A modern analog for paleoseismic studies
As a result of ~ 1.5 m of co-seismic subsidence associated to the giant 1960 Chile earthquake, the shoreline of Pangal, a beach-ridge plain located midway along the 1960 subsided area, retreated ~400 m landward by erosion. Twenty years later, by 1979, the shoreline started to prograde seaward constructing a series of beach ridges and a wide beach. Today, the shoreline is ~150 m seaward from its pre-1960 position. Stratigraphy recorded this story through a scarp buried at the most landward reach of the retreat. The scarp is marked by a truncated 1960 soil and concentrations of heavy minerals and organic wrack. All these features are now buried by the backmost ridge constructed after 1979. Remarkably, all this evolution occurred without evident post-1960 coastal level change.
Shoreline morphological evolution of Pangal was inferred using a set of pre-and post-1960 airphotos along with more recent satellite images. The spatial analysis was made with a GIS. While topographic profiles were made using a regular level and dGPS, surface topography was carried out by photogrammetry using a drone. We linked all the elevation data to a local mean sea level estimated by measuring the water level continuously for one week with a portable acoustic tide gauge. The resulting tide record was fitted to tidal predictions from the TPXO 8-atlas tidal model.
The stratigraphy of the area that was eroded and then recovered after 1960 was obtained from hand-auger holes and shovel pits. Pits were not very useful, because being the sand saturated with water, the sand used to collapse easily. Additionally, we performed GPR profiles using an antenna with a frequency of 250 mhz.
Our results allow us to characterize the shoreline geomorphological and stratigraphic effects generated by co-seismic subsidence in 1960. Because the traces left are so obvious, including a buried scarp and a beach ridge, they provide a modern analog for similar features found inland. If so, they promise for extending the earthquake history farther back in time. This research is funded by Fondecyt N° 1150321, and has been supported by Iniciativa Científica Milenio (ICM) through grant NC160025 "Millennium Nucleus CYCLO: The Seismic Cycle along Subduction Zones".
