate seismicity, and have steady slow slip that occurs downdip of the locked regions. The northern Costa Rica subduction zone also exhibits slow slip, but differs from Cascadia and Japan in that its plate boundary is relatively cold, more heterogeneous, with locked patches and abundant interplate microseismicity, and has both deep (~ 30 km) and shallow (~6 km) slow slip. This study focuses on comparing the spatiotemporal relationship between normal earthquakes and "slow slip" phenomena in northern Costa Rica. 

A major slow slip event, accompanied by seismic tremor, was recorded in May 2007 on a GPS and seismic network installed on the Nicoya Peninsula, Costa Rica. We manually scanned, picked P and S wave arrival times and located earthquakes for a time period spanning several months before and after initiation of the slow slip event. Analysis of the temporal evolution of events shows a fairly constant level of seismicity leading up to the slow slip event followed by a gradual decrease in the number of events after slow slip. The majority of events locates along the western coast of the Nicoya Peninsula and delineate the geometry of the subducting slab. A comparison of interplate event locations with slow slip and tremor reveals that slow slip phenomena occur at both the up- and down-dip edges of the seismogenic zone defined by the microseismicity and therefore at both the shallow and deep frictional transitions from stick-slip to stable sliding.