Geometry and state of stress of the Wadati‐Benioff zone in the Gulf of Tehuantepec, Mexico

Abstract

Data recorded in the Gulf of Tehuantepec and published moment tensor solutions were used to infer the geometry of the Wadati‐Benioff zone (W‐B zone) and the stress distribution of the subducted Cocos Plate. From northwest to southeast the subducted slab gently increases its dip, bends up at depths of the order of 100 km and about 240 km inland from the trench, then unbends and increases its dip toward the southeast below Chiapas. Contour lines of equal depth at 50 and 100 km show a significant contortion of the W‐B zone west of the Isthmus of Tehuantepec and coincide with significant changes in the topographic features of the crust. We calculated a Vp/Vs ratio of 1.75 for the whole area and a Moho depth in the Gulf of Tehuantepec of 28.5 ± 3.5 km. Near the trench, along the coupling zone, T axes of interplate and intraplate earthquakes are oblique to the interface and parallel to the dip direction of the subducted plate, respectively. At depths greater than 50 km the T axes are roughly horizontal. Best fit stresses using the Gephart and Forsyth [1984] code resulted in the following variation of the minimum stresses (T axes) from northwest to southeast as (plunge, azimuth) σ3(40°, 37°) to σ3(28°, 72°), then to σ3(11°, 54°) and finally to σ3(27°, 35°). We also analyzed recordings from the 30 September 1999, Oaxaca earthquake Mw = 7.5 and its aftershocks. We inferred an approximate fault area of 80 by 30 km inside the subducted Cocos Plate.