THE ANDEAN SUBDUCTION ZONE BETWEEN 22-DEGREES-S AND 25-DEGREES-S (NORTHERN CHILE) - PRECISE GEOMETRY AND STATE OF STRESS

One year of seismicity recorded by a local network is used to obtain m ore precision about the geometry and the stress regime of the Andean s ubduction between 22 and 25 degrees S in the northern Chile seismic ga p. A sharp image of the Wadati-Benioff Zone (WBZ) is obtained down to 270 km in depth. A seismically quasi-quiescent zone is observed in the WBZ below the volcanic are, between 150 and 210 km in depth. Hypocent res of distant intermediate depth earthquakes located with the local n etwork are compared with worldwide seismic network hypocentres in orde r to evaluate the accuracy of the WBZ image at depth greater than 100- 150 km in depth. No shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locall y at the deep root of the Atacama Fault. The stress field and the char acteristics of faulting along the subducted slab are investigated. Und erthrusting and localized reverse faulting earthquakes define the seis mically coupled plate interface from 20 to 50 km in depth (Locked zone ). Downdip, intra-slab normal faulting prevails (Tensile zone), but so me strike-slip faulting is observed. A transition between normal fault ing with variable fault azimuth and normal faulting with nearly homoge neous NNW- to NW-oriented fault plane is found at about 80 km in depth . It is found that the stress axes sigma(1) and sigma(3) in the Locked zone are oriented in the convergence direction (75-80 degrees E). Dow ndip, in the tensile zone, sigma(3) has a mean azimuth 60-65 degrees E . There, the slab is hence submitted to a tensional force (slab pull) oblique relatively to the convergence, The transition between seismic underthrusting and intraplate normal faulting downdip occurs at the de pth where the continental Moho encounters the Wadati-Benioff Zone, sug gesting that a relationship exists between seismic coupling and the pr esence of continental crust at the plate interface. The pre-seismic st ate of this segment of the Andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the Wad ati-Benioff Zone.