EVIDENCE FOR MULTIPLE GREAT EARTHQUAKES IN SOUTH CENTRAL CHILE

Strain is released along the Chilean megathrust by great earthquakes, including the greatest magnitude ever recorded, the 1960 M_w 9.5 rupture of the Valdivia segment. These earthquakes are characterised by intense, long duration shaking, significant land surface deformation and typically spawn trans-Pacific tsunamis. Records kept by Spanish settlers and visiting Europeans indicate four megathrust earthquakes ruptured all or part of the Valdivia segment over the last 500 years, but paleoseismic evidence records only some of these (Cisternas et al., 2005). We use sedimentary and microfossil evidence from tidal marshes in northern Chiloé to reconstruct multiple earthquake deformation cycles in the centre of the 1960 rupture zone. We interpret spatially consistent changes from organic-rich salt marsh peat to minerogenic units as evidence for repeated coseismic land subsidence. Microfossil analyses support this interpretation and our investigations of modern diatom distributions lead to quantitative assessments of coseismic land-level change. We estimate subsidence of 0.5 to 1.0 m associated with each of the last four recorded earthquakes. A detailed radiocarbon framework highlights that our record does not include the poorly understood 1737 and 1837 earthquakes, but rather features the ruptures of 1960, 1575 and two earthquakes preceding the start of Chilean written records.

Cisternas M., Atwater B. F., Torrejon F., Sawai Y., Machuca G., Lagos M., Eipert A., Youlton C., Salgado I., Kamataki T., Shishikura M., Rajendran C. P., Malik J. K., Rizal Y., and Husni M. 2005 Predecessors of the giant 1960 Chile earthquake. Nature 437, 404.