INTERMEDIATE-DEPTH EARTHQUAKES IN CENTRAL MEXICO - IMPLICATIONS FOR PLATE WAVES

Regional seismograms of intermediate-depth earthquakes (50 less than o r equal to H less than or equal to 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 b efore the S wave. This phase was previously interpreted as a seismic w ave refracted from a dipping interface below the source, thus apparent ly providing direct evidence of the structural location of the subduct ed Cocos plate below Mexico. The phase was called the plate wave. Rece nt intermediate-depth events have given rise to better quality data re corded 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 inform ation regarding the subducted plate.