Imaging the Northern Costa Rica Subducted Slab With Broadband Receiver Functions

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

This award supports investigation of the hydration state of subducted lithosphere at the Costa Rica convergent margin. Previous studies have suggested above average slab hydration in this region, which implies anomalous dehydration at depth. This is important because slab dehydration plays a critical role in producing arc lavas and in generating intermediate depth earthquakes within the downgoing slab. Slab dehydration at shallower depth may also be important in influencing seismogenic behavior along the plate interface. Metamorphic dehydration reactions in oceanic crust have been implicated in the serpentinization of the forearc mantle wedge and the termination of interplate earthquakes in some subduction zones. The Costa Rica Seismogenic Zone Experiment imaged a distinct pattern of geodetic locking and microseismicity along the plate boundary in northern Costa Rica. The onset of interplate microseismicity occurs down-dip of the region of geodetic locking, coincident with thermal models of the 200-250 degrees Centigrade isotherms. This has been interpreted as a down-dip weakening of the plate interface due to fluid production from low-grade metamorphic reactions in basaltic crust coupled with a decrease in permeability around 250 degrees Centigrade producing elevated pore pressure and sufficient weakening of the thrust interface to permit earthquake failure. The implication of slab dehydration in the initiation of interplate earthquakes has thus increased the importance of determining the state of hydration of subducting lithosphere. P-to-S converted seismic phases, generated at the crust-mantle (Moho) interface of the subducting Cocos slab, recorded by the Costa Rica Seismogenic Zone Experiment broadband seismometers positioned directly above the slab are being analyzed. Teleseismic converted phases provide a direct method to image the structure of the subducted slab, and are being used to determine velocity structure across the oceanic Moho and relating it to the extent of oceanic crustal hydration. In addition, the velocity structure in the vicinity of the plate boundary is being investigated and related to the onset of seismicity and the inferred weakening of the plate interface. The possible role of a large normal fault outer rise earthquake in hydrating the oceanic crust near the Nicoya Peninsula is being assessed. This work is enhancing the understanding of the velocity structure of the subducting lithosphere and plate interface and its material properties.