Spatial distribution and B value of intermediate‐depth earthquakes beneath central Peru

Abstract

We have investigated the spatial distribution of microseismicity at intermediate depth in the Wadati‐Benioff (W‐B) zone beneath central Peru. Within a 100 to 150 km depth range in the subducting Nazca plate, 67 event hypocenters define a horizontally subducting zone (horizontal zone) 450 to 600 km from the trench, and a zone of increasing dip (resubduction zone) 600 to 700 km from the trench. Although the shape of the W‐B zone is generally consistent with previous observations from teleseismic data, the distribution of seismicity is not. In a 550 km wide band extending from 400 to 750 km from the trench, the density of teleseismicity (mb > 4, 1971–1981) increases as a function of distance from the trench, while the density of microseismicity (1.6 < mb < 4.2) decreases. Combining local and teleseismic data, the b value changes from .84 in the horizontal zone to .57 in the resubduction zone. This result suggests that the minimum earthquake size and/or stress drop increases in the resubduction zone. Low b‐values may also be intrinsic to proximity to the seismic cut‐off at 150 km depth.