Annual Meeting - SCEC.org

Poster Abstracts
New paleoseismologic investigations at the Bidart Fan site, ~5 km SE of Wallace Creek, indicate that southern SAF in the
Carrizo Plain has apparently ruptured, on average, every 88 years (45-144 yr for individual intervals) between ~A.D. 1350 and
1857 (Akciz et al., 2010). B4 LiDAR (light detection and ranging) data analysis by Zielke et al. (2010) also found that only ~5.5
m of slip occurred along the SAF in the Carrizo Plain in 1857 and at least since ~A.D. 1400, and none of the earthquakes
generated displacements larger than 5 meters (Grant Ludwig et al., 2010).
Slip per event and earthquake timing constraints can be tested against slip rate information to assess the steadiness of slip.
Therefore, these new data and the geochronological limitations of the published slip-rate studies emphasize the need to
improve, if not confirm, the existing slip-rate estimates by providing additional geochronological constraints. In August, 20111,
we re-excavated T7 and T11 from Sieh and Jahns' study, photologged the trench walls (1:10) and collected a total of 30 new
detrital charcoal samples from different stratigraphic layers from both of the trenches. Trench logs and radiocarbon results
will be presented.
ONE-DIMENSIONAL ELASTIC MODELING OF FAULT SLIP RATES IN THE SALTON TROUGH,
CALIFORNIA (A-058)
E. Alaniz and S.F. McGill
We used site velocities from SCEC’s Crustal Motion Model 4 (CMM4) to conduct one-dimensional elastic modeling for fault
slip rates along a transect across the Pacific-North America plate boundary through the Coachella Valley, southern California.
Our best-fitting model has slip rates of 18 and 16 mm/yr for the San Andreas and San Jacinto faults, respectively, with a small
amount (2 mm/yr) of right-lateral slip inferred east of the San Andreas fault and 4 mm/yr on the Elsinore fault. Other models
that fit the site velocities reasonably well have slip rates ranging from 16-20 mm/yr for the San Andreas and 14-20 for the San
Jacinto fault. These results are for the Coyote Creek strand of the San Jacinto fault. Future models will also include the Clark
fault.
SEISMIC VELOCITY STRUCTURES IN THE SOUTHERN CALIFORNIA PLATE BOUNDARY
ENVIRONMENT FROM DOUBLE-DIFFERENCE TOMOGRAPHY (B-023)
A.A. Allam and Y. Ben-Zion
We present tomographic images of crustal structures in the southern California plate boundary area, with a focus on the San
Jacinto Fault Zone (SJFZ), based on double-difference inversions of earthquake arrival times. Absolute arrival times of over
359,410 P- and S- wave phase picks for 5493 earthquakes recorded at 139 stations in Southern California are used. In addition
to the absolute event-station traveltimes, the double-difference inversions utilize differential travel times for phases from
nearby event pairs. Starting with a layered 1D model, and continuing in later iterations with various updated initial models,
we invert the data for Vp and Vs in a 270 km long, 180 km wide and 35 km deep volume around the SJFZ using a spacevariable
grid with higher density around the SJFZ. The examined volume stretches from Cajon Pass to the northernmost
Imperial Fault Zone and includes portions of the southern San Andreas Fault (SAF), the Elsinore Fault, and the Brawley
Seismic Zone in the Salton Trough. Because differential traveltimes are most sensitive to near-source structures, we obtain
high resolution around the earthquake sources. After 24 iterations we improve the average travel time misfit by a factor of 20.
Though ray coverage is limited at shallow depths, we obtain high-resolution images of seismic velocities from 3 to 16 km
throughout much of the study area. Our final velocity model shows clear velocity contrasts across the SJFZ and the southern
SAF as well as zones of low-velocity and anomalous Vp/Vs ratios associated with various fault strands and sedimentary
basins. The NE side of the SJFZ has generally higher velocities than the SW block, and the contrasts of Vp are generally higher
(up to 20%) than the contrasts of Vs (up to 15%). Near the San Jacinto valley to the NW, and the SE end of the SJFZ close to the
Salton trough, the polarity of the velocity contrast in the shallow crust is reversed. For the southern SAF the NE side is
generally the block with lower seismic velocities. The velocity reductions in fault zone regions are generally highest in
geometrically complex areas (up to 25% in the top few km), and are higher for Vs than for Vp. The imaged features have
important implications for various aspects of earthquake and crustal dynamics in the region.
2011 SCEC Annual Meeting | 139