Active tectonics projects

Assessment of active faulting along the Agua Blanca and San Miguel-Vallecitos faults, northern Baja California, Mexico

Background: A great deal of work has been undertaken to unravel the Mesozoic and Tertiary tectonic histories of the Baja California peninsula. While this work has helped clarify the long-term geologic history of Baja California, relatively few active tectonics studies contributing Quaternary fault slip rates and earthquake recurrence estimates have been conducted, especially compared to north of the border (Figure 1). For this reason, the southern extensions of the primary faults threatening US populations are largely excluded from fault models and seismic hazard assessments for the region, thus rendering these models incomplete. Though west of the main plate boundary in the Peninsular Ranges, the active Agua Blanca and San Miguel-Vallecitos faults accommodate ~14% of relative plate motion (Box 2, Figure 1). Sparse Quaternary slip rates have been estimated for these faults, but they predate modern quantitative geochronologic methods and are thus associated with large uncertainties. Because both of these faults threaten US and Mexican populations, and because together they represent a unique opportunity to investigate fault interaction and slip partitioning, new investigations to improve our understanding of their recent tectonic history are overdue.

Progress: This project is currently being developed. We have submitted an NSF Catalyzing New International Collaborations grant to support a new collaboration between Dr. Behr’s group at UT and researchers at CICESE. If funded, this grant will support a planning meeting in May-June 2014 where we will finalize the details of a follow-up NSF-Tectonics proposal to support an investigation of slip rates and earthquake recurrence along the Agua Blanca and San Miguel-Vallecitos faults.

Figure 1: Fault map of the southern San Andreas fault system. Box 1 shows the location of the Dever’s Hill project (Part 1). Box 2 shows the location of the Agua Blanca-San Miguel-Vallecitos study area (Part 2). Black and white circles indicate locations of published geologic slip rates and paleoseismic trenches, respectively.