The destructive September tsunamis of 1985, 2017 and 2022 on the Pacific coast of Mexico: Numerical modelling and wave directivity

We have examined three prominent near-field tsunamis that devastatingly impacted the contiguous Pacific coasts of Mexico. The 1985 tsunami was generated by a major (M_w 8.0) earthquake off the Mexican state of Michoacán on September 19 that caused serious damage and killed more than 5000 people. Tsunami waves from this event were observed at numerous sites along the coast,  including Lázaro Cardenas, Zihuatanejo, Acapulco, and Manzanillo. The normal-fault earthquake that occurred on 8 September 2017 in the Gulf of Tehuantepec (Chiapas, Mexico) was an even stronger event (M_w 8.2), resulting in tsunami waves that were measured by a large number of high-resolution tide gauges on the Pacific coasts of California, Mexico and Central America and by three open-ocean DART stations located in the offshore region. The third tsunami was produced by a thrust fault M_w 7.6 earthquake on 19 September 2022 within the coastal zone of Michoacán, Mexico, i.e., on the same date and almost at the same location as the 1985 event. This tsunami was recorded by six coastal tide gauges and by DART 43412. All three tsunamis have been thoroughly examined and numerically simulated. Both the observational data and the modelling results demonstrate that the “strength” of the tsunami waves was mostly determined by the distance from the source rather than by the specific resonant characteristics of the individual recording sites. Numerical modelling of the events closely reproduced the coastal and offshore tsunami records. Our modelling also reveals markedly different anisotropic features for the tsunami energy radiation patterns, whereby the low-frequency energy was mostly concentrated in trapped edge modes propagating along the shelf while higher frequency leaky waves radiated tsunami energy outward from the source with the main beam directed like a “searchlight” normally to the mainland coast. The “trapping coefficients” – a measure of the trapped mode contribution to the tsunami wave energy – were estimated theoretically for all events, with values ranging from 60 to 80.