SCEC Award Number 12234 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Assessing the Earthquake Potential of the Ventura, Southern San Cayetano, and Pitas Point Fault System: Determination of Recent Siip Rates and Paleo-Earthquake Ages and Displacements
Investigator(s)
Name Organization
James Dolan University of Southern California John Shaw Harvard University
Other Participants Thomas Pratt, United States Geological Survey
SCEC Priorities 1a, 4a SCEC Groups Geology, USR, WGCEP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 As part of our long-term collaborative effort (USC, Harvard, USGS, UCLA), during 2012 we drilled a north-south transect of four cored boreholes and 13 cone-penetrometer tests across the active fold front forming above the Ventura fault, a large thrust ramp that may serve to link together major faults of the central and western Transverse Ranges to generate large-magnitude (Mw 7.7-8.1) earthquakes similar in size to San Andreas “Big Ones”. Our transect of 23-m-deep boreholes, which were drilled directly into the central portion of one of our 2010 high-resolution seismic reflection profiles, revealed a well-bedded stratigraphic section with many units that are traceable continuously along the entire 375 m length of the transect. Stratigraphic relationships indicate that the prominent topographic scarp at this site records ~6.5 m of uplift during fold growth in the most-recent earthquake on the underlying Ventura blind thrust ramp. Similarly, sedimentary growth observed in the borehole cross section indicates ~4.5 m of growth during the penultimate event, with an event horizon located at the base of a growth interval between 4 and 8.5 m depth south of the scarp. The large amounts of uplift in the two folding events indicate that they formed in large-displacement earthquakes, likely involving rupture of the Ventura fault together with thrust ramps to both the west (e.g., Pitas Point fault) and east (Southern and eastern San Cayetano faults). In addition to several radiocarbon ages, ongoing dating of 13 luminescence samples will provide age constraints on these two large-magnitude earthquakes.
Intellectual Merit This effort provides the first direct structural evidence of fold growth during earthquakes on the Ventura fault, a major seismic source beneath the Ventura Coast that is capable of generating large-magnitude events similar in size to the "Big Ones" generated by the San Andreas fault. Our measurements of uplift in these earthquakes, coupled with ongoing structural modeling of the Ventura fault, will allow us to assess the likely magnitudes of these events. Moreover, ongoing luminescence dating will refine the ages of these earthquakes, facilitating comparison of these events with those generated by nearby thrust faults. This, in turn, will allow us to assess the likelihood that the Ventura fault ruptures together with major reverse faults along strike to the east and west (e.g., San Cayetano fault).
Broader Impacts These data are of critical importance for seismic hazard assessment in southern California. The hazard associated with the major thrust faults in the Transverse Ranges remains under-studied relative to the major strands of the San Andreas system, yet preliminary evidence form a number of studies suggests that the Ventura and related faults may be capable of liking up to generate truly great earthquakes similar in size to major San Andreas events. The project forms the basis of part of Lee McAuliffe's USC Ph. D. dissertation. The companion harvard-led effort formed the basis of part of Judith Hubbard's Ph. D. dissertation. The research has also trained several other graduate students in various aspects of active tectonics and structural geologic research (e.g., USC Ph. D. student Chris Milliner). Finally, this inter-disciplinary effort has fostered closer interactions amongst four research institutions (USC, Harvard, USGS, and UCLA) and has been used in both our undergraduate and graduate courses at USC and Harvard.
Exemplary Figure Figure 6 of this report (borehole/CPT cross section)

Figure 6. Boreholes, CPTs, and preliminary stratigraphic correlations along the Day Road transect. Different colors denote different depositional units.  Black lines at surface denote projections of far-field surface slope of Day Road alluvial fan. The 6 meter difference between these two lines records uplift during the most recent fold-forming event above the Ventura thrust fault. Obvious growth within orange unit records onlapping of ≥4.5 m-tall, now buried fold scarp that formed during the penultimate uplift event. Black age data on cross section are calibrated radiocarbon ages in years before 1950 AD. Red stars denote OSL sample locations.

figure will be published in McAuliffe et al. (in prep.)
Linked Publications

Add missing publication or edit citation shown. Enter the SCEC project ID to link publication.