MISLOCATION OF MEXICAN EARTHQUAKES AS REPORTED IN INTERNATIONAL BULLETINS

Abstract

Comparison of locations of earthquakes determined from field seismographs or from special studies with those reported in PDE and ISC bulletins suggests that shallow coastal earthquakes in Mexico are systematically mislocated in these bulletins. In general the epicenters are shifted about 35 km towards N 35°-45°E. ISC, more often than PDE, reports depths based on depth phases. These depths, although slightly greater than the depths estimated from synthetic modeling or field data, are more accurate than the PDE depths. However, there is little to choose between the two bulletins as far as epicentral locations are concerned. A modest increase in the number of Mexican stations sending readings to PDE and ISC appears to improve the locations by about 10 km or so. The mislocations are most probably due to higher velocity of the Cocos plate below Mexico. The systematic mislocation should be taken into account in the use of PDE and ISC bulletins.