Do low-angle normal faults produce large earthquakes? A case study of the Cañada David Detachment of northern Baja California, Mexico

John M. Fletcher, Jaziel F. Cambron, Thomas K. Rockwell, Keene W. Karlsson, Paula M. Figueiredo, Ronald M. Spelz, Pierre G. Lachan, Ivan Peña Villa, Alejandro Leon Loya, Alejandro Hinojosa, Sambit Prasanajit Naik , & Lewis A. Owen

Submitted August 15, 2017, SCEC Contribution #7649, 2017 SCEC Annual Meeting Poster #142

The Cañada David detachment (CDD) controls the southern half of the Laguna Salada rift basin in northern Baja California and is a world-class example of an active low-angle normal fault (LANF). LANFs form one of the most poorly understood and controversial classes of faults because they defy many of the most basic principals of rock mechanics and the documentation of large earthquakes on them are exceedingly rare. We conducted an extensive campaign to map and trench fault scarps that cut a sequence of late Quaternary alluvial fan surfaces that are preserved along the 55 km length of the CDD. Our mapping- and trench-based paleoseismic studies reveal that all sections of the CDD have produced between two and four Holocene surface ruptures with recurrence intervals of 2-5 kyrs. Offset across principal scarps varies from 2 to 5 meters in single events and field relations permit events with up to 10m of coseismic slip. The mapping also demonstrates that the geometry of the scarp array is intimately related to the presence of megamullion corrugations of the detachment surface (DS), which have wavelengths varying form 1 to 20 km. Individual scarps change strike by up to ~60° over very short distances (100-200 m) and they also break up into en echelon arrays to follow the small scale megamullion curves of the DS. However, the scarp array is also observed to separate from the trace of the DS to follow more rectilinear splays that initiate new mountain fronts in the synformal flexures of the CDD where they form structural bridges that act to reduce the fault’s surface area making slip more mechanically favorable. In order to further examine the intimate structural relationships between the detachment fault and its coseismic scarps, we excavated a 7 m-deep by 30 m-long trench in a canyon bottom where an array of synthetic and antithetic high-angle faults, which cut historical, Holocene and late Pleistocene gravels, are observed to sole into the green chloritic gouge of the DS, which dips ~15° along the base of the trench. This trench provides unequivocal evidence that demonstrates that coseismic slip propagates to the surface along LANF’s, which occurred most recently in the 1892 or 1934 earthquakes. We conclude that despite the poor understanding of how they function mechanically, LANF’s produce a spectrum of large and moderate earthquakes at rates and frequencies that are indistinguishable from other active faults in the region.

Key Words
Low-angle normal fault, misoriented faults, earthquakes,

Citation
Fletcher, J. M., Cambron, J. F., Rockwell, T. K., Karlsson, K. W., Figueiredo, P. M., Spelz, R. M., Lachan, P. G., Peña Villa, I., Leon Loya, A., Hinojosa, A., Prasanajit Naik , S., & Owen, L. A. (2017, 08). Do low-angle normal faults produce large earthquakes? A case study of the Cañada David Detachment of northern Baja California, Mexico. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology