Tectonic significance of an earthquake sequence in the Zacoalco half-graben, Jalisco, Mexico

We studied a sequence of small earthquakes that occurred during the months of April and May of 1997, in Jalisco, southwestern Mexico. The earthquakes were located along a set of active faults that form the Zacoalco half-grabe n (La Lima fault system), west of lake Chapala, within the rift-rift-rift t riple junction. A total of 33 events were located, with magnitudes ranging from 1.5 to 3.5, recorded by a portable array of broadband seismographs. We identified two groups of events: one corresponding to a shallow normal fau lt, synthetic to La Lima fault system, and another group associated with a deeper fault. The events that occurred on the synthetic fault show normal f aulting oriented on a NW-SE plane, dipping shallowly towards the SW. The ot her group of mechanisms showed either a normal fault oriented NW-SE and dip ping steeply to the NE, or a very shallow-dipping normal Fault, dipping to the SW. Earthquake distribution and fault plane solutions suggest that the Zacoalco half-graben developed from blocks that rotate as slip occurs on li stric faults. These mechanisms could represent the type of motion expected for larger earthquakes in the area, like the one that occurred in 1568. (C) 1999 Elsevier Science Ltd. All rights reserved.