Shallow seismicity in subduction zones

During 1965 the University of San Agustin operated a net of 9 high‐sensitivity seismographs in southern Peru. Stable hypocenters for local earthquakes were determined by an interactive method utilizing both the P and the S arrival times. The hypocenters delineate a 30° dipping Benioff zone but also indicate considerable intraplate activity. Of particular interest is the existence of an aseismic wedge between the shallower earthquakes in the subducting lithosphere and those in the continental lithosphere. Investigations of other regions, e.g., the Aleutians and Japan, suggest that this may be a global phenomenon. We suggest that this aseismic wedge occurs because the compressive stress due to the interaction of the oceanic and continental plates is counterbalanced by tension caused by the downwarping of the continental lithosphere due to the drag of subduction. The unstressed zone (at depths less than 40 km) includes the coastal areas and explains the absence of large shallow earthquakes in that region. Further inland, beyond the region of downwarping, stresses are again adequate to cause large shallow earthquakes.