In a new way to identify and date earthquakes geologically, coastal erosion and recovery in a beach-ridge plain provides evidence for great megathrust earthquakes in south-central Chile. The earthquake history obtained thus far accords with a 2.000-year reconstruction from nearby marshes and shows promise for extending farther back in time.
Pangal plain, midway along the giant 1960 Chile earthquake area, covers ~25 km2 of sand beach ridges intercalated by swales. The ridges consist of beach berms formed at the back edges of former beaches. The plain progrades westward to Maullin Bay, while the ridges arc northward to the Maullin River and southward to a tidal inlet. The plain is 6 km west of the Chuyaquen marsh where evidence for seven pre-1960 earthquakes was previously encountered.
Comparison of historical air photos shows that the ~1.5-m subsidence in 1960 was followed by ~400 m of retreat of the plain front. By 1979 the plain front started to prograde into the bay, building ridges and a wide beach. Conversely, the flanks of the plain have eroded since 1960 as the river and the inlet have widened. These erosional banks expose scarps covered by overhanging topsoil and toppled soil blocks. Sand at the base of the scarps contains conspicuous heavy minerals and organic wrack beds marking the high-tide swash.
These scarps, overhanging soils, soil blocks, and accompanying heavy minerals and wrack provide modern analogs for similar features found buried beneath the plain itself. We observed the fossil features along drainage ditches and GPR profiles. The soil blocks preserve horizontal, woody roots in growth position. Over the soils there are tabular sand sheets capped in turn by thin, poorly developed soils. These sequences are buried by massive parallel-laminated sand with abundant heavy-mineral and wrack layers that grades upward into lighter sand with progressively steeper, landward-dipping laminae that form a capping ridge. Ages of the roots resemble those of the events inferred in Chuyaquen.
We accordingly interpret the buried features as produced by co-seismic subsidence events, and the covering tabular sand sheets as deposited by the associated tsunamis. The poorly developed soils capping the sheets developed in the relatively short period between the tsunami and the erosion by the retreating coast. Subsequently, each sequence was buried by sand of the prograding coast. Work funded by Fondecyt (1150321) and Millenium Nucleus CYCLO (NC160025).
