Abstract
The Chilean tsunami of 22 May 1960 reamed out a breach and built up a fan as it flowed across a sparsely inhabited beach-ridge plain near Maullín, midway along the length of the tsunami source. Eyewitnesses to the flooding, interviewed mainly in 1988 and 1989, identified levels that the tsunami had reached on high ground, trees, and buildings. The maximum levels fell, from about 10 m to 2 m, between the mouth of the tidal Río Maullín and an inundation limit nearly 5 km inland across the plain. Along this profile at Caulle, where the maximum flow depth was a few meters deep, airphotos taken in 1961 show breaches across a road on a sandy beach ridge. Inland from one of these breaches is a fan with branched distributaries. Today its breach holds a pond that has been changing into a marsh. The 1960 fan deposits, as much as 60 cm thick, are traceable inland for 120 m from the breach. They rest on a pasture soil above two additional sand bodies, each atop its own buried soil. The earlier of the pre-1960 sand bodies probably dates to AD 1270-1400, in which case its age is not statistically different from that of a sand sheet previously dated elsewhere near Maullín. The breach likely originated then and has been freshened twice. Evidence that the breach was freshened in 1960 includes a near-basal interval of cobble-size clasts of sediment and soil, most of them probably derived from the organic fill of pre-1960 breach. The cobbly interval is overlain by sand with ripple-drift laminae that record landward flow. The fan of another breach near Maullín, at Chanhué, also provides stratigraphic evidence for recurrent tsunamis, though not necessarily for the repeated use of the breach. These findings were anticipated a half century ago by description of paired breaches and fans that the 1960 Chilean tsunami produced in Japan. Breaches and their fans may provide lasting evidence for tsunami inundation of beach-ridge plains. The breaches might be detectable by remote sensing, and the thickness of the fan deposits might help them outlast an ordinary tsunami sand sheet.
Highlights
This paper offers two kinds of findings about tsunami history
Among descriptions of the waves, high-water marks, and losses of life from the Pacific Ocean tsunami that originated during the giant Chile earthquake of 22 May 1960, most refer to distant effects in Hawaii (Eaton et al, 1961; Dudley and Lee, 1998) and Japan (Japan Meteorological Agency, 1961; The Committee for Field Investigation of the Chilean Tsunami of 1960, 1961)
Fan deposits nearly a meter thick in the Chilean near field were later reported from Mehuín (Fig. 2b), where they formed behind a seaside beach ridge that the 1960 tsunami breached (Bourgeois and Reinhart, 1989)
Summary
This paper offers two kinds of findings about tsunami history. It adds to previous Chilean descriptions of the 1960 tsunami. The paper offers geological findings that may have international application in tsunami research and preparedness. A tsunami flows through a sandy beach ridge, either for the first time or by reaming a breach that an earlier tsunami created. The landward-directed outflow from the breach builds a sandy fan as it spreads out on a plain, or it builds a delta if it runs into water already standing there. The combination of breach and fan may endure for centuries as a geological forewarning of the area’s catastrophic tsunami. We collected eyewitness accounts and examined breaches and fans near the town of Maullín, southcentral Chile. This study area includes the estuary of the Río Maullín and beach-ridge plains that adjoin it (Fig. 2)
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