Seismicity and State of Stress in the La Paz-Los Cabos Region, Baja California Sur, Mexico

Abstract

We present the results of an analysis of seismic data collected for seven years in the La Paz–Los Cabos region. The data set includes earthquakes with magnitudes of up to 3.6 and focal depths mostly between 2 and 14 km. The results show that some located epicenters correlated with known faults of the region, but others did not, suggesting faults that have not yet been recognized. In addition, even though earthquakes occurred all over the study area, zones of stress concentration were identified. In such zones, the seismicity often developed as earthquake swarms. Composite fault-plane solutions were prepared by using data of those areas. The resulting mechanisms for events of the northern part of the study area indicated normal faulting (east-side down). In such case, the P axis had a mean vertical angle of 55° in the N12° W direction, whereas the T axis was nearly horizontal and with a N68° E average trend. This part of the study area is found to be under a predominant tensional stress regime. To the southwest of the area, the focal mechanisms showed predominant components of strike-slip faulting. There, the T axes were still subhorizontal, but the P axes were more horizontal, resulting in a mean plunge of about 20° in the N38° W direction. On the other hand, highly consistent directions of P and T axes from 13 representative events of the Gulf of California fault system led to average axes that are close approximations to the principal stresses that drive such a fault system. A comparison of these axes with average P and T axes of our region shows that the latter are about 16 to 23 degrees more westerly than the former. We propose then either that the earthquakes in the study area occur more in response to local tectonic forces, or that our inferred stress axes are only approximations to the principal stresses. The complex tectonic situation of the region makes the first possibility feasible, but the fact that the earthquakes occurred on pre-existing faults favors the second one. Future seismic data of the region will help to elucidate this uncertainty. Finally, the microearthquakes studied were not generated by the Gulf of California fault system that forms the boundary between the Pacific and North America plates. Our results, then, are evidence of a wide zone of deformation across which subsidiary faulting accommodates part of the relative plates motion.