Seismic barrier for MW >8 earthquakes acting as asperity for MW about 7 at the Atiquipa region (southern Peru) from satellite GOCE data

Abstract

Mapping and modeling the heterogeneities distributed along the megathrust region in subduction zones is a key factor in understanding the seismic cycle and studying earthquake rupture behavior. Heterogeneities that control seismicity can be either structural, such as fractures or faulting, or compositional. The spatial resolution of Earth gravity field satellite-only models derived from the GOCE satellite ranges from 60 to 80 km at full degree/order, making them suitable for studying coseismic slips in large earthquakes (MW ≥ 8.0). In southern Peru, the Nazca Ridge subducts, a broad, smooth, and shallow seafloor feature characterized as a seismic barrier. Using the vertical gravity gradient, we have mapped mass heterogeneities in this region that appear to be spatially related to seismogenic behavior. Positive and high relative values in the long-wavelength vertical gradient are associated with barriers to the propagation of seismic energy during large megathrust earthquakes (MW > 8.0). In contrast, low-vertical-gradient zones with an elliptical shape are qualitatively linked to asperities. These seismic barriers are associated with forearc strike-slip faults, while the asperities are linked to submarine basins. The positive gravity gradient mapped in the southern region of the subducted Nazca Ridge exhibits dual behavior: as a barrier to large earthquakes and as a seismic asperity for MW about 7 earthquakes.