The earthquake swarm of february 1981 in Mexico City

Abstract

Between February 4 and 15, 1981, a particularly well recorded earthquake swarm occurred in Mexico City. The 5 largest events analyzed here (2.7 <ML<3.2) had hypocenters within a volume of 1 km3 with mean location at 19.3780N-99.1960W, and a depth of 1.2 km. Relative locations of these events showed a migration from 0.7 to 1.6 km on a nearly vertical plane. A normal fault plane solution with the tensional axis almost east-west was obtained using local and regional stations. Spectral analysis gave seismic moments in the range 0.5 - 2.0 x 1020 dyne-cm and rather low stress drops (1 to 5 bars). Similar parameters were obtained for the largest event occurring in a swarm in the same area in February 1980. It seems most likely that these events occur as a result of accumulating regional tension; however, the sinking of the Valley of Mexico could possibly produce similar oriented tensions, which, if not causing the earthquakes, might trigger them.