Colloquium: Susan Schwartz

Susan Schwartz

Professor, Seismology Laboratory, Earth & Planetary Sciences, University of California Santa Cruz

Abstract - The 5 September 2012, M_w 7.6 Nicoya, Costa Rica megathrust earthquake occurred within a well-recognized seismic gap, directly beneath a network of seismic and continuous GPS stations and within an area of focused geophysical research for the last 15 years. This provides a unique opportunity to study the seismogenic behavior of the plate boundary throughout the earthquake cycle in exceptional detail. We compare locations of Nicoya Earthquake coseismic slip, geodetically determined locking, microseismicity, and slow slip and tremor activity to develop an understanding of the mechanics of the shallow plate interface and compare it with structural parameters of the subduction system. The mainshock slip extends ~50 km along strike and ~30 km along dip and corresponds very closely to the region of interseismic strain accumulation. Geodetically detected slow slip events occur about every year in distinct regions of the plate boundary from those accumulating strain and hosting seismic slip. We find an along strike northwest to southeast transition in the mechanical behavior of the plate interface. Coseismic slip is constrained to the northwest part of the Peninsula where slow slip occurs at both its up- and down-dip edges. The largest geodetically detected slow slip events occur beneath the southeastern portion of the Nicoya Peninsula. This change from strong to weak plate coupling corresponds with a contrast from lower to higher forearc vp/vs ratios suggesting that upper plate properties control the segmentation in seismogenic behavior.