SEISMIC RESPONSES TO RIDGE-TRANSFORM SUBDUCTION - CHILE TRIPLE JUNCTION

The first local seismic network established in the remote region borde ring the Chile triple junction monitored earthquakes at a rate of thre e per day with magnitudes M = 0 to M = 4, far smaller than any recorde d previously from that region. Focal mechanisms were obtained by the a daptation of the relative amplitude method to local earthquakes. Epice ntral locations define a lineament corresponding to the active segment of a subducted transform fault, and events corresponding to the predi cted position of a subducted ridge show extensional mechanisms, with t he exclusion of thrusting and trench-parallel mechanisms. These data s uggest that oceanic spreading and seismic activity continue in the sub ducted ridge-transform system. Extension in the slab influences the ov erlying crust, giving rise to dextral transtension. Over the past 10 m .y., the locus of ridge subduction has migrated northward; we postulat e that the pattern through time of extension and basin development in the overlying crust has mirrored this.