The 21 May 1960 M-w 8.1 Concepcion Earthquake: A Deep Megathrust Foreshock That Started the 1960 Central-South Chilean Seismic Sequence

One of the most notable seismic sequences in modern times was recorded in May 1960 along the southern Chilean subduction zone. The sequence started on 21 May with the M-w 8.1 Concepcion earthquake; 33 hr later the M-w 9.5 Valdivia megathrust earthquake occurred, the largest ever recorded in the instrumental period. These events changed the geomorphology of the coast along more than 1000 km, generated extensive structural damage in the main cities of central-south Chile, and triggered a Trans-Pacific tsunami. Observed land-level changes due to both earthquakes were reported in 1970. These observations were ascribed to both events but have been used to study only the general source properties of the 22 May Valdivia mainshock. Here, we separate these data to constrain for the first time the slip distribution of the 21 May Concepcion earthquake, applying a Bayesian approach that considers uncertainties in the data. Our results show that the M-w 8.1 Concepcion earthquake ruptured a deep segment of the megathrust, concentrated in a compact zone below the Arauco peninsula between depths of 20 and 50 km. Tsunami generation from this deep source agrees well with the tsunami arrival times and small amplitudes recorded by tide gauges along the Chilean coast. Our study highlights the importance of the 21 May 1960 Concepcion earthquake in the context of large historical Chilean earthquakes.