Active faulting south of the border; the other half of the big bend domain of the Pacific-North American plate margin

John M. Fletcher, Alejandro Gonzalez-Ortega, Thomas K. Rockwell, Peter O. Gold, & Michael E. Oskin

Submitted August 15, 2019, SCEC Contribution #9769, 2019 SCEC Annual Meeting Talk on Wed 0900

The central portion of the Pacific-North American plate margin known as the Big Bend domain (BBD) contains kinematically and geometrically diverse arrays of faults that accommodate the transfer of shearing from the Gulf of California to more widely distributed belts in the north including the coastal region of California, Walker Lane and Great Basin. It turns out that an international border divides this interesting domain and artificially separates faults of the SCEC natural laboratory from their close family members in northern Baja California. In this talk we will present an overview of on-going multidisciplinary studies in the BBD. The great diversity of faulting in the BBD allows researchers to evaluate the factors that affect regional stress and directions of fault slip including (1) relative slip of lithospheric plates, (2) topographic gradients and (3) the regional partitioning of transtensional and transpressional slip components. Geologic studies demonstrate that the BBD is a geodynamically active region where pronounced decelerations of slip on well established faults like the southern San Andreas are compensated by the Plio-Pleistocene birth of new faults (e.g., San Jacinto, Elsinore, San Miguel Vallecitos, Agua Blanca faults) as well as the dramatic acceleration of slip on others (e.g., Laguna Salada, CaƱada David and San Pedro Martir faults). These changes in the patterns of long-term slip rate imply important changes in boundary conditions, applied stress and lithospheric rheology throughout the BBD. Within the BBD, plate-margin shearing changes from transtension in the south to dominantly transpression in the north. Both shear regimes require three-dimensional strain, which can only be accommodated by networks of intersecting faults with diverse orientations and slip directions. Recent multifault earthquakes like the 2010 Mw7.2 Mayor Cucapah event demonstrate that the stability of interlocking fault networks and the limit of differential stress in the seismogenic crust are controlled by cross-cutting, misoriented, keystone faults that regulate slip on adjacent optimally oriented faults. All recent events including the 2019 Mw 7.1 Ridgecrest earthquake demonstrate that significant seismic risk is associated with unmapped faults that have very subtle geomorphic expression, and thus multidisciplinary studies on both sides of the international border require close coordination to discover such faults and refine existing fault models.

Key Words
big bend domain, transtension, transpression, slip rate, fault mechanics, keystone faults

Citation
Fletcher, J. M., Gonzalez-Ortega, A., Rockwell, T. K., Gold, P. O., & Oskin, M. E. (2019, 08). Active faulting south of the border; the other half of the big bend domain of the Pacific-North American plate margin. Oral Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology