The world's largest tsunamigenic earthquakes have historically occurred along subduction zones, with devastating events exceeding magnitude 9 recorded in regions lacking comprehensive instrumental records. While the 1000-kilometer-long Mexican subduction zone lacks instrumental records of such events, historical and geological data suggest the possibility of a magnitude 8.6 tsunamigenic earthquake. The Guerrero seismic gap (GSG), a 200-kilometer section of this zone, has been relatively quiet for over a century, raising concerns about potential future seismic activity. A rupture across the entire gap could generate an earthquake of Mw 8.4, posing significant risks to the Mexico City metropolitan area and coastal communities like Acapulco. Recent hypotheses suggest that the GSG's rheology might favor slow slip over fast slip, potentially reducing the frequency of large earthquakes. However, this hypothesis is based on an incomplete assessment of historical and late Holocene tsunamigenic earthquakes. This study addresses this gap by presenting geologic evidence of past large tsunamis in the GSG, offering critical insights into the region's seismic potential and the need for a comprehensive hazard assessment that incorporates both long-term geological data and instrumental observations.

Previous research on the Guerrero seismic gap has focused on various aspects of its tectonic setting and potential for large earthquakes. Studies such as Suarez et al. (1990) examined the geometry of subduction and the depth of the seismogenic zone. More recently, Husker et al. (2018) investigated the geological factors contributing to transient slip within the seismogenic zone. Other studies have highlighted the challenges of anticipating megathrust earthquakes and tsunamis using coastal geology (Sawai et al., 2012), and have explored the geological records of similar events in other subduction zones, including Chile (Cisternas et al., 2005), Sumatra (Monecke et al., 2008), and Cascadia (Atwater et al., 1995). Ramírez-Herrera et al. (2007) provided previous evidence for late and mid-Holocene earthquakes and associated tsunamis in the GSG. Černý et al. (2020) examined seafloor morphology in the region to understand earthquake implications. The current study builds upon this existing body of work by providing more detailed geological evidence of past tsunamis and integrating it with numerical modeling to estimate the magnitude of the triggering earthquakes.

The research involved a multi-faceted approach combining field surveys, stratigraphic and sediment analysis, microfossil analysis (diatoms), magnetic property analysis (anisotropy of magnetic susceptibility or AMS), and radiometric dating (210Pb, radiocarbon, and optically stimulated luminescence or OSL). Field surveys covered approximately 55 kilometers of the Guerrero coast, focusing on low-lying coastal plains with high potential for tsunami deposit preservation. At multiple sites, stratigraphic logs and sediment cores were collected to examine sedimentary layers. Detailed analyses of grain size, diatom assemblages, and geochemistry were performed on selected cores to understand depositional environments and identify marine incursions associated with tsunami events. AMS analysis of sediment fabric was used to reconstruct flow characteristics during suspected high-energy events like tsunamis. Radiocarbon dating of organic matter and OSL dating of quartz grains provided age constraints on the identified events. Numerical modeling using the Funwave code simulated coseismic deformation and tsunami inundation based on various earthquake rupture scenarios to assess the consistency between modeled events and the observed geological record. The models incorporated detailed bathymetric and topographic data.

The analysis of sediment cores revealed evidence of at least four large tsunami events in the Guerrero seismic gap over the past 2000 years. The most significant event, associated with sand unit 3, is estimated to have occurred between 1240 and 1370 AD. This event showed clear evidence of coseismic coastal subsidence of approximately 1 meter, consistent with a large earthquake located near the trench. Other events also exhibited evidence of coastal uplift or subsidence. The diatom assemblages provided valuable information on changes in salinity levels, indicating shifts in depositional environments related to tsunami inundation and coseismic deformation. The magnetic fabric analysis (AMS) supported the high-energy nature of the identified tsunami events, with particular magnetic fabric patterns indicative of the flow directions and intensities. Age estimations based on radiocarbon, OSL, and 210Pb dating provided a chronological framework for the tsunami events. Tsunami modeling supported the geologic findings, with a best-fit scenario (T5) involving a Mw 8.6 earthquake located approximately 20 km from the coast generating a tsunami capable of inundating the study area, reaching amplitudes exceeding 10 m at the coast and inundating over 1 km inland. The modeled coastal subsidence associated with this scenario aligned with the geological observations from sand unit 3. The variable recurrence interval of these events over the past 2000 years suggests that the occurrence of large tsunamigenic earthquakes is highly variable in this region.

The findings strongly suggest a significant potential for large, tsunamigenic earthquakes in the Guerrero seismic gap, even greater than what has been recorded instrumentally in the last century. The discovered evidence of large-magnitude earthquake events exceeding magnitude 8, coupled with the numerical modeling results, counters the assumption of primarily slow-slip behavior in the GSG and highlights the region's significant seismic hazard. The recurrence of large events over the past 2000 years indicates that the long-term hazard assessment must be integrated with short-term instrumental observations. The observation of coseismic coastal subsidence associated with the ~1300 AD event, which is distinct from the commonly observed coastal uplift in recent instrumentally recorded events, indicates variable rupture modes in the GSG and necessitates a more refined understanding of earthquake processes in this region. The consistency between the geological data and the numerical models supports the reliability of the magnitude estimate for the largest inferred event. This research adds to a growing body of work emphasizing the importance of integrating long-term geological data into seismic hazard assessments for subduction zones globally.

This study provides compelling geological evidence of large tsunamigenic earthquakes in the Guerrero seismic gap, challenging existing assumptions about the region's seismic behavior. The discovery of at least four significant tsunami events over the past 2000 years, and particularly the modeled Mw > 8 event around 1300 AD, underscores the region's high seismic potential and necessitates a comprehensive re-evaluation of earthquake and tsunami hazards in the area. Further research should focus on improving the dating of some of the identified tsunami deposits and expanding the spatial extent of the investigation to better define the recurrence patterns and spatial distribution of past earthquake ruptures. The integration of geological and instrumental data is crucial for improved forecasting of future events and mitigation strategies for the affected communities.

While the study provides robust evidence for large tsunami events and their probable association with major earthquakes, some limitations exist. The dating of one event (sand unit 4) remains inconclusive, requiring further investigation to refine the age estimate. The maximum inland extent of tsunami deposits may be a conservative estimate due to the complexity of factors influencing sediment preservation and deposition. Further research could investigate whether the three minor events observed exhibit any permanent deformation that is not readily apparent. While the numerical modeling provides valuable insights, the models rely on certain assumptions and simplifications regarding earthquake rupture parameters and tsunami propagation, which may affect the accuracy of the magnitude estimations and tsunami inundation maps. The study area is focused along a particular section of the GSG; a broader regional investigation is needed for a complete understanding of the spatial variation in earthquake recurrence and tsunami hazards.