US-Mexico Planning Visit: Collaborative Study of Active Faulting in Baja California, Mexico

University Of Texas At Austin, Austin TX

Investigators

Abstract

US-Mexico Planning Visit: Collaborative Study of Active Faulting in Baja California, Mexico Project Summary NSF-CNIC funds will support the initiation of a collaboration between geoscientists at the University of Texas, Austin, and the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE). The long-term research focus of this collaboration will be working toward a better understanding of rates of active faulting and seismic hazard for faults in northern Baja California, which are part of the southern San Andreas Fault system. The primary goal of the initial planning meeting will be to bring together an interdisciplinary team of scientists that will work towards addressing these research goals by first (1) examine existing datasets to highlight critical scientific gaps, (2) compile existing Spanish- and English-language published and unpublished literature, (3) determine how to combine resources to answer key scientific questions and develop new research methods, (4) conduct exploratory field work, and (5) design strategies for disseminating what we learn to local. From a hazard perpective, the collaboration will be focused on the city of Ensenada since it sits above and just north of the Agua Blanca Fault, the largest of two faults in northern Baja California that transfer displacement away from the Pacific-North American tectonic plate boundary that lies to the east. Despite having been recognized as an important structure over half a century ago, because of its location in northwestern Mexico on the periphery of San Andreas system it has garnered little attention. However, recent faulting nearby has served as a reminder that this part of the plate boundary system is too poorly understood given its role in plate boundary deformation. Earthquakes on the Agua Blanca Fault are a primary concern, but the first type of measurements we plan to collect will also help to illuminate its role in transferring slip off shore to other active faults and will lend insight into the proportion of tectonic plate motion that is expressed as earthquake-generating brittle faulting versus more benign diffuse folding. In greater detail, the focus of this project will be to measure new geologic slip rates for the Agua Blanca Fault and by doing so to fill three existing gaps in our understanding of southern San Andreas (sSAF) plate boundary system. First, the new slip rates will inform estimates of the earthquake hazard this fault poses for US and Mexican cities. Existing slip rate estimates for the ABF are highly imprecise, but suggest that it may accommodate as much as 14% of total plate boundary slip. These estimates as well as the geomorphic expression of the ABF are suggestive of a fault that is capable of producing damaging surface ruptures, but the new geologic slip rates are needed to confirm this. Second, new slip rates for the ABF constrain on the amount of plate boundary slip accommodated by the poorly understood network of faults off shore of southern California and northern Baja California. Previous estimates suggest a 2-20% of total plate boundary slip may be accommodated off shore, but confirming this by collecting sub-marine slip rates is complex and expensive. However, since the ABF is the primary fault feeding slip offshore, its slip rate constrains the minimum offshore slip rate. Third, new ABF slip rates will help to reveal the proportion of plate boundary slip that is absorbed by off-fault distributed deformation rather than brittle faulting. Geodetic and geologic slip rates indicate that the major onshore faults in this region accommodate roughly 80% of total relative plate motion. Since the new ABF slip rates will constrain how much plate boundary slip is accommodated off shore, whatever remains of the 20% slip deficit after both on- and offshore slip is accounted for can be attributed to undetected off-fault distributed deformation. In these ways, this we hope to harness the strengths of multiple scientists from universities in both the US and Mexico to answer key outstanding questions regarding seismic hazard and slip partitioning in a prominent but poorly understood section of the southern San Andreas Fault system.