Seismic subduction of the Nazca Ridge as shown by the 1996-97 Peru earthquakes

By rupturing more than half of the shallow subduction interface of the Nazc a Ridge, the great November 12, 1996 Peruvian earthquake contradicts the hy pothesis that oceanic ridges subduct aseismically. The mainshock's rupture has a length of about 200 km and has an average slip of about 1.4 m. Its mo ment is 1.5 x 10(28) dyne-cm and the corresponding M-w is 8.0. The mainshoc k registered three major episodes of moment release as shown by a finite fa ult inversion of teleseismically recorded broadband body waves. About 55% o f the mainshock's total moment release occurred south of the Nazca Ridge, a nd the remaining moment release occurred at the southern half of the subduc tion interface of the Nazca Ridge. The rupture south of the Nazca Ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth. Because the axis of the Nazca Ridge is at a high angle t o the plate convergence direction, the subducting Nazca Ridge has a large s outhwards component of motion, 5 cm/yr parallel to the coast. The 900-1200 m relief of the southwards sweeping Nazca Ridge is interpreted to act as a "rigid indenter," causing the greatest coupling south of the ridge's leadin g edge and leading to the large observed slip. The mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inver ts local and teleseismic data. Most aftershocks were within the outline of the Nazca Ridge. A three-month delayed aftershock cluster occurred at the n orthern part of the subducting Nazca Ridge. Aftershocks were notably lackin g at the zone of greatest moment release, to the south of the Nazca Ridge. However, a lone foreshock at the southern end of this zone, some 140 km dow nstrike of the mainshock's epicenter, implies that conditions existed for r upture into that zone. The 1996 earthquake ruptured much of the inferred so urce zone of the M-w 7.9-8.2 earthquake of 1942, although the latter was a slightly larger earthquake. The rupture zone of the 1996 earthquake is imme diately north of the seismic gap left by the great earthquakes (M-w similar to 8.8-9.1) of 1868 and 1877. The M-w 8.0 Antofagasta earthquake of 1995 o ccurred at the southern end of this great seismic gap. The M-w 8.2 deep-foc us Bolivian earthquake of 1994 occurred directly downdip of the 1868 portio n of that gap. The recent occurrence of three significant earthquakes on th e periphery of the great seismic gap of the 1868 and 1877 events, among oth er factors, may signal an increased seismic potential for that zone.