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 Juchit\ue1n, 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 Ca\uf1\uf3n del Sumidero and Tuxtla Guti\ue9rrez 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 Guti\ue9rrez and Juchit\ue1n, 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 Guti\ue9rrez 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 Juchit\ue1n, 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

Effects of Pleistocene to Holocene seismicity on the landforms and fluvial-lacustrine sequences of the Ixtlahuaca paleobasin, and their possible relation with the Acambay graben: Implications for the seismic hazard assessment of Central Mexico

We mapped Plio-Pleistocene lake deposits in the Ixtlahuaca paleobasin in Central Mexico, located 51 km south of the Acambay Graben and 69 km west of Mexico City. Within the Acambay basin, Pleistocene to Holocene lake sediments record primary and secondary 25 environmental effects of strong local earthquakes as a consequence of Quaternary activity of the major normal border faults. Likewise, the Ixtlahuaca paleobasin has been controlled mainly by the Quaternary evolution of the Perales and Ixtlahuaca faults, even if there is no record of local historical earthquakes, and therefore the area has not been considered as potentially seismic. However, the application of the ESI 2007 Scale to deformation structures as effects of seismic events on the geological-natural environment in the Tierras Blancas - San Bartolo Lanzados basins and San Pedro El Alto, within the Acambay graben, and its comparison with seismites identified in the Ixtlahuaca paleobasin, show that past earthquakes similar to the Mw 6.5 to 7 surface faulting events generated by the main faults of the Acambay graben have also occurred in the Ixtlahuaca area

Evidencias geol\uf3gicas, geomorfol\uf3gicas y geof\uedsicas de deformaci\uf3n asociada a la falla Cerritos y su implicaci\uf3n en el peligro s\uedsmico de Morelia, Michoac\ue1n, M\ue9xico

The Cerritos fault is located SW of Morelia city, in the state of Michoac\ue1n, M\ue9xico. The fault belongs to the western part of the Morelia-Acambay Fault System, an area with several active segmented faults that form various grabens and half-grabens. In this area, faulting affects Miocene to Holocene lithologies; some of these faults even control the distribution of regional monogenetic volcanoes. This work is an interdisciplinary approach to characterize the Cerritos fault, an important structure with a high seismic hazard potential. The Cerritos fault is a 12 km-long oblique fault (normal-left lateral) with a 130\ub15 m-high topographic relief (100 m of surface displacement + 30 m of subsurface displacement estimated from the inverted resistivity study). This fault is ENE-WSW oriented (255\ub0), with a 76\ub0 NNW dip. In a well\u2010exposed cross section of the Cerritos fault, a few meters from the master fault trace, a colluvial wedge with organic material was identified, yielding a radiocarbon age of 3.37-3.21 cal ka BP. Our geologic and geomorphological analyses indicate that the Cerritos fault is a young, tectonically-active fault, especially in its eastern part. The vertical and horizontal linearity of the fault scarp and the accumulation of lake deposits in the down-thrown block suggest active vertical motion (uplift and subsidence, respectively) along this fault. Geophysical surveys, including seismic refraction, terrestrial magnetometry, and electrical resistivity tomography, show the subsurface geometry of the fault to be characterized by a main listric fault plane and a damage zone in the footwall block, extending as far as 75 m from the main scarp. The damage zone is characterized by secondary, synthetic, and antithetic faults, forming roll-over anticlines and two crestal collapse grabens that accumulate colluvial material. Paleoseismic estimates of activity and seismic hazard potential indicate that the Cerritos fault has a slip-rate of 0.03\ub10.01 mm/yr, with mean vertical displacements of 0.5 m per event and a mean recurrence interval of 16 700 years. The Cerritos fault can generate single-segment ruptures with magnitudes of MW 6.2 to 6.6. Still, in a worst-case scenario, it could also rupture with the subparallel and adjacent Morelia and Cointzio faults, generating earthquake magnitudes up to MW 6.9

Geomorphic characterization of faults as earthquake sources in the Cuitzeo Lake basin, central Mexico

The Trans-Mexican Volcanic Belt is an active continental volcanic arc characterized by several arc-parallel Miocene-Holocene tectonic lake basins, such as the Cuitzeo, Zacapu, Chapala, and Zacoalco. Normal faults and related continental seismicity shape these basins. In the central part of this volcanic arc, the Cuitzeo Lake basin presents neotectonic fault activity, accommodating oblique extension through ENE-striking normal fault zones of the Morelia-Acambay Fault System and transfer faulting. Specific faults have been described in the basin, and various geological and structural studies have outlined its kinematics and structural geometry. However, these fault zones have not been fully characterized. Characterizing earthquake sources in the Cuitzeo Lake basin is essential to constrain the regional potential seismic hazard, aiming toward better territory planning. This study presents the first regional fault compilation in the Cuitzeo Lake basin that characterizes 21 fault zones from geomorphic data. We integrated previous volcanic and tectonic studies with new geomorphic and paleoseismic evaluations to assess potential seismic sources in the Cuitzeo Lake basin. Furthermore, we evaluate fault zone segmentation, fault slip distribution, transtensional partitioning, seismic hazard implications, and basin development. We improved the current understanding of the basin\u2019s transformation, subsidence, and sedimentation with all this information. Here, fault zones present a mean vertical slip-rate of 0.17 \ub1 0.27 mm/yr for 1 Ma, capable of generating Mw 6.2 to 7.0 earthquakes and average single-event displacements from 0.2 to 1.2 m. Multi-fault ruptures could be up to 63 km-long, capable of generating Mw 7.0 to 7.2 earthquakes, representing an important regional seismic hazard