We have estimated the P-wave velocity structure under the Northwestern South America Andean region (NWSAA) by
applying a tomographic method to 5671 P-wave arrival times from 499 local events recorded by sixteen permanent seismic stations of the Seismological Network of Colombia (SNC) from December 1992 to June 1999. Seven layers at depths of 5.0 km, 15.0 km, 25.0 km, 40.0 km, 60.0 km, 80.0 km and 100.0 km were postulated to obtain the Vp structure along the plate boundary between the Nazca, South America and Caribbean plates. The Vp structure suggests the existence of low velocity zones beneath volcanic regions. A high velocity area at 77°W – 4°N interrupting the low velocity zones (partially melted rocks) seems to arrive from a shallow anticline related to the upper mantle.
This area of velocity contrasts marks the boundary between the more steeply dipping subduction to the north and shallower dipping subduction to the south. The Caribbean plate and the Chocó block join in the north and overlap the Nazca plate.