Modern earthquake ruptures on contrasting fault systems in the Laguna Salada region of northwestern Mexico.

Ronald M. Spelz, John M. Fletcher, Thomas K. Rockwell, Alejandro Hinojosa, Michael E. Oskin, Lewis A. Owen, Jaziel F. Cambron, Víctor H. Villaverde, Laura V. Vallin, Abel A. Gutierrez, & Keene W. Karlsson

Published August 15, 2016, SCEC Contribution #6966, 2016 SCEC Annual Meeting Poster #073

The Laguna Salada region of northwestern Mexico is located near the axis of the Pacific-North American plate margin and hosts a wide variety of active faults that accommodate the three dimensional strain of transtensional shearing. Individual fault segments can be classified by their slip tendency, which is high for optimally oriented faults that form angles of ~30° from the maximum compressive stress and ranges to very low values for severely misoriented faults such as low angle normal faults. We recognize three fundamentally different types of fault systems that have contrasting mechanical properties, and we are undertaking studies to determine if they also have contrasting seismogenesis as reflected by their paleoseismic records of large earthquake production during the past ~30 kyr. In 2010 the Mw 7.2 El Mayor-Cucapah earthquake activated a complex fault system composed of individual faults that have a wide range of orientations, extend short distances along strike (<30 km), and typically end at intersections with other faults. Because each fault is linked through multiple intersections with other faults, it is likely that none of them cuts the entire seismogenic upper crust and failure of the entire network thus should depend on a misoriented fault that requires the largest differential stress (Fletcher et al., 2016, Nature Geoscience). In 1892, the Mw7.2 Laguna Salada earthquake activated the Laguna Salada fault, which controls the northern half of the Laguna Salada basin and extends at least 75 km along strike, and thus is a simple fault system that must cut the entire seismogenic upper crust. Along most of its length, the Laguna Salada fault is optimally oriented and should slip under relatively low magnitudes of applied differential stress. This contrasts markedly with the Cañada David detachment, which is the master fault that controls rifting in the southern half of the Laguna Salada basin. With a strike length of ~55 km, this fault also likely cuts the entire seismogenic crust, but its uniformly shallow dip (<20°) at the surface shows that it is severely misoriented. Therefore, it should require much higher magnitudes of differential stress in order to slip. Systematic mapping of Quaternary scarps and fan surfaces along the CDD led to the discovery of the surface rupture produced by a ML = 6.5 earthquake in 1934. Although its focal mechanism is characterized by subvertical nodal planes with pure strike slip kinematics, we can show that the event actually originated on the CDD. This discovery raises the worldwide total number of modern earthquakes on low-angle normal faults to only nine events and thus it provides an important opportunity to explain the dichotomy posed by the paucity of seismologic evidence versus the abundance of geologic evidence for large earthquakes on low angle normal faults.

In summary, simple fault systems are those that cut the entire seismogenic crust and we distinguish between the optimally oriented Laguna Salada fault from the severely misoriented Cañada David detachment. Complex fault systems, like the fault array of the Sierra Cucapah, are composed of individual faults that span a wide range of orientation-controlled slip tendency. Individual faults have multiple intersections with others and failure of the network is controlled by misoriented keystone faults (Fletcher 2016, Nature Geoscience). Each of these three contrasting fault systems has produced a modern earthquake rupture and hosts a rich record of older earthquakes during the past ~30 kyrs, which will illuminate potentially significant differences in their mechanics and seismogenesis.

Key Words
Laguna Salada, complex-fault networks, misoriented faults, low-angle normal fault, multi-fault rupture, paleoseismic record, Baja California

Citation
Spelz, R. M., Fletcher, J. M., Rockwell, T. K., Hinojosa, A., Oskin, M. E., Owen, L. A., Cambron, J. F., Villaverde, V. H., Vallin, L. V., Gutierrez, A. A., & Karlsson, K. W. (2016, 08). Modern earthquake ruptures on contrasting fault systems in the Laguna Salada region of northwestern Mexico.. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)