COSMISMIC STRATIGRAPHY IN LACUSTRINE SEQUENCES OF THE CVTM, SAN PEDRO EL ALTO CASE

La estratigraf\ueda cos\uedsmica se define como el registro sedimentario modificado por la actividad s\uedsmica ocurrida en el momento de su dep\uf3sito, sin expresi\uf3n superficial aparente. Esta actividad influye en la evoluci\uf3n de la cuenca y a su vez se ve reflejada en el paisaje. En cuencas lacustres de origen tect\uf3nico o vulcanotect\uf3nico como las del Cintur\uf3n Volc\ue1nico TransMexicano (CVTM), los efectos de cambio clim\ue1tico, tasas de sedimentaci\uf3n, influencia antropog\ue9nica, entre otros, son factores importantes en su evoluci\uf3n, no obstante, deben considerarse factores ex\uf3genos como sismicidad y vulcanismo, los cuales modifican las condiciones paleoecol\uf3gicas y paleolimnol\uf3gicas de la cuenca. Por lo anterior, es necesario llevar el estudio bajo dos puntos de vista, el estratigr\ue1fico y el s\uedsmico ya que adicionalmente a las rupturas cos\uedsmicas, las propiedades f\uedsicas de los materiales son modificadas por los efectos primarios y secundarios de un sismo. El graben de Acambay, perteneciente al CVTM y una de las zonas representativas de la actividad s\uedsmica reciente en el pa\ueds, almacena el registro sedimentario de los eventos que han sido parte de la evoluci\uf3n de las cuencas en el Cuaternario. Tal registro se corrobora mediante la deformaci\uf3n de los estratos que en algunas ocasiones y sitios espec\uedficos han desarrollado estructuras que ofrecen informaci\uf3n sobre el tipo de evento que las gener\uf3. La cuenca de San Pedro El Alto, presenta una serie de secuencias sedimentarias en donde se han registrado eventos s\uedsmicos que han influenciado su evoluci\uf3n. En este trabajo se pretende definir la evoluci\uf3n del paisaje s\uedsmico de la cuenca ligada al Graben de Acambay con base en el estudio detallado de la estratigraf\ueda y la identificaci\uf3n de paleosismos (estratigraf\ueda cos\uedsmica)

COSEISMIC STRATIGRAPHY: A USEFUL APPROACH IN THE STUDY OF ACTIVE TECTONIC BASIN SCENARIOS IN CENTRAL MEXICO

We propose the concept of coseismic stratigraphy, an approach that could be highly useful for studying lacustrine basins in zones with important tectonic activity. Extant and past lakes have been studied in many parts of the world with paleolimnological objectives, for knowing climate evolution, or for detecting anthropogenic effects. Our research team working in lakes of the Trans Mexican Volcanic Belt has noticed that many of the secondary effects of important earthquakes (>5) have been either omitted or interpreted as effects of climate and human processes. Through the analysis of stratigraphy and the examination of secondary structures generated by earthquakes or by coseismic ruptures, we propose the term coseismic stratigraphy in order to call the attention of students of sediments in basins within scenarios that have been modified by seismic events that changed the conditions of sediment deposition and sediment structure

Evidence of seismogenic activity of Perales Fault in the Ixtlahuaca Basin, Mexico

The study area is located in central Mexico, in the Trans-Mexican Volcanic Belt (TMVB) one of the areas with highest geological activity. The structural framework that controls the kinematics of the studied area is linked to the Acambay graben, characterized by structures mainly oriented E-W, where the November 19, 1912, Ms 6.9 earthquake took place. The lacustrine and fluvial sequences of Ixtlahuaca basin are currently affected by faults and uplift of Perales fault footwall, which has migrated into the basin during the Late Quaternary. The Ixtlahuaca-basin has been considered as aseismic zone, however, soft sediment deformations and paleoliquefaction features are clearly genetically related with synsedimentary faulting of the lake basin floor. The identified structures and the application of the ESI 2007 scale (Michetti et al, 2007) suggests a seismically induced origin; the deformational structures are similar and the intensity in the two areas is from VII to IX

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

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

Coseismic stratigraphy in Holocene lacustrine sequences of San Pedro El Alto, Estate of Mexico

The Mexican Volcanic Belt contains a series of coseismic registers in lacustrian sequences in which the consequence of seismic collisions due to fault movements left deformed structures. The scope of this paper is to define the geological and biological evolution of the lacustrian depression of San Pedro el Alto, immersed in the Acambay graben. The area presents morphological evidence of current coseismic ruptures such as vertical height differences, fault edges, structural drainage control and counterslopes. The natural conditions, as well as the time and space relations in which seismic events took place will be demonstrated, underpinned by a detailed study of the area\u2019s stratigraphy and fossil diatoms. Furthermore, the fluctuations in the level of the lake can provide guidelines to understand the environmental factors linked to coseismal ruptures occurred during the Miocene

