SCEC Award Number 17197 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Dating past earthquakes revealed in two trenches across the Agua Blanca Fault, northern Baja California, Mexico
Investigator(s)
Name Organization
Whitney Behr University of Texas at Austin
Other Participants Tom Rockwell (SDSU, collaborator)
John Fletcher (CICESE, collaborator)
Lewis Owen (University of Cincinnati, collaborator)
Peter Gold (UT Austin, PhD student)
SCEC Priorities 1a, 2e, 5c SCEC Groups Geology, SAFS, WGCEP
Report Due Date 06/15/2018 Date Report Submitted 06/10/2018
Project Abstract
 As much as 14% of Pacific-North American relative slip is accommodated by subsidiary faults of the San Andreas plate boundary system that traverse the Peninsular Ranges of northern Baja California, Mexico. At least half of this slip, a minimum of ~2.8-3.4 mm/a (based on other work by PI Behr), is accommodated by the Agua Blanca Fault, a primarily dextral strike-slip fault that transfers slip to the offshore Palos Verdes-Coronado Bank Fault paralleling the coast of southern California and northern Mexico. We present the first multi-event Holocene earthquake history from two trenches across the central section of the Agua Blanca Fault. Seven earthquakes since ~6.7 ka suggest average earthquake recurrence of ~1000 years, with a coefficient of variation of ~0.55. This variability may be real or the result of an incomplete record, however geologic evidence from a second site records coseismic lateral slip-per-event for the past two surface ruptures of ~2.5 m, suggesting regular slip at least recently and requiring a long recurrence interval to keep pace with the 3 mm/a Late Quaternary slip rate. Estimates from surface exposure dating require the penultimate and most recent earthquakes to post-date ~1.6 ka, consistent with paleoseismic evidence for a rupture after 1486 years before present (ybp) and after 1089 ybp. The paleoseismic, slip-per-event and slip rate measurements over the past ~1.6-1.4 ka are mutually consistent and suggest that whether or not displacement is regular or variable, the ABF is likely near the middle of an earthquake cycle and thus is unlikely to produce a strong earthquake in the immediate future.
Intellectual Merit Long-term geologic slip histories are critical datasets for answering fundamental questions regarding fault behavior and patterns of strain release over geologic time scales (Schwartz and Coppersmith, 1984). Estimates of past rupture timing reveal the frequency and constancy of strain release and, in a complete slip history, compliment time-averaged geologic fault slip rates and estimates of rupture size (magnitude, total slip, rupture length). Whether earthquakes of similar magnitude occur along faults at essentially regular intervals or whether ruptures are essentially random in size and/or recurrence interval is key for seismic hazard estimates as well as for understanding how strain is accommodated through complex, multi-fault systems like the southern San Andreas Fault. Factors affecting long-term fault slip are numerous, but include fault length, total slip, slip rate, initiation timing, and along-strike geometry and segmentation, characteristics that vary widely among important active structures within the southern San Andreas system. Earthquake timing measurements for the Agua Blanca Fault benefit our broader understanding of fault behavior because (1) the size of the past two earthquakes and the Holocene-Late Pleistocene slip rate are known (complimentary, and partially SCEC funded work by P.I. Behr and former UT Ph.D. student Peter Gold), and (2) because the ABF is an interesting example of a young (1.5-3 Ma) fault with a slow (3 mm/a) slip rate that has been constant over 1.6, 11.7, 21.8 and 65.1 ka time scales, is highly segmented, and microseismically quiet.
Broader Impacts This project constitutes roughly a quarter of UT Ph.D. student Peter Gold’s dissertation project, and has contributed to furthering his training as a Quaternary geoscientist. It has also provided valuable field experience in techniques not taught at UT for UT undergraduate student Daniel Ortega Arroyo, and has continued to foster collaboration between professors and students at UT, San Diego State University and the Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) in northern Baja California, Mexico. The results of this project compliment additional SCEC, USGS and NSF funded efforts along the Agua Blanca Fault, which together provide the first reliable constraints on which to base hazard estimates for the greater Ensenada region (population > 500k).
Exemplary Figure Figure 5