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.