Abstract
The 1960, and 2010 Chilean great earthquakes provide modern analogues for the sedimentary signatures of the largest megathrust events and their accompanying tsunamis. This paper presents lithological and diatom assemblage data from five sites and provides key insights for the development of longer earthquake chronologies, essential for assessing the seismic hazards associated with a subduction zone. We find that the 1960 and 2010 tsunami deposits are fragmentary, variable and have no unique, diagnostic diatom assemblage. Where rapid postseismic sedimentation occurs, our diatom-based transfer function model gives estimates of coseismic deformation that agree with independent estimates of land-level change. Sedimentary hiatuses at two sites following the 2010 earthquake suggest that the magnitude of coseismic deformation may be underestimated in fossil records. Where sediment accumulation allows, criteria for distinguishing between seismic and non-seismic stratigraphies based on evidence for the largest plate boundary earthquakes are corroborated by the lesser magnitude earthquake of 2010. The key to reconstructing earthquake characteristics, such as rupture magnitude and differences between plate-boundary and upper plate sources, depends on applying explicit stratigraphic assessment criteria at multiple sites in order to identify the spatial pattern of deformation associated with each earthquake.
Highlights
The Chilean megathrust creates great earthquakes exceeding moment magnitude (Mw) 8, including the greatest magnitude ever recorded, the 1960 Mw 9.5 rupture of the Valdivia segment, and the Mw 8.8 Maule earthquake of 27th February 2010
In this paper we aim to develop quantitative reconstructions of relative land surface deformation during the 1960 and 2010 earthquakes based on lithostratigraphy and diatom assemblages from five sites
Our transect of short cores from a tidal marsh close to the pre-earthquake river mouth shows the 2010 tsunami deposit to be of variable thickness
Summary
The Chilean megathrust creates great earthquakes exceeding moment magnitude (Mw) 8, including the greatest magnitude ever recorded, the 1960 Mw 9.5 rupture of the Valdivia segment, and the Mw 8.8 Maule earthquake of 27th February 2010. These earthquakes are characterised by intense, long duration shaking, significant land surface deformation, generation of near-field tsunamis along the Chilean coast and may spawn destructive trans-Pacific tsunamis. Differences between the historical and paleoseismic evidence may reflect variations in the size of the rupture zones of megathrust earthquakes; alternatively, interseismic land uplift may lead to low sediment accumulation or erosion of tidal marshes so the sediments record only a partial chronology of great earthquakes. Analysis of the sedimentary record of the 1960 and 2010 earthquakes provides potential modern analogues for building century to millennial scale paleoseismic records for different segments of the Chilean megathrust
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