Building a Geologic Record of Earthquakes and Tsunamis of the Guerrero Seismic Gap, Mexican Subduction

Date: 4/25/2019

Time: 11:15 AM

Room: Cascade II

The Guerrero seismic gap (GSG) is located along the Mexican subduction zone (MSZ). No large subduction thrust earthquakes have occurred in the NW part of the gap since 1911. Historical M>7 earthquakes occurred in A.D. 1899. If the gap were to rupture in a single earthquake, it would give rise to an event of magnitude Mw of 8.1 - 8.4. Because of the seismic hazard to large cities such as Acapulco and to Mexico City (>20 million people), this region has been highly instrumented onshore and now offshore relatively to other segments of the MSZ.

GPS evidence suggests that long-term slow slip events (SSEs) occur every 4 yr, invading the seismogenic zone within the GSG, with equivalent Mw up to 7.6, the largest measured in the world. Understanding this phenomenology, which seems to be present in different subduction zones, results of critical importance to produce reliable hazard assessments for future earthquakes and tsunamis. Achieving this in the GSG requires identifying whether or not large earthquakes and accompanying tsunamis have occurred in pre- and historic time, their chronology, and eventually to determine their recurrence.

Previous paleoseismology studies in the GSG documented geologic evidence of coseismic deformation and tsunamis extending back to 3400 – 3500 BP. We aim to extend this record to better constrain the timing of great earthquakes and tsunamis in the GSG. We collected samples (1,800 samples) from five geoslices (2.5 m long each) along coast-perpendicular transect at 2 different sites in the coastal area of GSG. To evaluate possible stratigraphic evidence of coseismic land-level changes and tsunamis, we employed sedimentological, microfossil, geochemical, magnetic properties, and radiometric dating – C14 and Pb210 analyses. Sharp basal contacts between stratigraphic units with concurrent anomalous sand beds and diatom evidence indicative of environmental change and marine inundation suggest the occurrence of three additional(?) potential earthquakes and tsunamis in the GSG segment. Ongoing dating would confirm the precise timing of these events.

Presenting Author: María-Teresa Ramírez-Herrera

Authors