1960 Chile: Too Soon for Its Size
The year 1960 belongs to a bygone era in earthquake and tsunami studies. It predates the recognition of plate tectonics, the installation of the world-wide standardized seismographic network system, the computer simulation of tsunami generation and propagation, and the establishment of international tsunami warning systems. Today’s still-meager understanding of the giant 1960 Chile earthquake—of its size, tectonics, and tsunami generation—accordingly took decades to achieve. This understanding, as outlined below, now includes recognition that the earthquake’s predecessors varied greatly in size, and that approximately 300 years elapsed, on average, between the largest of these earthquakes in the past 2000 years .The giant 1960 Chile earthquake culminated a series of fault displacements that began 29h earlier, with a foreshock of Mw 8.1 (Cifuentes 1989). The series also included a slow earthquake ∼15 min before the mainshock. This puzzling precursor, which may account for a belt of uplift inland from the mainshock’s coseismic subsidence (Linde & Silver 1989), had nearly as much seismic moment as did the mainshock(Kanamori & Cipar 1974, Kanamori & Anderson 1975, Cifuentes & Silver 1989).The combined seismic moment of the slow precursor and the mainshock has been estimated as 4–6 × 10^23 Nm.
The mainshock itself has a range of estimated sizes. Kanamori (1977) used 2 × 10^23 Nm as an average estimate of seismic moment; the corresponding moment magnitude of 9.5 has become the widely accepted number. However, the seismic moments estimated from free oscillations and strain seismograms span the range 1–3 × 10^23 Nm, equivalent to Mw 9.4–9.6 (Kanamori & Cipar 1974, Kanamori &Anderson 1975, Cifuentes 1989, Cifuentes & Silver 1989). This range implies average slip between about 20 and 30 m if the rupture length (estimated from aftershockdistribution) is close to 900 km and the rupture width is between about 60 and 290 km(Cifuentes 1989). However, as judged from land-level changes inferred to have accompanied the mainshock (Plafker & Savage 1970), the seismic moment is less than1 × 10^23 Nm, either with uniform slip of 17 m on a 850 km × 130 km wide fault, o rwith variable slip as great as 40 m (Barrientos & Ward 1990).
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