Triggered seismicity in the Valley of Mexico from major Mexican earthquakes

Abstract

We have made a systematic examination of seismograms and bulletins from Mexico City since 1920 to examine whether major and great subduction zone earthquakes along the ·Pacific coast of Mexico caused increases of local seismicity in the Valley of Mexico, which is situated in the Trans Mexican Volcanic Belt. Of ten coastal events analyzed through 1957, seven events are followed by seismicity increases in the valley, although the increase in one of these cases· is small. The probability that these increases occur by coincidence is negligible. Eight large coastal earthquakes since 1957 were also examined. For these events the results are more ambiguous due to an increase in the cultural noise and the necessity to use different seismic stations. It is clear, however, that the two 1985 earthquakes were followed by an increase in local events. A small increase in the activity is noted following events in 1978 and 1979. During the events which were followed by increased activity, the computed shear strain amplitudes and dilatations were greater than or equal to 3 and 2 micwstrains, respectively, while for the events that caused no increase in seismicity, the computed strains were less thari or equal to these values. These observations are consistent with the possibility that the activity in the Valley of Mexico was triggered. They are further consistent with the existence of some threshold in strain that needs to be exceeded to cause remote triggering of earthquakes. The observed delays in the triggered seismicity could be caused by either creep or fluid flow processes.