Modeling Coseismic Slip of the 2012 Nicoya Peninsula Earthquake, Costa Rica: Roles of Megathrust Geometry and Surface Displacement

Abstract:

Geomorphic and geodetic measurements of coseismic ground surface displacement due to the 2012 M_w=7.6 Nicoya, Costa Rica Earthquake are unprecedented datasets due to their proximal location relative to the megathrust seismogenic zone. This unique situation presents an opportunity for in-depth analysis of the plate interface geometry on which the earthquake occurred, and the surface displacement data that constrain coseismic slip models. Estimated coseismic slip on a strike-and-dip variant surface based on the USGS Slab1.0 model yielded a M_w=7.7 earthquake, producing 1.3x more moment than published values using dip-variant segmented planes. This is consistent with the depth contrast between Slab1.0 and the segmented planes, where the deeper surface geometry requires more slip to predict an equivalent amount of surface displacement. This result presents the possibility that the earthquake was larger than originally estimated, which has relevance to evaluating future seismic hazards and emphasizes the importance of considering the true plate interface geometry when estimating earthquake slip. 

Densely spaced geomorphic measurements of coseismic vertical displacement along the SW coast of the Nicoya Peninsula reveal local heterogeneities in deformation not captured by sparser geodetic data. Model-predicted vertical displacements better fit the geomorphic data as compared to the geodetic data, both in the context of spatial distribution and magnitude. This signifies that the slip distribution models using a strike-and-dip variant plate interface geometry can successfully fit local heterogeneities revealed by an independent dataset. This research has relevance to both Nicoya seismicity and earthquake research worldwide, as large-scale and interdisciplinary analysis of these unprecedented data provides insight into how we model earthquakes and analyze their hazards.