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

Because the neighbouring 2010 and 2015 tsunamis resulted from large fault slip right offshore of the affected coasts (Moreno et al., 2012; Tilmann et al., 2016), the nearly 300-year tsunami gap of Chile's metropolitan coast suggests the 1822, 1906, and 1985 main slips were then not large enough or did not occur right offshore. The latter is supported by the profusely reported coastal uplift generated by each of the last three earthquakes, implying relatively deep ruptures, beneath the coast, and lower tsunamigenic power. Lack of known historical records of coastal level changes in 1730 hinders comparison of its possible slip with that of the later events. However, the size of the 1730 ensuing tsunami, 11 m locally and up to 2 m in Japan, implies large offshore uplift and little if any vertical motions of the coast (Carvajal et al., 2017b). Allowing for large offshore slip in 1730, deeper slips in the subsequent earthquakes, a very locked fault (Saillard et al., 2017) and a plate convergence at 6.55 cm/yr (Jarrin et al., 2023), nearly 20 m of shallow, tsunamigenic slip deficit could be accumulated since 1730 offshore metropolitan coast. This hypothetical area of shallow deficit is perhaps independently evinced by the region´s megathrust microseismicity. It defines three largely aseismic, trenchward, crescent- shaped patches that might be building up stress and be in a late phase of the seismic cycle (Sippl et al., 2021). In this context, determining recurrence of the 1730- type events is crucial to assess tsunami hazard on Chile’s metropolitan coast. However, it is not possible to achieve this through written history, as it begun in Chile in 1541 and recorded the 1730 tsunami as the only major tsunami on this coast. Alternatively, the geological record left by past tsunamis could provide a proxy if a modern analogue is available (Dawson et al., 2020). The only such study for the metropolitan coast suggests large tsunamis recurred between ~200 and ~650 years, with an average interval of ~500 years, during a mid-Holocene, 2600-year time window (Dura et al., 2015). Evidence was found in a marsh fringing the southern shore of Quintero Bay, 30 km north of Valparaíso. It contains a sequence of relict peat intercepted by six clean sand sheets interpreted as deposited by large tsunamis. Sheets were radiocarbon dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP. Because the sequence did not record later events, including the 1730 tsunami, no modern analogue is available for comparison with either the mid-Holocene sequence itself or with historical inundation reports.