SZ4D International Webinars: Unearthing geological and historical evidence of large earthquakes and tsunamis on the Mexican Subduction – results, problems and future studies
María-Teresa Ramírez-Herrera - Full time Professor, Ph.D. Geomorphology - Paleoseismology, Earth Scientist - Active Tectonics & Tsunami Geology-Paleoseismology, Tsunami and Paleoseismology Lab Instituto de Geografía UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO
September 25th, 2020 at 9 AM PDT
Globally, instrumentally based assessments of tsunamigenic potential of subduction zones have underestimated the magnitude and frequency of great events because of their short time record. Historical and sediment records of large earthquakes and tsunamis have expanded the temporal data and estimated size of these events. Instrumental records suggest that the Mexican Subduction earthquakes produce relatively small tsunamis, however historical records and now geologic evidence suggest that great earthquakes and tsunamis have whipped the Pacific coast of Mexico in the past. The sediment marks of centuries-old-tsunamis validate historical records and indicate that large tsunamigenic earthquakes have shaken the Guerrero-Oaxaca region in southern Mexico and had an impact on a bigger stretch of the coast than previously suspected. We present the first geologic evidence of great tsunamis near the trench of a subduction zone previously underestimated as a potential source for great earthquakes and tsunamis. Two sandy tsunami deposits extend over 1.5 km inland of the coast. The youngest tsunami deposit is associated with the 1787 great earthquake, M 8.6, producing a giant tsunami that poured over the coast flooding 500 km alongshore the Mexican Pacific coast and up to 6 km inland. The oldest event from a less historically documented event
occurred in 1537. The 1787 earthquake, and tsunami and a probable predecessor in 1537, suggest a plausible recurrence interval of 250 years. We have also reported on geologic evidence of other historical and prehistorical earthquakes and their tsunamis, and of coastal coseismic deformation on the Oaxaca, Guerrero and Jalisco coast of the Mexican subduction zone. Furthermore, our observations based on a combination of geologic and historic evidence, together with modelling results, imply that this subduction zone is also subject to long-fault ruptures near the trench capable of producing large tsunamis. Indeed subduction zones might have variable rupture modes, long- and short-fault ruptures, along the Middle American trench. We show that the common belief that great earthquakes and tsunamis do not occur on the Mexican Pacific coast cannot be sustained.
Paleoseismology and tsunami deposits studies have provided new insights into the understanding of the Mexican and other subduction zones, and still remain several unresolved questions and challenges. We will summarize questions, challenges, and strategies that could help solve some of them.
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