Annual Meeting - SCEC.org

Poster Abstracts
local conditions (e.g., rural and urban soil locales, as well as weathered and competent rock sites), conventional field
procedures are often modified ad-hoc to fit the unanticipated complexity at each location. For the majority of sites (>80%),
fundamental-mode Rayleigh wave dispersion-based techniques are deployed and where complex geology is encountered,
multiple test locations are made. Due to the presence of high velocity layers, about five percent of the locations require multimode
inversion of Rayleigh wave (MASW-based) data or 3-D array-based inversion of SASW dispersion data, in combination
with shallow P-wave seismic refraction and/or HVSR results. Where a strong impedance contrast (i.e. soil over rock) exists at
shallow depth (about 10% of sites), dominant higher modes limit the use of Rayleigh wave dispersion techniques. Here, use of
the Love wave dispersion technique, along with seismic refraction and/or HVSR data, is required to model the presence of
shallow bedrock. At a small percentage of the sites, surface wave techniques are found not suitable for stand-alone
deployment and site characterization is limited to the use of the seismic refraction technique. A USGS Open File Report-describing
the surface geology, Vs profile and the calculated Vs30 for each site--will be prepared after the completion of the
project in November 2011.
ASSESSING STRAIN ACCUMULATION RATES ACROSS THE PACIFIC-NORTH AMERICAN PLATE
BOUNDARY NEAR SAN BERNARDINO, CALIFORNIA (A-053)
R.F. Mason, E.P. Upton, A. Grijalva, K.L. Muhammad, A. Torrens-Bonano, S.F. McGill, E. Alaniz, J. Spinler, R.A. and
Bennett
The goal of this project was to achieve a more precise understanding of the strain accumulation rates of the various faults that
make up the Pacific and North American plate boundaries in Southern California. We are particularly interested in the San
Andreas Fault (SAF) and the San Jacinto Fault (SJF), which are likely sources of impending large earthquakes in Southern
California. To do this, we used precise Global Positioning System (GPS) surveying equipment to measure site positions at 26
pre-established benchmarks throughout the San Bernardino Mountains and surrounding area. Using a spreadsheet, we then
conducted one-dimensional elastic modeling of our velocity data, along with site velocities from SCEC’s Crustal Motion
Model 4 (CMM4), within a transect across the plate boundary passing through the San Bernardino Mountains. We tested
about 3.2 million slip-rate combinations to find which ones produced a good fit to the measured site velocities. From the
combinations that produced the best-fitting curves, we concluded that the SJF accounts for more movement (~12 mm/yr)
along the plate boundary than does the SAF (~10 mm/yr). From combinations producing the reasonably well-fitting curves,
our results show that the faults west of the SJF system could have slip-rates between 6-12 mm/yr, the SAF and SJF combined
could have rates between 18-28 mm/yr (with most reasonable fits between 20-24mm/yr), and the ECSZ could have rates
between 12-16 mm/yr.
CHARACTERIZING THE RECENT BEHAVIOR AND EARTHQUAKE POTENTIAL OF THE BLIND
WESTERN SAN CAYETANO AND VENTURA FAULT SYSTEMS (A-124)
L.J. McAuliffe, J.F. Dolan, J. Hubbard, and J.H. Shaw
We are studying the activity and paleoearthquake history of the blind Ventura and western San Cayetano faults through a
multidisciplinary analysis of strata that have been folded above the fault tiplines. These two thrust faults form the middle
section of a >200-km-long, east-west belt of large, interconnected reverse faults that extends across southern California.
Although each of these faults represents a major seismic source in its own right, we are exploring the possibility of even
larger-magnitude, multi-segment ruptures that may link these faults to other major faults in the Transverse Ranges system.
The proximity of this large reverse-fault system to several major population centers emphasizes the importance of
understanding the behavior of these faults for seismic hazard assessment.
During the summer of 2010 we used a mini-vibrator source to acquire four, one- to three-km-long, high-resolution seismic
reflection profiles. The profiles were collected along the locus of active folding above the blind, western San Cayetano and
Ventura faults - specifically, across prominent fold scarps that have developed in response to recent slip on the underlying
thrust ramps.
Our initial efforts to document the earthquake history and slip-rate of this large, multi-fault reverse fault system focus on a site
above the blind, western San Cayetano thrust ramp. At Briggs Road ~14 km east of Ventura, a high-resolution profile across
the locus of recent folding reveals a well-defined north-dipping active synclinal axial surface in growth strata that extends to
the surface at a prominent south-facing fold scarp lying at the topographic range front. During August 2011, we drilled 11
hollow-stem boreholes and cone-penetrometer tests along the same alignment as the reflection profile, providing overlap
between the data sets. Preliminary analysis of the borehole data reveals a fine-grained section dominated by thinly bedded
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