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 55Њ in the N12Њ W direction, whereas the T axis was nearly horizontal and with a N68Њ 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 20Њ in the N38Њ 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.

Active Deformation along the Southern End of the Tosco-Abreojos Fault System: New Insights from Multibeam Swath Bathymetry

“…The relative displacement between the Pacific plate and Baja California Sur is consistent with a right lateral motion along the margin and with the focal mechanisms of earthquakes occurring in this region (Fig. 2;DEMETS and DIXON, 1999;DIXON et al, 2000;MUNGUIA et al, 2006); however, its rate is still debated. MICHAUD et al (2004) show that the Pacific plate moves relative to Baja California Sur at a rate of 3-7 mm/year along a direction of motion of N130°(i.e.…”

Late Quaternary faulting history of the Carrizal and related faults, La Paz region, Baja California Sur, Mexico

“…The Gulf of California is an active oblique-divergent plate boundary (Lonsdale, 1989;Plattner et al, 2007) that formed between the North American and Pacifi c plates (Stock and Hodges, 1989), and has largely localized plate motion in the past 2-3 million years. Despite this fault localization along the dominant plate boundary, faults remain active in the southwestern gulf-margin fault system along the southeastern Baja California peninsula and offshore near the peninsula (Fletcher and Munguía, 2000;Martínez-Gutiérrez and Jorajuria-Lara, 2003;Munguía et al, 2006;Busch et al, 2011). Understanding the relation of the gulf-margin system within the plate boundary as a whole will lead us to better understand the process of localization of faulting at divergent plate boundaries and the rift to drift transition process.…”

Geometry and evolution of rift-margin, normal-fault–bounded basins from gravity and geology, La Paz–Los Cabos region, Baja California Sur, Mexico

“…The spreading rate across the Alarcón rise in the southern Gulf of California is slower than that of the Pacifi c-North American plate boundary (~90% of total), indicating that structures in addition to the transform faults and spreading centers in the gulf also accommodate deformation in this region (DeMets, 1995;Fletcher and Munguía, 2000;Plattner et al, 2007). Seismicity (e.g., Munguía et al, 2006) and geomorphic relationships (Fletcher and Munguía, 2000;Busch et al, 2006Busch et al, , 2007Maloney et al, 2007) indicate that the array of normal faults rupturing the southern end of the Baja California peninsula are active and act as a weak distributed shear zone that contributes to translating the Baja California peninsula blocks away from mainland Mexico (Plattner et al, 2007). The gulf-margin is a zone of distributed extension as well as decreasing elevation and thinning crust (Lizarralde et al, 2007).…”