Uncovering a medieval tsunami: Multidisciplinary evidence for a ∼1300 CE tsunamigenic earthquake and coseismic subsidence in the Guerrero seismic gap, Mexico

Reconstructing long-term seismic behavior using geological records along subduction zones, where the largest tsunamigenic earthquakes occur, is key in understanding earthquake and tsunami hazard. To assess whether the Guerrero seismic gap, a segment of the Mexican Subduction Zone, can generate great megathrust earthquakes, we present original geomorphological, sediment characteristics, geochemical, microfossil, magnetic properties, and radiocarbon data, documenting a tsunamigenic earthquake at approximately 1300 CE from Boca de Mitla, Guerrero. The sand deposit displays basal erosional contact, rip-up clasts, flame-like basal structures, marine geochemical signal, and brackish-marine diatoms. Diatom assemblages show evidence of coseismic subsidence of ∼21 cm. Radiocarbon constrain the event at ∼1300 CE, accordant with a prehistoric tsunami previously identified approximately 30 km WNW of Boca de Mitla. The reconstructed long-term seismic prehistory shows that the Guerrero seismic gap can produce large tsunamigenic earthquakes (Mw > 8). The new evidence enhances our understanding of the region's seismic behavior by integrating the paleoseismic and paleotsunami evidence in the assessment of earthquake and tsunami hazard along the Mexican subduction zone, and underscores the importance of geomorphological, geological and geochronological research in this and other active convergent margins globally.