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 sens e of motion on these faults are consistent with the mapped faults and the a lignment 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 a lignment in, roughly, a NE direction. The focal mechanisms and seismic mome nts 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% o f the solutions are left-lateral strike-slip mechanisms with a normal compo nent. The preferred fault plane strikes NE. Another group of events, probab ly caused by triggered seismicity on the Chapala-Oaxaca fault zone, shows l eft-lateral strike-slip mechanisms with a large-thrust component on NW-tren ding faults. S wave splitting shows 1-2.5% crustal-anisotropy. The directio n of the anisotropy coincides with the NE alignment of events, and the pref erred nodal plane. This is also the alignment of cinder cones, suggesting t hat preexisting fractures and cracks are responsible for the seismicity and anisotropic behavior of the crust. The resulting stress orientation; NE co mpression, is the one expected for the fore-are region. We conclude that al though Michoacan Triangle lies in the Trans-Mexican Volcanic Belt, it does not form part of this stress province where the stress orientation is NS ex tension. (C) 1999 Elsevier Science Ltd. All rights reserved.