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

418 PATA Days 2024 R E S U LT S A N D D I S C U S S I O N Our study has unearthed compelling evidence of large tsunamis and the probable local earthquakes responsible, shedding light on the potential for future similar events within this sector of the Mexican subduction zone (MSZ). Situated approximately 800 meters inland and 2 meters above sea level within an estuary, our study site boasts favourable conditions for the preservation of tsunami deposits. Through stratigraphic logs of geoslices and grain size analysis, we have identified three distinct sand units at multiple sites along the coastline, spanning approximately 55 kilometres. These sand units, particularly Sand Unit 2, exhibit unique characteristics such as marine diatoms and elevated levels of marine source geochemical elements indicative of a marine influence. Magnetic susceptibility analysis of Sand Unit 2 further supports our hypothesis of a high-energy event, likely a tsunami inundation. Radiocarbon dating of Sand Unit 2 aligns closely with historical records of earthquakes and tsunamis in the late 1950s to early 1960s, suggesting a correlation between this deposit and those events. SandUnit 3, observed at several sites, displays features such as syndepositional soft-sediment deformation and marine diatom fragments, pointing to another tsunami event. Magnetic susceptibility analysis indicates a higher energy environment for the deposition of this unit compared to Sand Unit 2. This 1300 AD event (unit 3) is likely a remarkable event that produced considerable coastal deformation (~ 1 m subsidence) and may reflect a rupture closer to the trench. Sand Units 4 and 5 further contribute to our understanding of past seismic activity and its impact on coastal environments. Sand Unit 4 suggests a marine flooding event, while Sand Unit 5 exhibits signs of apparent liquefaction, likely triggered by an earthquake preceding the 6th century AD (Fig. 2). Fig. 2: Synoptic results of microfossil analyses for site A1. Colors indicate depositional environments. The relative abundance of diatoms is given as a percent of the total count. A1 stratigraphic column to the left shows sand units 2, 3, 4 and 5. Blue belts show inferred tsunami events. (Modified from Ramirez-Herrera et al., 2024).