Diatom-based reconstructions of megathrust earthquake deformation and tsunami inundation in central and southcentral Chile.

摘要

The timespan of instrumental and historical records of earthquakes and tsunamis limit our understanding of long-term subduction zone behavior. Coastal stratigraphy, incorporating diatom sea-level indicators, has provided some of the most detailed geological reconstructions of earthquake induced land-level change and tsunami inundation, often over multiple seismic cycles.;In central Chile, I extended the seismic history through a study of a lowland stratigraphic sequence at Quintero (32.5°S). I documented six laterally continuous sand beds dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP, probably deposited by high tsunamis. Sediment properties and diatom assemblages of the sand beds---anomalous marine planktonic diatoms and upward fining of silt-sized diatom valves and sediments---point to a marine sediment source and high-energy deposition. I inferred coseismic uplift concurrent with the deposition of the sand beds based on an increase in freshwater siliceous microfossils in units overlying the beds. Our record indicates the recurrence interval of large tsunamigenic earthquakes in central Chile is ~500 years, implying that the frequency of historical earthquakes (~80 year recurrence) in central Chile is not representative of the greatest earthquakes the subduction zone can produce. My study sites in south-central Chile are located in the overlap of the 1960 (Mw 9.5) Valdivia segment and the 2010 (Mw 8.8) Maule segment ruptures. My paleoseismic investigations from the Tirua (38.3° S) and Quidico (38.1°S) rivers were consistent with eyewitness accounts of tsunami inundation (AD 2010 and 1960) and historical accounts of coseismic land-level change (AD 2010, 1960, 1835, 1751, and 1575). The vertical deformation inferred from diatom analysis suggests that Maule segment earthquakes result in uplift at our sites (e.g., 2010, 1835, and 1751), and Valdivia segment earthquakes result in no deformation (e.g., 1960) or subsidence (e.g., 1575). I identified four prehistoric tsunami deposits dated to AD 1457-1575, 1443-1547, 256-461, and 176-336. Diatoms indicate uplift coincident with sand deposition in AD 1457-1575 and AD 256-461, which we attribute to the Maule segment. Subsidence coincident with sand deposition in AD 1443-1547, and no change in elevation in AD 176-336, is attributed to the Valdivia segment.