Stratigraphic evidence of relative sea level changes produced by megathrust earthquakes in the Jalisco subduction zone, Mexico. The signature of the 1995 Colima-Jalisco earthquake (Mw 8) as a modern analogue

Abstract

Geological evidence of megathrust earthquakes along the Mexican Pacific coast relies predominantly on tsunami deposits. Records of coseismic relative sea-level changes are scarce even though such evidence can complement and constrain tsunami records, providing information to reconstruct past earthquake rupture dimensions to improve the earthquake hazard assessment of the region. This paper provides the first diatom-based quantitative reconstruction of coastal subsidence along the Mexican subduction zone through the analysis of coastal wetland sediments. We use the stratigraphic signature of the 1995 Colima-Jalisco earthquake (Mw 8.0) as this earthquake is instrumentally well-constrained, allowing us to evaluate our findings and establish approaches to investigate earlier earthquakes. Deposits beneath the wetland Estero Potrero Grande reveal a stratigraphic sequence, < 28 cm beneath the wetland surface, that resembles a signature of rapid submergence. This sequence consists of a horizontal bed of silt with highly humified organic matter abruptly overlain by a horizontal bed of grey silt. The abrupt stratigraphic contact between the two units extends across >1 km of the wetland. Using 14C ages and 137Cs chronohorizons to build an age-depth model, we estimate the age range (1985–2003 CE) and mean age (1995 CE) of this abrupt stratigraphic contact, coincident with the year of the earthquake. Three elemental log-ratios (S/Zn, Br/Zn and Ca/Zn) indicate an abrupt increase of salinity across the stratigraphic contact, which we interpret as marsh submergence recording coseismic coastal subsidence, reflected as relative sea-level rise. Fossil diatom assemblages confirm this trend, and through quantitative approaches we estimate its magnitude (−0.06 ± 0.08 m and − 0.11 ± 0.23 m), which is comparable with coastal subsidence measured by geodetic instruments. Our records reveal a millimetric spike of sand, which we interpret as a signature of the tsunami that accompanied this great earthquake. These findings demonstrate that coastal wetland sediments from the Mexican Pacific coast can record tectonic induced relative sea-level changes. This approach can allow us to better understand the long-term spatial and temporal behavior of this subduction zone.