Tectonic implications of the microearthquake seismicity and fault plane solutions in southern Peru

Abstract

Using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern Peru, we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions. A series of tests evaluating the effects of elevation corrections, erroneous velocity structure, and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km. At shallow depths (<60 km) the seismicity is concentrated in three belts: one offshore, one beneath the coast, and one beneath the Western Cordillera. There is also scattered but infrequent shallow activity near the Altiplano. The belts offshore and beneath the coast are apparently associated with slip along the Nazca‐South American plate boundary. Although the style of deformation is complicated, one can infer that beneath the coast the active boundary between the Nazca and South American plates is within the mantle rather than at the base of the South American crust. The belt beneath the Western Cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km. Fault plane solutions suggest that crustal shortening is taking place in a northwest‐southeast direction in this region. The significance of this activity is difficult to explain, but these earthquakes provide no clear evidence of “tectonic erosion” in this area. A composite fault plane solution from the few events near the Altiplano shows normal faulting, which is typical of the active tectonics of that region. The results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern Peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30°. They also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion. Moreover, the volcanic line ends at the contortion, which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism. Fault plane solutions for events between 60‐and 100‐km depth show a large scatter, but some suggest a detachment of a sliver of the downgoing slab. Solutions for deeper events are more consistent with one another with the T axes for most events being nearly horizontal and generally normal to the projections of the seismic zone.