Abstract

The aftershocks of the 3 October 1974, Mw 8.1 earthquake have been relocated using data from a 10-station temporary seismic network. These aftershocks are among the best-observed of great circum-Pacific subduction earthquakes. Recent modelings of the rupture history of the mainshock, done in other studies, indicate a bilateral rupture on the thrust interface updip to the Peru-Chile Trench. Preseismicity was predominately downdip of the mainshock rupture, consistent with the mainshock's asperities being loaded by sinking of the Nazca plate. The aftershocks, including an Ms 7.1 event, are located primarily near the downdip periphery of the mainshock's shallow-dipping rupture surface. For a concentration of aftershocks furthest downdip, a composite focal mechanism shows east-trending, right-lateral, strike-slip displacement, corresponding to the dominant moment release of the mainshock being north of the strike-slip deformation. At a depth of 20 to 25 km, the 11° NE-dipping Nazca plate increases in dip to 30° NE. Although a postmainshock episode of plate motion must have bridged both this hinge and the bend at the Peru-Chile Trench, the associated bending moments were aseismic. The Lima Basin is directly over the downdip edge of the shallow-dipping, strongly coupled interface and the aftershocks there, similar in location to basins in outer rises of other regions. The space-time history of the aftershocks shows an alternation between heightened activity in the updip-of-hinge and downdip-of-hinge zones, with many earthquakes near the pending rupture of the 9 November 1974, Ms 7.1 aftershock, and a regional seismic quiescence following this large aftershock.

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