BEACH RIDGES, BURIED EROSIONAL SCARPS AND OVERHANGING SOILS EVIDENCE RECURRING PAST CO-SEISMIC SUBSIDENCE MIDWAY ALONG THE AREA OF THE GIANT 1960 CHILE EARTHQUAKE

A sandy beach-ridge plain, midway along the 1960 M9.5 Chile earthquake area, recorded a sequence of co-seismic subsidence events, and possible inter-seismic uplift, during the last millennium. Pangal plain is adjacent to Maullín and ~6 km west of Chuyaquen, where geologic evidence of seven earthquakes prior to 1960 was previously reported. The ~25 km² plain is composed of sand beach ridges ~5 m above mean sea level (amsl), intercalated by swales ~2 m amsl. The ridges consist of beach berms and low dunes formed at the back edges of former beaches. The plain progrades westward into Maullin Bay, while the ridges arc northeastward to the Maullin River and southwestward to a tidal inlet.

Air photos show that the 1960 subsidence produced rapid, strong retrogradation of the plain front facing the bay. However, by 1979 the plain started to prograde into the bay, building a tight sequence of parallel ridges and a beach, although no post-1960 land-level change is evident in the area. Conversely, the flanks of the plain have eroded as the river and inlet have widened since 1960. Erosional scarps in the banks, capped by overhanging topsoil and toppled soil blocks, resulting from caving of the basal sand, concentrate heavy minerals and organic wrack marking the high-tide swash zone.

We found a series of buried scarps, overhanging soils and toppled blocks along drainage ditches and GPR profiles across the plain. The lowest blocks, below the water table, preserve horizontal, woody roots in growth position. The scarps and soils are buried by parallel-laminated sand with abundant heavy-mineral and wrack layers that grades upward into lighter sand with progressively steeper, landward-dipping laminae, which occasionally forms a capping ridge.

We interpret the scarps, soils and ridges as produced by a series of co-seismic subsidence events, and possible subsequent emergence, based on: i) the modern analog from 1960, ii) the lateral extension of the buried features along the paleocoasts, and iii) the age match with some of the events recorded at Chuyaquen. We base this match on radiocarbon dating of the roots—likely killed by subsidence that lowered them below the water table or into the intertidal zone. Currently, we are searching for additional evidence and bracketing the ages by cross-dating root rings with regional tree-ring master chronologies.