Abstract

Lateral velocity variations in the Andean foreland near San Juan Province, Argentina, were detected by analyzing the station corrections computed as part of the joint location of local earthquake data. The events, recorded by a portable network (PANDA), have intermediate and shallow depths (about 100 km and less than 40 km, respectively). The JHD technique was applied to intermediate-depth events and to two subsets of shallow events that concentrate on two distinct geologic units (Precordillera and Pampean Ranges). The computed station corrections have a strong dependence on the data processed. Analysis of the different patterns of corrections shows that the corrections are consistent with the presence of large-scale lateral velocity variations bounded by major fault systems. Based on these corrections, a qualitative velocity model is proposed that has relatively high velocities between the eastern Precordillera and a westerly dipping plane on the eastern edge of Pie de Palo (one of the mountain blocks of the Pampean Ranges). Velocities are lower immediately east of this plane and between the central and eastern Precordillera. This model has important tectonic implications, because it suggests that there is a major boundary between the central and eastern Precordillera and that the Pampean basement, whose extent is unknown, underlies the eastern Precordillera. The existence of large velocity variations across the Precordillera is consistent with a westward shift of almost 10 km of the relocated intermediate-depth events and with the removal of an apparent westward dip (opposite to the direction of subduction) observed in the Wadati - Benioff zone as determined from single-event location. The analysis of the intermediate-depth events also exemplifies the deleterious effect on the JHD results introduced by the cancellation of small singular values in the computation of the station corrections.

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