Sismicidad y mecanismos focales en el Valle Mexicali-Imperial (1973-1985)

Abstract

We report the relocation of ninety hypocenters at the Mexicali seismic zone and the determinations of sixty focal mechanisms of earthquakes that occurred in the Imperial-Mexicali Valley. Stations providing the data are from both sides of the intemational border; seismic catalogs and other determinations found in the literatura are also used in the analysis. The earthquakes with magnitudes less than 5.0 are mostly confined to the MexicaJi and Brawley seismic zones. Several bursts of seismicity occurred in the Valley during the last fifty years. The last one, which took place between 1973 and 1981, has its larger swarms and aftershocks located on non-overlapping sections of the seismic zones. The seismic foei are mainly situated between 5 km and 13 km depth. Main shock epicenters of the most recent four earthquakes with ML > 6.0 occurred over the Cerro Prieto or Imperial fault traces, just outside the seismic zones. Focal mechanisms solutions of these four earthquakes indicate NW right-lateral strike-slip motion. This type of focal mechanism predominates in the Valley. However, the seismic zones show strike-slip, normal and a few reverse faultings. A cluster of events may contain focal mechanisms of different types, even within a period of a few hours. The global features are consistent with tectonics dominated by the relative motion between the Pacific and North American plates.