Low‐frequency source characteristics of the great 1960 Chilean earthquake

Abstract

The low‐frequency nature of the great 1960 Chilean earthquake, the Earth's largest recorded event, is characterized, and a source model composed of three events with a combined seismic moment of 5.5 × 1023 N m and a duration of 1500 s is presented. The results show conclusively that large‐scale slip, which is only observed at low frequencies, preceded the main shock. Complex amplitude measurements of normal mode data in the band 1.0–5.0 mHz, recorded at eight vertical component stations, comprise the data set. In this band the earthquake source is assumed to be described by a line source propagating at a constant rupture velocity. The data set is the basis for a sequence of least squares inversions to determine the seismic moment, rupture velocity, spatial slip distribution, and the temporal slip function. The data are best explained by a source model that begins 1150 s before the main shock with an event of moment 1.9 × 1023 N m and rise time of 300 s, a main shock with moment of 3.2 × 1023 N m and for which most of the low‐frequency energy was released beginning about 50 s after the short‐period origin time, and an event 350 s later with moment of 0.4 × 1023 N m. The uncertainty in the moment estimate of the great 1960 Chilean earthquake is about 50% and is due primarily to the probable range of fault plane dip, which is not well constrained at this time.