Paleoseismic evidence for the 1730 Mw ≥9 Chile earthquake in a former coastal lagoon: an infrequent, shallow, tsunamigenic rupture with plausible near-term recurrence

We report the first paleoseismic evidence jointly documenting coseismic subsidence and tsunami inundation from the 1730 Chile earthquake (Mw ≥ 9) and its trans-Pacific tsunami. At Campiche, a former coastal lagoon in Chile's Metropolitan Region, multiproxy stratigraphic, sedimentological, and microfossil data reveal a laterally continuous tsunami sand sheet that extends ∼2 km inland, sharply disrupts lagoonal mud, and shows an erosional lower contact, rip-up clasts, and mud drapes from waning flow. Accompanying shifts from freshwater to brackish–marine diatom assemblages and the sudden appearance of salt-tolerant plant remains record a persistent increase in tidal influence, indicating coseismic subsidence. Radiocarbon and luminescence ages constrain its deposition to 1698–1782 CE, consistent with historical accounts of the 1730 tsunami and the absence of any other comparable event in the written record. Campiche thus complements previously reported uplift-dominated mid-Holocene records by showing that infrequent, subsidence-generating shallow ruptures—not just deeper, uplift-producing earthquakes—are an integral component of central Chile's megathrust behavior. Remarkably, this paleoseismic archive, formed during a brief window within a ∼4000-year marine-to-terrestrial transition and preserved in an emergent, semiarid, preservation-limited margin, suggests that similar evidence may exist in other unfavorable settings. Integration of the Campiche record with historical, geophysical, and geodetic data indicates that a shallow slip deficit of ∼20 m may have accumulated since 1730, consistent with highly coupled shallow asperities, the recent shift from coastal stability to gradual subsidence, and proposed 200–650 yr recurrence intervals for large tsunamis. Taken together, these lines of evidence suggest that Chile's Metropolitan Region now lies within a plausible near-term window for another large tsunamigenic rupture. These findings underscore the need to integrate paleoseismic records and deep–shallow rupture interplay—including infrequent shallow Mw ≥ 9 events superimposed on more frequent Mw ∼8 deeper earthquakes—into tsunami-hazard models for Chile and the wider Pacific. • First paleoseismic evidence that the 1730 Chile quake was a shallow, tsunamigenic rupture. • Small emergent semiarid coastal lagoons can preserve tsunami and land-level signals. • Interplay of frequent deep and rare shallow ruptures promotes a large shallow slip deficit. • Campiche evidence places Chile's Metropolitan Region within a near-term window for a large tsunamigenic event.