A narrowly spaced double-seismic zone in the subducting Nazca plate

Abstract

High-precision relocations of intermediate-depth earthquakes (80-130 km) below the Central Andes reveal a fine-scale double-layered Wadati-Benioffzone (WBZ). Upper and lower band of seismicity are separated by about 9 km and occur at the top of the oceanic crust and in the uppermost oceanic mantle, respectively. Analysis of focal mechanisms and waveform similarities indicate that fluid processes are causing the events. Earthquakes in the oceanic crust occur on pre-existing normal faults due to hydraulic embrittlement from metamorphic dehydration, and on subvertical faults that connect the two layers in a narrow depth range. Extensional faulting predominates in both layers, indicating that stab pull forces are the dominant stress source superseding possible unbending forces in this segment of the Nazca plate. Copyright 2004 by the American Geophysical Union.