SCEC Award Number 14179 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Re-evaluating the long term geologic slip rate of the western Agua Blanca fault near Ensenada, northern Baja California, Mexico
Investigator(s)
Name Organization
Whitney Behr University of Texas at Austin Thomas Rockwell San Diego State University
Other Participants Peter O. Gold
SCEC Priorities 4a, 4b, 4c SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 The goal of this project is to clarify three existing deficiencies in our understanding of the southern San Andreas transform plate boundary system by measuring precise, quantitatively constrained geologic slip rates for the Agua Blanca Fault (ABF) in northern Baja California, Mexico (Fig. 1). Specifically, the slip rate of the ABF helps constrain 1) the amount of slip transferred west from the plate boundary and in turn the earthquake hazard posed by this fault and the sub-parallel San Miguel Vallecitos fault to proximal US and Mexican populations, 2) the amount of slip transferred to off-shore continental borderland faults, and 3) the amount of plate boundary slip taken up as diffuse, off-fault deformation. We formally started this project in May of 2014 by initiating a new collaboration between geologists at UT Austin and CICESE. We conducted exploratory fieldwork during this time, primarily along the western reach of the ABF. The abundance of well preserved geomorphic features suggestive of active, right-lateral slip that were observable in the field and using Google Earth was promising for a focused slip rate study, therefore we leveraged money from PI Behr’s UT startup funds and co-sponsored with CICESE the collection of airborne LiDAR data for the western 70 km of the ABF. NCALM collected this data in late summer 2014, and we took receipt of it in early 2015. In May 2014, we also collected samples for 10Be exposure dating during the exploratory field work, which will be processed at ASU and measured at LLNL early this summer.. Funding received from the NSF’s Catalyzing New International Collaborations (CNIC) program will support additional summer 2015 field mapping and sample collection at previously known and newly discovered slip rate sites; new, preliminary quantitative slip rates should be forthcoming by the end of 2015. This new project follows nearly completed SCEC-funded work measuring the slip rate for the Banning strand of the southern San Andreas Fault, which is described below and in SCEC publication 2083 submitted March 4, 2015 to Journal of Geophysical Research- Solid Earth by UT graduate student Peter Gold, PI Behr, PI Rockwell and others,.
Intellectual Merit The complexity of faulting on- and off-shore of northwestern Baja California also characterizes much of the Pacific-North American plate boundary, where in many places relative motion is accommodated by multiple kinematically diverse, overlapping or parallel faults. Determining the spatial and temporal scales over which slip is partitioned between multiple active faults is key to understanding how complex fault systems evolve as they accommodate strain, which in turn may aid our understanding of fault structure in the upper crust. Constructing fault slip histories with which to investigate fault interaction requires fault slip rates over multiple time scales to complement Holocene paleoseismic data. This intellectual merit of this study derives from its contribution to a broader investigation of slip partitioning and complex fault zone evolution in the southern San Andreas Fault plate boundary system
Broader Impacts The most immediate impact of new slip rates for the Agua Blanca Fault is that they will reveal the proportion of total plate boundary slip, and thus the relative hazard posed, by this fault, as well as faults offshore that are difficult to investigate. Although we have expended significant resources estimating hazard along faults in southern California, as a community we have paid relatively little attention to fault that are less conveniently investigated, especially those in northern Mexico. However, these faults, while ambiguously continuous with southern California faults at the surface, almost certainty connect at depth and as demonstrated in 1892 and 2010, can produce large surface rupturing earthquakes. Therefore we cannot have a complete understanding of the southern San Andreas Fault system with little constraint on fault activity south of the border. Our new measurements will help to determine more carefully the hazard potential the Agua Blanca Fault poses for Mexican populations as well as for southern California residents. This study is also supporting an early career researcher (PI Behr) and a Ph.D. candidate (Gold), and is facilitating a new long-term collaboration between researchers at UT Austin and CISESE in Ensenada.
Exemplary Figure Figure 2 in project report.

Caption
Figure 2
A. Fault map of the Peninsular Ranges of northern Baja California, Mexico, showing the locations of the slip rate sites shown in B, C, and D. Slip rate for the ABF critically constrain regional earthquake hazard, the amount of slip being transferred west, the amount of slip fed off shore and contribute to a more complete total brittle strain summation. LiDAR-derived hillshade images shown in B, C, and D were made using airbore lidar collected by NCALM and funded independently by CICESE and PI Behr’s startup funding from UT.