Distributions of stresses and geometry of the Wadati-Benioff zone under Chiapas, Mexico

Abstract

Seismicity recorded from June 1994 to May 1995 on a portable network of analog and digital seismic stations defines the geometry of the Wadati-Benioff zone under Chiapas and the distributions of the principal stresses along the subducted slab. Seismicity at the outer-rise region is not well located. A Poisson’s ratio of 0.24 was calculated for the crust of Chiapas. We located 321 earthquakes in the magnitude range from 3.3 to 5.6. Hypocenters define a Wadati-Benioff zone 39±4 km thick dipping 40° ± 3° to N45° E. Earthquake depths range from 10 km to 300 km. The Cocos plate dips nearly 25° in Oaxaca, 30° to 35° under the Gulf of Tehuantepec and 40° to the Southeast. A major change in dip occurs at the intersection of the Tehuantepec Ridge with the Middle America Trench. Seismicity in this zone is diffuse to depths of 200 km. Our data suggests a gradual increase of the dip of the Cocos plate from Oaxaca to Chiapas. Focal mechanisms of events with depths less than 50 km indicate that the subducted crust is under tension in this region, while at depths between 50 and 200 km there is a mixed pattern of stress distribution. At depths from 200 to 300 km the subducted lithosphere is under down-dip compression.