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 C hile is analysed using focal mechanism solutions obtained from the inv ersion 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 c hange from tensional to compressional stress field along the upper par t 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 inte rplate events (40 +/- 10 km), probably meaning that, at depths of betw een 40 and 60 km, large low-dip angle thrust events do not nucleate. S eismic slip, however, probably extends down to 40 km in depth. The sha llow 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 tha n 200-250km from the trench. This change, observed from about 21 degre es S, could be associated with a younger and probably more buoyant lit hosphere than that observed to the north of this latitude. There are t wo gaps located between the three main clusters of seismicity; these g aps are clearly not related to detachments ill the descending litosphe re. The first cluster is located in and beneath the seismogenic interp late contact, and is characterized by reverse and thrust faulting even ts over a scarce tensional activity. In the second cluster, the compre ssional seismicity is scarce for teleseismic events and is located ben eath the normal faulting events. The third cluster corresponds to tens ional events. Therefore, these gaps in seismicity could be associated with alternating changes from compressional to tensional stress field in the subducting slab.