Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

clasts. The sand of the sheet resembles that of the bay´s modern beach in texture, magnetic susceptibility, minerals, and grain size. Peelings of the sheet reveal a sequence of inner layers and primary sedimentary structures exhibiting a sequence composed by three sandy subunits separated by two very thin, delicate, continuous mud horizons. We interpreted this interbedded sand sheet, its subunits, and the associated erosion features as produced by a large tsunami composed by at least three waves able to reach the former lagoon and two intervening phases of stagnation. Diatom assemblages preserved locally in the sediment changed after the tsunami. Freshwater diatoms were abruptly replaced by brackish water diatoms. Similarly, benthic diatoms were replaced by planktonic diatoms, which live in the water column. Both changes, coupled with lithologic variations, suggest the lagoon level rose after the tsunami and remained there for decades, submerging previously emerged shorelines. Radiocarbon dates of plant remains from the mud immediately below and above the sand sheet and luminescence dating of the sand itself constrains its deposition age to historical times. Because the only large tsunami reported in historical times was that of 1730, we ascribe the widespread sand sheet and the evidence of strong erosion to this event. To produce this evidence, the 1730 tsunami had overrun both the landward inundation limit of the official tsunami inundation map and the area where large industrial and harbor infrastructure is founded today. The resultsofournumeric experiments leave intact thehypothesis of a large amount of slip offshore the coast of metropolitan Chile in 1730. A systematic test of tsunami inundation, predicted from tens of rupture scenarios, show that to reach the inland extent of both the sand deposit at Campiche, and the inundation reported in Valparaíso a rupture with slip larger than 10 m right offshore metropolitan Chile is required. A side-effect of such a rupture may have been the coastal coseismic subsidence by a few decimeters. If these effects occurred in 1730, the next metropolitan Chile’s earthquake could set out a troublesome tsunami scenario for the country’s most populated coast. A C K N O W L E D G E M E N T S This research is funded by Fondecyt projects Nº1240681 and N°1231735.