[S08-2] Paleoseismology and paleotsunami studies: Their potential and limitation II

[S08-2-03] The Great 1787 Earthquake (M 8.6) and Tsunami along the Mexican Subduction Zone – history, geology and tsunami hazard assessment

^○Maria Teresa Ramirez Herrera¹, Marcelo Lagos², Avto Goguitchaichvili³, Maria Luisa Machain⁴, Ana-Carolina Ruiz-Fernandez⁴, Gerardo Suarez⁵, Maria Ortuno⁶, Margarita Caballero⁵ (1.Laboratorio Universitario de Geofisica Ambiental & Instituto de Geografia, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico, 2.Instituto de Geografia, Laboratorio de Tsunamis, Pontificia Universidad Catolica de Chile, Santiago, Chile, 3.LIMNA, Instituto de Geofisica, Unidad Michoacan, UNAM, Campus Morelia, Morelia, Mexico, 4.Instituto de Ciencias del Mar y Limnologia, UNAM, Ciudad Universitaria, Ciudad de Mexico, Mexico, 5.Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Ciudad de Mexico, Mexico, 6.Ciencias de la Tierra, Universidad de Barcelona, Barcelona, Spain)

invited

The 1787 great earthquake (M 8.6) triggered a deadly tsunami that poured over the coast of Oaxaca, Guerrero, and Chiapas, along more than 500 km of the Mexican Pacific coast and up to 6 km inland. This tsunami, according with historical documents, destroyed mostly farmlands and livestock, and damaged few villages since the density of population was sparse at the time.

We report first on geological evidence from the Corralero lagoon and adjacent coastal plain that seem in agreement with historical accounts. The deposit left by the 1787 tsunami can be traced along a transect of cores and test pits from the coastline and up to 1.6 km inland. The test pits showed an anomalous sand layer that was deposited in a single event in the swales of a series of beach ridges. The anomalous layer is almost continuous along the transect, about a 1000 m-long, and is formed of coarse to medium sand, at variable depths up to 36 to 64 cm, with variable thickness, reaching about 28 cm and pinching up with the distance from the coastline. We used stratigraphy, grain size, microfossils (foraminifera and diatoms), magnetic susceptibility and anisotropy of magnetic susceptibility proxies to reveal the nature of this anomalous sand layer. Stratigraphy, abrupt contacts, and magnetic susceptibility support a sudden and rapid event, consisting of sands transported most probably by an extreme sea-wave far inland. Furthermore, based on the accounts of the 1787 earthquake (M 8.6) and tsunami, and estimates from Pb210 sedimentation rates, we suggest that this is the tsunami deposit left by the 1787 event. Tsunami modeling will further enhance the hazard and risk assessment of this area in Mexico.