Determination of crustal thickness beneath Chiapas, Mexico usingSandSpwaves

Abstract

S U M M A R YWe 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, México. 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 Comitán (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 Tacaná 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.

CO2 and He degassing at El Chichón volcano, Chiapas, Mexico: gas flux, origin and relationship with local and regional tectonics

“…The Tehuantepec Ridge became strongly hydrated by the sea water infiltration 10-15 Ma BP (Manea and Manea 2008). Magma generation beneath El Chichón mainly results from a massive dehydration of the serpentinized oceanic lithosphere of the Tehuantepec Ridge at great depths (90% water loss at 180-200 km), coinciding with the slab depth beneath El Chichón (Ponce et al 1992;Rebollar et al 1999;Narcía-López et al 2004). This substantial fluid flux towards the deeper mantle wedge could cause the observed K-enrichment in the generated magma.…”

“…1a). The Tehuantepec Ridge clearly delimits two separate tectonic provinces within the subducting Cocos Plate: Northwest of the ridge, the subduction angle is nearhorizontal, while to the southeast the subduction angle increases to ∼40° (Ponce et al 1992;Narcía-López et al 2004). The slab depth beneath El Chichón is 200-220 km, with a subduction angle of 40±2° (Rebollar et al 1999;Bravo et al 2004), and the distance from the trench is 325 km, which is larger than in most arcs (Manea and Manea 2008).…”

Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite

“…The overlying crustal thickness increases from $25 km beneath the Nicaraguan VF to $46 km beneath the Guatemalan VF and with increasing distance behind the VF, for example to $46 km in the Nicaragua back-arc (e.g. Ligorria and Molina, 1997;Narcía-Lopez et al, 2004;Auger et al, 2006;MacKenzie et al, 2008).…”

Crustal thickness in Vrancea area, Romania from S to P converted waves

“…It is best observed at stations projected in the proximity of a nodal plane on the focal hemisphere, as it is also suggested by the relative amplitude ratios on synthetic seismograms obtained by the reflectivity method (Wang 1999). Such converted phases have been extensively observed in various regions of the world and used to map the lithosphere-astenosphere boundary (Sacks and Snoke 1977), the crustal thickness (Regnier et al 1994;Narcía-López et al 2004), the location of the upper slab-astenosphere interface (Nakamura et al 1998), or the geometry of the upper boundary of the plates (Ohmi and Hori 2000). In this paper, the time difference between the converted phase (sp) and the direct p wave is corrected to a first order for epicentral distance and for the event depth by using the local velocity model (LVM) routinely used in earthquake location by the Romanian National Institute for Earth Physics (NIEP) (Oncescu 1984).…”