60 years after the 1960 Central-South Chilean earthquakes: what have we learned and what we still need to understand

Abstract:

One of the most notable seismic sequences in modern times was recorded during May 1960 along the southern Chilean subduction zone. Unfortunately, the regional seismic records were saturated and few stations across the world were capable to register this sequence to analyzed the surface waves and free oscillations normal modes. The sequence started on 21 May with the Mw 8.1 Concepción earthquake; and only 33 h after the Mw 9.5 Valdivia megathrust earthquake occurred. These great subduction earthquakes along the Chilean margin were capable to reproduce a significant coseismic land-level changes along more than 1000 km. The regional-scale deformation was compiled and reported by Plafker and Savage (1970) and includes inland deformation obtained from geodetic surveys. These observations were ascribed to both events but have been used to study only the source properties of the 22 May Valdivia mainshock. Here, we reinterpret the coastal vertical displacements related to both events to constrain the slip distribution of the 21 May Concepción earthquake, by using a Bayesian inversion approach that considers the uncertainties in the reported data. Our results show that the seismic sequence started with a Mw 8.1 earthquake that ruptured deep into the megathrust, concentrated mainly in a compact zone about 20 and 50 km depth. Such a deep source is consistent with the tsunami arrival times and small amplitudes recorded by five tide gauges along the Chilean coast. During the 33 h between the main deep precursor Mw 8.1 and the Mw 9.5 mainshock, at least 8 events M > 5.8 were reported which the largest one of magnitude Ms 7.8, and a long-period nucleation phase of large-scale slip began, which likely triggered the Mw 9.5 Valdivia mainshock. Recent studies proposed that the mainshock event could have included an important strike-slip component, presumably associated with the Liquiñe-Ofqui Fault Zone, a ~1000 km-long right-lateral shear system along the continental lithosphere who accommodate the crustal deformation in southern Chile. Undoubtedly, the greatest earthquake recorded in our instrumental era still has many unsolved mysteries due to the lack of data. However, in order to better understand what happened in the past, it is important to re-study these events with the new hypotheses that we have been building in recent times.