During the past years, significant work has been done for studying the crustal anisotropy and state of stress of the Mexican subduction zone. At the same time, there is new evidence of the geometry of the subducted slab proposing subduction tearing. Here, we present a study of the Earth crust using three different methods: azimuthal anisotropy based on ambient noise, shear-wave splitting of tectonic tremors, and moment tensor inversions of the earthquakes of 7 September 2017 Mw 8.2 Tehuantepec, Mexico. This earthquake initiated a seismic sequence that triggered shallow seismicity and aftershocks. The shallow earthquakes fall into a region where there were few published focal mechanism higher than Mw 4.5. Two slab tearings: in the Michoacán–Guerrero border and in central Oaxaca, best represent the slab geometry of the Mexican subduction zone. At the Michoacán–Guerrero, the subducted slab is subhorizontal, whereas in central Oaxaca the plate is characterized by northeast vergence. We interpret that the mantle’s flow in this part of the subducted slab produces multiple alignments in the crust and differentiates the tectonostratigraphic terranes of the southern region of Mexico.
In this article we studied the northern part of the Laramide foreland of the Chihuahua Trough. The purpose of this work is twofold; fi rst we studied whether the deformation involves or not the basement along crustal faults (thin-or thick-skinned deformation), and second, we studied the nature of the principal shortening directions in the Chihuahua Trough. In this region, style of deformation changes from motion on moderate to low angle thrust and reverse faults within the interior of the basin to basement involved reverse faulting on the adjacent platform. Shortening directions estimated from the geometry of folds and faults and inversion of fault slip data indicate that both basement involved structures and faults within the basin record a similar Laramide deformation style. Map scale relationships indicate that motion on high angle basement involved thrusts post dates low angle thrusting. This is consistent with the two sets of faults forming during a single progressive deformation with in -sequence -thrusting migrating out of the basin onto the platform.We found that the style of deformation in the Chihuahua trough is variable. In places such as the East Potrillo Mountains and Indio Mountains is typical of the thin-skinned style, associated with the Cordilleran thrust belt, while in other places, the thick -skinned deformation present is typical of the Laramide orogeny in the southern Rocky Mountains. The Franklin Mountains record the transition from thick-to thin -skinned deformation. We notice that this difference in the style of deformation is related to the thickness of the Cretaceous section within the Chihuahua trough. On the other hand, the orientation of the shortening direction can be explained based on the geometry of the trough and especially the strike of its eastern margin. Along strike variations in shortening direction and kinematics are controlled by the curved northeast margin of the trough and refl ect stress reorientation along the weak interface between the strong platform and weak basin interior. These processes were wide spread affecting the 300 km long eastern margin of the Chihuahua trough between El Paso and the Big Bend region of west Texas.Key words: Chihuahua trough, kinematic analysis, Laramide orogeny, thin-skinned deformation, thick-skinned deformation, stress inversion.
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ResumenEn este trabajo se presenta un estudio probabilístico de peligro sísmico en la parte central de la Península de Baja California, la cual comprende principalmente la región de la Reserva de la Biosfera del Vizcaíno. La zona ha sido poco estudiada debido a su baja densidad de población; sin embargo, en esta parte de la península se han registrado las mayores aceleraciones de Baja California Sur de los últimos 15 años. Se observa que en esta región las fallas transformantes son de mayor importancia para periodos de recurrencia cortos (< 1000 años), mientras que las fallas peninsulares controlan el peligro para grandes periodos de retorno (> 1000 años). Los resultados muestran que es importante conocer la localización de las fallas activas especialmente en el valle del Vizcaíno, donde los espesores sedimentarios cubren su localización. Se propone escoger periodos de recurrencia a partir de la información geológica. Además, se describe la dificultad de conciliar los modelos Gutenberg-Richter y Característicos, debido a que el modelo Característico no puede ser expresado con una distribución de Poisson para periodos típicos de los mapas de peligro sísmico en esta zona, porque en general las fallas cuaternarias tienen recurrencias de decenas de miles de años, mientras que la información contenida en los mapas de peligro sísmico se refieren a pocos cientos de años. La alternativa que se presenta es simple y se recomienda para la toma de decisiones en la construcción de estructuras esenciales (Tipo A). Esta alternativa recomienda usar como periodo de retorno del mapa aquel que corresponda al del ciclo del modelo de la falla característica más cercana al sitio. De otra forma los mapas de peligro sísmico se encuentran sub-valuados a pesar de que se apliquen factores de seguridad en el diseño.
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