Long-Lived Tsunami Edge Waves and Shelf Resonance From the M8.2 Tehuantepec Earthquake

The 8 September 2017 M8.2 Tehuantepec, Mexico, earthquake ruptured an similar to 150-km-long high-angle normal fault below the subduction zone megathrust. A tsunami was generated by the event with surveyed runup as large as 3m. Tide gauges in the region show a remarkably long duration of the tsunami with oscillations within the very wide and shallow Tehuantepec shelf lasting as long as 3days. Here we produce a model of the tsunami and validate it by comparing it to the tsunami survey and to the time and frequency domain features of regional tide gauges. We analyze the model results and show that the long-lived oscillations are a result of wholesale resonance of the shelf as well as very efficient trapping of edge waves at the shore. These resonant features are the result of the Tehuantepec shelf morphology and illuminate a previously unidentified tsunami hazard for the region. Plain Language Summary The Tehuantepec earthquake generated a tsunami that, while modest in size, lasted a very long time. Specialized instruments, called tide gauges, measure the sea-level and registered tsunami waves for as long as 3days in the region. In order to understand why the tsunami lasted so long, we model it using specialized computer code and analyze the results of the model. We find that the long duration is most likely due to where the tsunami happened. It occurred inside of a very flat and shallow part of the ocean called the Tehuantepec shelf. Because of its shape, the Tehuantepec shelf traps tsunami waves and these slosh around the region for a very long time. This trapping effect had not been identified in the region before, and it is important to understand because future earthquakes in the region might generate much larger tsunamis which will have the added hazard of also lasting a very long time. Future tsunami planning efforts should take these results into account.