Intermediate‐depth earthquakes in central Mexico: Implications for plate waves

Abstract

Regional seismograms of intermediate‐depth earthquakes (50≤H≤80 km) that occur below the Central Mexican Plateau show a phase which, at epicentral distances of 150 to 450 km, arrives about 15 to 20 sec after the P wave and 5 to 12 sec before the S wave. This phase was previously interpreted as a seismic wave refracted from a dipping interface below the source, thus apparently providing direct evidence of the structural location of the subducted Cocos plate below México. The phase was called the plate wave. Recent intermediate‐depth events have given rise to better quality data recorded by some newly‐installed broadband seismographs. An analysis of these and previous data strongly suggests that the phase is an S‐to‐P converted phase at the free surface and, therefore, provides no information regarding the subducted plate.