Determination of crustal thickness beneath Chiapas, Mexico usingSandSpwaves

Abstract

SUMMARY We used arrival time differences between direct S waves and converted Sp waves to determine the crustal thickness and the mean shear wave velocities of the crust and upper mantle beneath Chiapas, Mexico. For this purpose, we used four seismological stations that operated in the region from 1994 to 1998. The data set consists of 35 digital records from moderate magnitude earthquakes (3.6 < Mb < 5.3) with focal depths between 50 and 300 km. To identify the seismic phases more accurately, the records were rotated in the longitudinal and vertical directions of motion. The results show differences, in both thickness of the crust and upper mantle velocity, between stations located near the trench and those located inland. Beneath Tapachula (Ta) and Arriaga (Ar), the nearest stations to the Middle American trench, we found the crustal thickness to be 17 km and 28 km, respectively. Under Tuxtla (Tu) and Comitan (Co), the thickness is 49 km and 53 km, respectively. Similarly, we estimated that the upper mantle S-wave velocity under Ar and Ta is 4.1 km s−1, while under Tu and Co, 5.7 km s−1 and 5.6 km s−1, respectively. The crustal S-wave velocity also varies from zone to zone: under Tu and Co it is 3.5 km s−1 and 3.7 km s−1, respectively, and beneath Ar it is 3.8 km s−1. It is likely that the presence of the Chiapas granitic massif may influence the velocity estimates. Under Ta, located near the Tacana volcano and close to North American and Caribbean plate border, a crustal velocity of 3.3 km s−1 was calculated. In general, the thickness of the crust in Chiapas increases towards the northeast. This increment is probably related to the subduction zone process, since we observe similarities between the depth of the Moho and the geometry of the Benioff–Wadati zone that had been made for the Cocos plate.