Tectonic implications of the earthquake swarm of 1997 in the Michoacan Triangle, Mexico

An earthquake swarm occurred during February and March 1997 in the vicinity of the Tancitaro Volcano, in the southern part of the tectonically complex Michoacan Triangle. A study of these events provides an opportunity to map the active faults in the area and to learn if the orientation and the sense of motion on these faults are consistent with the mapped faults and the alignment of cinder cones in the region. The foci of 230 earthquakes, which could be located, are distributed between 10 and 18 km depth, and show an alignment in, roughly, a NE direction. The focal mechanisms and seismic moments of the 27 best-recorded events were determined by waveform modeling of P and S waves. These mechanisms show two distinct patterns. More than 50% of the solutions are left-lateral strike-slip mechanisms with a normal component. The preferred fault plane strikes NE. Another group of events, probably caused by triggered seismicity on the Chapala-Oaxaca fault zone, shows left-lateral strike-slip mechanisms with a large-thrust component on NW-trending faults. S wave splitting shows 1-2.5% crustal-anisotropy. The direction of the anisotropy coincides with the NE alignment of events, and the preferred nodal plane. This is also the alignment of cinder cones, suggesting that preexisting fractures and cracks are responsible for the seismicity and anisotropic behavior of the crust. The resulting stress orientation; NE compression, is the one expected for the fore-are region. We conclude that although Michoacan Triangle lies in the Trans-Mexican Volcanic Belt, it does not form part of this stress province where the stress orientation is NS extension. (C) 1999 Elsevier Science Ltd. All rights reserved.