Environmental Effects and ESI-07 intensity of the September 8, 2017, Mw 8.2 Tehuantepec-earthquake: geological signature of a large intraslab event

The September 8, 2017, Mw 8.2 earthquake with epicenter in the Gulf of Tehuantepec offshore, 133 km Southwest of Pijijiapan, Oaxaca, Mexico, generated considerable environmental effects in a vast region including central-southern Mexico and Guatemala. Immediately after the event, we carried out field reconnaissance surveys of the geological phenomena induced by the event. In the following months, we analyzed and interpreted additional data made available by the National Geological Survey of Mexico and other sources, and those published in the literature meanwhile. This allowed us to compile the first database of the earthquake-induced environmental effects and ESI-07 macroseismic field for an event due to slab bending and rupture. In fact, the Tehuantepec event represents an intraslab earthquake characterized by a hypocentral depth of 45.9 km, and a focal mechanism indicating a WNW-trending, subvertical, normal fault. The total area affected by secondary effects is in the order of 54.000 km2. Intensity of VII to IXESI-07 has been assigned in the Gulf of Tehuantepec region. In the epicentral area, coseismic coastal subsidence up to 0.5 m has been documented at Salina Cruz and Juchitán, Oaxaca (figure 1). Oceanic tsunami reached Puerto Chiapas (run-up of 3.4 m above tide level), Salina Cruz (1.89 m), and Huatulco (1.37 m; MNSS). Intensity of XESI-07 has been assigned to the epicentral area and IXESI-07 to this coastal area. VIIIESI-07 is assigned to Cañón del Sumidero and Tuxtla Gutiérrez city, ground effects included landslides and rockfalls up to 104 m3. In beachfront areas, cracks on alluvial soils reached 40 cm of width and 1.40 m of depth (figure 2). A value VIIESI-07 is assigned to Tuxtla Gutiérrez and Juchitán, the most notable ground effects were lateral spreading with up to 10 cm wide longitudinal fractures in paved roads and 20 cm wide in riverbanks. In Tuxtla Gutiérrez city, rock falls and landslides up to 102 m3 occurred. Lateral spreading and longitudinal and transversal fractures up to 10 cm wide are observed along paved roads in Juchitán, Oaxaca. The attenuation of ESI-07 intensity was more pronounced to the ESE than to the opposite direction, in agreement with rupture directivity deduced from instrumental data. The application of ESI-07 scale will provide data for the generation of a new intensity map and will allow its comparison with published macroseismic data based on traditional intensity scales (MM, EMS). Such map will contain reliable information according to the distribution and size of damage in the natural environment, as a scenario to take into account in future hazard and seismic risk studies in the country.