Thesis Chapters by Gino Figueroa
Estimación de paleo-subsidencia costera asociada a los predecesores del terremoto gigante de 1960 en el centro-sur de Chile: utilización de la regla de Bruun mediante un análogo moderno
This paper presents new morpho-stratigraphic evidence of subsidence caused by the 1960 Valdivia e... more This paper presents new morpho-stratigraphic evidence of subsidence caused by the 1960 Valdivia earthquake in Pangal, a coastal strand plain located approximately 60 km southwest of the city of Puerto Montt. With a magnitude of 9.5, the 1960 earthquake, the largest instrumentally recorded earthquake in the world, subsided the Pangal area about 1.5 m and generated a tsunami that inundated up to 5 km inland. The morpho-stratigraphic evidence includes: (1) an erosional scarp produced by the receding coastline after the earthquake and during the next two decades, (2) a layer of sand between two layers of soil associated with the tsunami, and (3) the build of a new beach ridges covering the scarp and extending toward the ocean. A few meters landward of these evidences are located the maximum retreat lines of paleocoasts associated to coseismic subsidence (PMRS) of each ancient event recorded in the area. Similar morpho-stratigraphic records are also recognized in the inland part of the plain and are associated with subsidence events caused by past earthquakes. Using the evidence of 1960 event as a modern analog together with the location of the PMRS of past events, this allows us to determine the retreat values and consequent subsidence associated with earthquakes occurred during the last two millennia in the center of the rupture area of the giant earthquake of 1960. To estimate the rate of longitudinal change of the coastline, we combined aerial photographs with satellite images in a geographic information system (GIS), using a digital shoreline analysis system (DSAS). Using a differential GPS and elevation data provided by SHOA, we built a digital elevation model (DEM) and topography profiles (Swath Profile), both referenced to local sea level. For the stratigraphic description of the plain we used the geological record of pits and wells, together with ground-penetrating radar profiles. Additionally, we used a new methodology for determining paleosubsidence: Bruun's rule, a theory for estimating the magnitude of shoreline retreat in response to sea level rise (in our case, caused by coastal subsidence). This methodology was previously calibrated with the subsidence and retreat observed for the 1960 earthquake (modern analogue). The results show that the plain retreated, on average, 330 m after the 1960 earthquake, between ~1961 and ~1980. Subsequently, the coastline motion changed direction and commenced to prograde, a process that continues to today. Actually, the coast today is even more seaward than before the earthquake. During the progradation, three distinct beach ridges were formed. The one located farther inland, marking the 1960 PMRS, buried an erosional scarp that cut the stratigraphic sequence, consisting of soil from 1960, an intermediate sand layer interpreted as the 1960 tsunami deposit, and thin soil developed in the time between the earthquake and maximum coastal retreat. By linking this morphostratigraphic evidence with the 1.5 m of subsidence generated by 1960 earthquake in Pangal, we used the PMRS of past earthquakes to estimate paleosubsidence. Compared to 1960, the ~1300 and 1575 AD earthquakes generated smaller subsidence, and the other two, in ~800 and ~1100 AD, generated larger subsidence. The paleoseismological approach applied in this work seems promising as a new approach for other subduction zones.
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Thesis Chapters by Gino Figueroa