Two major near-field tsunamis (2017 and 2022) on the coast of Mexico: Observations, spectral properties and numerical modelling

Two prominent near-field tsunamis impacted the nearby coasts of Mexico. The first tsunami was   generated by a major (M_w 8.2) intraplate normal-fault earthquake on 8 September 2017 in the Gulf of Tehuantepec (Chiapas, Mexico). Tsunami waves from this event were measured by a large number of high-resolution coastal tide gauges located along the coasts of California, Mexico and Central America, by three open-ocean DART stations anchored offshore from the affected region and by several distant DARTs. The second tsunami was produced by a thrust fault M_w 7.6 earthquake on 19 September 2022 within the coastal zone of Michoacán, Mexico. The 2022 tsunami was recorded by six coastal tide gauges and a single offshore DART station. All seven instruments were located within 250 km of the source. No tsunami was detected at larger distances along the coasts of North and Central America, but the tsunami signal was detected at the Hawaii and Samoa islands. All available coastal and open-ocean data were used for comprehensive analyses of these two events. Maximum trough-to-crest wave heights for the 2017 tsunami were recorded at Puerto Chiapas (351 cm), Salina Cruz (209 cm), Acapulco (160 cm) and Huatulco (137 cm), while for the 2022 tsunami they were observed at Manzanillo (172 cm) and Zihuatanejo (102 cm). For both events, the “strengths” of the recorded tsunami waves were mostly determined by distance from the source rather than by the specific resonant characteristics of individual sites. Estimates of the frequency content (“colour”) of the two tsunami events revealed that the 2017 tsunami was mostly long-period (“reddish”), with 87% of the total tsunami energy at periods >35 min, while the 2022 tsunami was short-period (“bluish”) with 91% of energy at periods <35 min. A noteworthy feature of the 2022 event was the seismically generated 7 min period seiche observed at Puerto Vallarta that began immediately after the main earthquake shock and persisted for about one hour. Numerical modelling of the events closely reproduced the coastal and offshore tsunami records and demonstrated the markedly different character of the tsunami energy radiation patterns: the 2017 tsunami spread energy widely in a semicircular pattern emanating from the source whereas  the main beam of offshore energy radiating outward from the 2022 event was directed like a “searchlight” oriented normally to the mainland coast.