Microsismicidad en Ia zona norte de Ia falla La Paz, Baja California Sur, Mexico

Abstract

A five-station network of portable analog seismographs was installed along the northern segment of the La Paz fault zone, for a 40-day period of recording. Around 150 events were recorded with magnitudes up to 2.7, but only 46 events could be located on the basis of reliable measurements of P- and S-wave arrival times. 27 of these events correspond to an earthquake swarm which occurred during July 10 to 11, 1989 east of Espiritu Santo Island, near. the northern end of the La Paz fault and in the Gulf of California. The estimated focal depths for these earthquakes range from 8 to 16 km, with an average depth of 12 km. The remaining epicenters are scattered throughout the entire recording period,. and may be considered as indicative of the regional background seismicity. A composite fault plane solution, derived from frrst-r.notion plots for the swarm, suggested two possible mechanisms:a) left-lateral strike-slip faulting, on a vertical plane striking N1•80W and, b) normal faulting, on a plane of N28°W strike and dipping 60° to the northeast. The main difference betwe~ these two possible solutions lies in the dipping angle of the fault plane. On the• basis of the tectonic characteristics of the region, the strike-slip mechanism (allowing a slight component of normal faulting) is •preferred as a better approximation to the focal mechanism for earthquakes in the swarm.