ESTIMATED OF INTENSITY PALEOSEISMIC STRUCTURES UNDER ESI2007 SCALE IN THE ACAMBAY AND IXTLAHUACA GRABEN, MEXICO

La regi\uf3n central de M\ue9xico se caracteriza por sus grandes depresiones lacustres con orientaci\uf3n E-O, las cuales son controladas por fallas normales del Cuaternario. Los grabenes de Acambay e Ixtlahuaca representan unas de las cuencas intra-arco m\ue1s grandes del Cintur\uf3n Volc\ue1nico Trans-Mexicano. La evoluci\uf3n de la regi\uf3n Acambay-Ixtlahuaca es tambi\ue9n caracterizada por eventos s\uedsmicos recurrentes moderados a fuertes, como lo constat\uf3 el sismo de Acambay el 19 de noviembre de 1912, Ms 6.9. En el presente trabajo se realiza una comparaci\uf3n de datos macros\uedsmicos publicados a partir del sismo de Acambay de 1912, Ms 6.9, con nuevos an\ue1lisis de evidencia paleos\uedsmica preservada en la estratigraf\ueda y en la geomorfolog\ueda de tres sub-cuencas lacustres del Pleistoceno-Holoceno (San Pedro El Alto y San Bartolo Lanzados, en el \ue1rea epicentral del evento de 1912; e Ixtlahuaca de Ray\uf3n, a lo largo de la falla Perales, 40 km aproximadamente al S del epicentro de 1912 y en donde no se tiene registro de sismos hist\uf3ricos y por lo tanto considerada como zona as\uedsmica) bajo la aplicaci\uf3n de la escala ESI2007. Las tres sub-cuencas estudiadas son controladas por el mismo sistema extensivo de estructuras durante el Cuaternario, las tres muestran una tect\uf3nica y caracter\uedsticas geom\uf3rficas similares, incluyendo las fallas Pastores, Acambay- Tixmadej\ue9 y el sistema Temascalcingo, las cuales rompieron durante el sismo de 1912 adem\ue1s de la falla Perales. As\ued mismo, en las tres sub-cuencas, la intensidad preliminar para los efectos paleos\uedsmicos en el ambiente es comparable y consistente con la intensidad epicentral del evento de 1912 (I0 = X en la escala de Cancani). La comparaci\uf3n de intensidades ESI2007 hist\uf3ricas y paleos\uedsmicas en las subcuencas de San Pedro El Alto y San Bartolo Lanzados, muestra que la magnitud de sismos antiguos fue similar al de 1912, de tal manera, la intensidad epicentral de las estructuras paelos\uedsmicas de la cuenca de Ixtlahuaca es del orden de IX en la escala ESI2007. No obstante, lo mismo se puede afirmar para las paleosismitas lacustres analizadas en la paleocuenca de Ixtlahuaca de Ray\uf3n; las estructuras de licuefacci\uf3n, slumps, fallamiento sinsedimentario, sugieren que las estructuras tienen un origen s\uedsmicamente inducido y que es una fuente s\uedsmica local la que ha producido algunos de los paleoeventos deformacionales observados. De acuerdo a lo anterior, sugerimos que dentro de tal paisaje s\uedsmico, la intensidad ESI2007 manejada para las caracter\uedsticas paleos\uedsmicas en la paleocuenca de Ixtlahuaca indica un potencial s\uedsmico del orden de Mw 7 para la falla Perales,equivalente al demostrado por las fallas maestras del graben de Acambay

ESI2007 ASSESSMENT OF PALEOSEISMIC FEATURES IN THE ACAMBAY AND IXTLAHUACA GRABEN, MEXICO: EVIDENCE FOR CAPABILITY ALONG THE PERALES FAULT

We compare published macroseismic data from the Nov. 19th, 1912, Ms 6.9 Acambay earthquake with new analyses of paleoseismic evidence preserved in the stratigraphy and geomorphology of three Mid-Pleistocene to Holocene lacustrine subbasins (San Pedro El Alto and San Bartolo Lanzados, in the epicentral area of the 1912 event; and Ixtlahuaca de Ray\uf3n, along the Perales Fault, ca. 40 km S of the 1912 epicenter) using the ESI 2007 scale. In the Ixtlahuaca paleobasin (Mid Pleistocene?), liquefaction, megaslumping features, and synsedimentary surface faulting are genetically associated, suggesting A) a seismically induced origin, and B) a local earthquake source, at least for some of the observed deformational paleoevents. In the studied subbasins, the preliminary intensity assessment from the paleoseismic environmental effects is comparable, and consistent with the epicentral intensity of the 1912 event (Io = X in the Cancani scale). The three sub-basins are controlled by the growth of the same system of extensional structures during the Quaternary, showing very similar tectonic and geomorphic features. This includes the Pastores, Temascalcingo and Acambay-Tixmadej\ue9 faults, which ruptured during the 1912 earthquake, and the Perales Fault. We argue that within such a coherent seismic landscape the ESI 2007 intensity assessed from paleoseismic features in the Ixtlahuaca paleobasin indicates a seismic potential in the order of Mw 7 for the Perales Fault, equivalent to the one demonstrated by the master faults of the Acambay graben

PRELIMINARY GEOMORPHOLOGIC AND PALEOSEISMIC INVESTIGATIONS ALONG THE AGUA FRIA FAULT IN THE LOS AZUFRES VOLCANIC EDIFICE: AN ATTEMPT TOWARDS SEISMIC HAZARD ASSESSMENT IN CENTRAL MEXICO

The Morelia\u2013Acambay Fault System (MAFS) is a < 260 km long fault system bounded by Acambay Graben to the east and the Cuitzeo Graben to the west. Till date, most of the paleoseismic investigations were focus on the epicentral area of known crustal earthquakes, in the Acambay and Cuitzeo grabens. However there are several undocumented crustal faults between these grabens that are long enough to generate large magnitude earthquakes. In this study, we used a multidisciplinary approach (Geomorphology and Paleoseismology) to characterize the earthquake history of the Agua Fria Fault, one of the largest faults of the MAFS central segment. The fault traces and associated landforms like triangular facets, ponding and scarps were mapped using high resolution satellite imageries and topographic data. The geomorphological data indicate that the fault scarp is at least 13 km long striking E-W and is height varies from few meters to hundreds of meters. Three trenches were excavated at three different sites along the Agua Fria fault. Preliminary results indicate that various earthquakes were produced by this fault during Pleistocene