STRESS-DISTRIBUTION AND GEOMETRY OF THE SUBDUCTING NAZCA PLATE IN NORTHERN CHILE USING TELESEISMICALLY RECORDED EARTHQUAKES

The stress distribution along the subducting Nazca plate in northern Chile is analysed using focal mechanism solutions obtained from the inversion of long-period P, SV, and SH waveforms of 15 earthquakes (m(b) greater than or equal to 5.5), and from 212 events with reported focal mechanisms, which occurred between 1962 and 1993. A joint hypocentral determination was carried out to control the depth of 261 events (m(b) greater than or equal to 5.0) recorded at teleseismic distances. A change from tensional to compressional stress field along the upper part of the subucting slab is associated with the maximum depth extent of the coupled zone. This change occurs in northern Chile at similar to 200-250km from the trench, at depths of similar to 60 +/- 10 km. This depth is larger than the maximum depth observed for the thrusting interplate events (40 +/- 10 km), probably meaning that, at depths of between 40 and 60 km, large low-dip angle thrust events do not nucleate. Seismic slip, however, probably extends down to 40 km in depth. The shallow dip angle (up to 60 km in depth) of the Wadati-Benioff zone does not show variations along the strike of the trench. However, a gradual southward flattening of the slab is observed at distances greater than 200-250km from the trench. This change, observed from about 21 degrees S, could be associated with a younger and probably more buoyant lithosphere than that observed to the north of this latitude. There are two gaps located between the three main clusters of seismicity; these gaps are clearly not related to detachments ill the descending litosphere. The first cluster is located in and beneath the seismogenic interplate contact, and is characterized by reverse and thrust faulting events over a scarce tensional activity. In the second cluster, the compressional seismicity is scarce for teleseismic events and is located beneath the normal faulting events. The third cluster corresponds to tensional events. Therefore, these gaps in seismicity could be associated with alternating changes from compressional to tensional stress field in the subducting slab.