We report the first geological evidence for the largest historical tsunami that has struck Chile´s Metropolitan coast. Although official inundation maps for the area were recently updated to account for the worst plausible scenario, the 1730 central Chile tsunami, our evidence shows that this event exceeded the inundation limits redefined for Quintero Bay. This bay holds major industrial and port complexes and is located just 30 km north of Valparaíso and Viña del Mar, the country’s largest coastal urban area.
At Campiche, a valley dammed by a sand spit and low dunes fringing the northern part of Quintero Bay, we studied a tabular sand sheet, interbedded in clayed mud that infilled a former lagoon. The ~1-15 cm thick sand sheet sharply punctuates the mud and extends and pinches out 2 km inland. The sand resembles that of the modern swash zone and contains a monospecific assemblage of well-preserved foraminifera (Ammonia tepida) from the nearshore environment. Locally, the sheet exhibits an erosive lower contact and contains rip-up clasts made of the underlying mud. Apparently massive when observed on the pits walls, sediment peels reveal that the sheet is in fact composed of three internal sand layers. These layers are separated by two subtle mud layers, and display primary sedimentary structures. Radiocarbon dates of plant remains from the mud below and above the sand sheet constrain the deposit´s age to AD 1482-1663 and AD 1673-1952, respectively. OSL analysis of the sand itself brackets the age to AD 1600-1710. Because the metropolitan written history is continuous since 1541, and the only large tsunami since then occurred on July 8, 1730, we ascribe the local erosion of the mud and the widespread deposition of the sand sheet to this event.
Tsunami inundation models for a suite of hypothetical ruptures show that significant shallow megathrust slip offshore of Quintero best explains the inland extent of the sand sheet. To gain more insight into the 1730 event we are conducting specific tsunami modeling aimed to explain the sand sheet’s internal layers that we interpret as particular phases of the 1730 tsunami. Preliminary, we conclude that the 1730 tsunami was larger than the current worst-case scenario officially considered for the metropolitan coast of Chile and that such a large size was likely produced by shallow slip facing the country´s most populated coast. Work funded by Fondecyt (1150321) and Millenium Nucleus CYCLO (NC160025).
