The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

No 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.

The hypocentral distribution of locally recorded aftershocks of the great (Ms=8.1) Michoacan, Mexico, earthquake of September 19, 1985, defines a narrow Wadati‐Benioff zone structure, roughly 10 km thick, dipping 14° at N23°E. This is in good agreement with the source geometry obtained by waveform modeling of the 1985 Michoacan mainshock and the large 1979 Petatlán earthquake in the adjoining region. We inverted for the crustal velocity structure in the epicentral region by applying the Levenberg‐Marquardt non‐linear least squares algorithm to our local aftershock data. The velocity model consists of a layer with linearly increasing velocity in depth overlying a dipping, constant velocity halfspace. Our hypocentral location program uses a velocity model of the same form together with ray tracing. The earthquake hypocentral resolution obtained with this program is significantly better than that from conventional approaches (HYPO) and looks very promising for use in velocity structures with an important dipping interface like subduction zones.

No abstract