Velocity structure of the Andes of central Peru from locally recorded earthquakes

Arrival-times of local events recorded during two short term seismic ficd investigations in ccntral Peru are used to determine the P and S velocity structures above the slab, in a segment of the Andes where the subduction is horizontal. The velocities in the upper-crust can be related to the sedimentary cover. The Moho depth is estimated to be nbuut 50 km below the western part of the sub-Andean zone, and deepens gently to the west, reaching 60 km under the Western Cordillera. An uplift of high velocity material is observed under the coastal zone, associated with a cluster of earthquakes in and above the slab. The asymmeeical strucNre of the crust below the Eastern Cordillera reflects the convergence process in this part of the Andes: the shortening occurs along active reverse steeply west dipping faults involving the crust in its entirety on the eastern border of the cordillera, the compression then decreases toward the east.