Sand deposits reveal great earthquakes and tsunamis at Mexican Pacific Coast

Abstract

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 suggests 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 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 prove that the common believe that great tsunamis do not occur on the Mexican Pacific coast cannot be sustained. Worldwide, instrumentally based assessments of tsunamigenic potential of subduction zones have underestimated the magnitude and frequency of great events 1-3 , to some extent because great earthquakes and tsunamis are infrequent 1-5 and instrumental seismic data are relatively short. Historical and sediment records of large earthquakes and tsunamis have expanded the temporal data and estimated size of these events but mainly where direct observations of great tsunamis have been possible 1,3,4,6. This means that little is still known of great earthquakes and tsunami generation potential of other subduction zones 7-11. Additionally, at the centre of the problem is still the question as to whether subduction zones, despite their relatively short instrumentally seismic history, could generate great earthquakes and tsunamis. We reveal the first geologic evidence, and validate historical records, of great tsunamis and earthquakes near the trench of the Mexican subduction zone previously underestimated as potential source for great earthquakes and tsunamis. Here, we focus on the Corralero coastal plain, in southwestern Mexico, where a great earthquake, M 8.6, triggered a giant tsunami that poured over the coast of Oaxaca, Guerrero, and Chiapas, flooding 500 km along open

The Risk of Tsunamis in Mexico

“…Interestingly, a day after the main earthquake, a M7.5 aftershock hit the zone; it is thought the generated tsunami affected a local fishing community with waves ranging from 2 to 3 m high [15]. More recently, it has been found that instrumentally based assessments of "tsunamigenic" possibility of subduction zones in the Pacific coast have underestimated the frequency and magnitude of great earthquakes and tsunamis [25]. The authors argue that geological evidence shows that in fact great tsunamis (and earthquakes) have occurred in the subduction zone in the past, i.e., the stretch of the coasts of Guerrero and Oaxaca, the southern region of Mexico.…”

“…For example, it has been found evidence of two sand tsunami deposits, 1.5 km inland of the coast [25]. Further, it is believed that an earthquake of M8.6 of magnitude occurred in 1787 and produced a giant tsunami that flooded up to 6 km inland.…”

The 23 June 2020 Mw 7.4 La Crucecita, Oaxaca, Mexico Earthquake and Tsunami: A Rapid Response Field Survey during COVID-19 Crisis

“…occurred in historical time and geological evidence of the 1787 and probable predecessor in 1537 have flooded the southwest coast of Mexico (Ramírez-Herrera et al, 2020). However, because of the short instrumental record, tsunami hazard has been minimized and incorrectly evaluated on the Pacific coast of México.…”

“…predecessor of 1537 (Ramírez-Herrera et al, 2020). Instrumental data unfortunately do not capture in their short record (ca.…”

“…Even when earthquakes produced coastal uplift, as it happened during the 19 September 1985 Mw 8.1 (Bodin and Klinger, 1986) and 20 September 1985 Mw7.5 earthquakes, two tsunamis flooded the coast of Michoacan and Guerrero, Mexico (Sanchez and Farreras, 1993) leaving geologic evidence (Ramírez-Herrera et al, 2012). It is also possible that shallow events near the trench might cause coastal subsidence and large tsunamis such as the 1787 event (Ramírez-Herrera et al, 2020) and the more recent 1995 Mw 8.0 Colima-Jalisco earthquake (Pacheco et al, 1997;Hjörleifsdóttir et al, 2018). Tsunami modeling exercises may aid in estimating tsunami amplitudes, however due to the lack of detailed bathymetry and topography, local coastal morphology effects are missed in models.…”