SCEC efforts related to geology in 2004 fall into five principal areas: 1) directed geologic studies and compilation efforts that contribute to the Community Fault Model (CFM); 2) geologic, geomorphic and paleoseismic studies in the LA Basin, Eastern California Shear Zone (ECSZ), and southern San Andreas fault system, with emphases on: a) acquisition of long records to understand recurrence models and the constancy of strain release; b) efforts related to resolving differences between geologic and geodetic rates and the possible role of off-fault deformation in the southern San Andreas system and the ECSZ; and c) chronologic efforts to precisely date past earthquakes and develop a catalog of probability density functions for large prehistorical earthquakes for the RELM project; 3) activities related to the Fault Activity Database (FAD and Fault Information System (FIS); 4) development of a vertical motion database and Community Vertical Motion Map (VMM) for southern California; 5) other geologic studies that relate to rock mechanics, issues in source physics, etc. This summary will focus primarily on those elements not covered elsewhere, which are new advances in paleoseismic, geomorphic and slip rate studies, the vertical motion database, and the geochronologic efforts.

New Geologic, Paleoseismic and Geomorphic Results

Several studies were funded by SCEC that deal directly or indirectly with paleoseismic issues, in addition to the funding supplied to LLNL and Lewis Owen for dating paleoseismic events. Each area is discussed briefly.

Eastern California Shear Zone – Two new paleoseismic studies are underway this year in the ECSZ, both related to understanding the current high rate of strain identified by GPS across the central Mojave. Specifically, 12-14 mm/yr of strain appears to be concentrated over the Calico-Blackwater fault zones, although geologic and geomorphic evidence for high rates is lacking (Oskin, 2004). Trenches are being emplaced across the Calico (Seitz) and Blackwater (Madden) faults to establish their Holocene earthquake history and compare the intermediate Holocene rate to the long-term geologic rate (<1 mm/yr) and the GPS rate. The fieldwork should be completed by the end of 2004, and dates of events will be available in early 2005. Chronologic funding was also supplied for other projects – the completion of the Pinto Mountain fault study (Cadena et al) and for the western Garlock fault (Madden and Dolan).

San Andreas System Faults – The Work Continues- Four projects were funded that deal either directly with paleoseismic studies on the San Andreas fault system, or with issues of improving event correlations. Biasi and Weldon are working on modeling strategies for southern San Andreas earthquake ruptures, in association with the RELM effort, and Rockwell is working on a similar project for the San Jacinto fault. McGill and Weldon are working to improve slip per event information for the San Andreas along the San Bernardino segment. All of these studies are being incorporated into the RELM.

The excavations at Hog Lake in the Anza seismicity gap (Rockwell, Seitz, Dawson) were very successful at acquiring a deeper and longer record. After draining the 2002 trenches of water, the excavation was substantially enlarged and deepended to about 6 m (twice the original depth, revealing evidence for an additional 5-6 events (see Figure III.1). This makes the Hog Lake record the longest and most complete for any site along the San Jacinto fault, and similar to the Pallet Creek and Wrightwood records on the San Andreas fault. The most recent 6 events all occurred in the past 1000 years, but preliminary radiocarbon results for the deeper section show that the previous 5-6 events span an additional 2000 or more years, suggesting non-periodic recurrence. Further, The lull in seismic activity at Anza appears to correlate with the “flurry” of earthquakes documented for the San Andreas fault at Wrightwood. The work continues (in spite of early winter rains) to attempt acquisition of a 15 event record spanning the past 3-4 ka.

Larger imageLos Angeles Basin and Western Transverse Ranges – Two studies were funded that involve collection or compilation of data on vertical motions in LA Basin (Mueller) and the region from Ventura Basin to LA Basin (Neimi and Oskin). Mueller’s study involves compilation of aquifer data to provide deformation surfaces for inclusion in the CFM and vertical motion data to be included in the Vertical Motion Database and Mapping effort (discussed below). The Neimi and Oskin study involves a geomorphic analysis to determine uplift rates over a broad region, as high-lighted in the next section.

Geomorphic analysis - Active thrust faults and associated folds present challenges for seismic hazard assessment because slip rates are difficult to measure via traditional field investigations and may vary significantly along strike. Using empirical and theoretical relationships that relate rock uplift rate to stream power, Neimi and Oskin developed and applied a spatial analysis technique to extract relative uplift rates on blind faults by deriving power values of streams that overlie active folds. Morphologic measurements of catchment area and slope were derived for > 30,000 stream segments from 10 meter-resolution digital elevation models. They calibrated the stream power relationship by comparing analyses on structures with known uplift rates. Preliminary results indicate that stream power increases dramatically across the axes of active structures (see Figure III.2). Stream power values in the Ventura anticline, South Mountain, and Oakridge system support very high uplift rates in the Ventura basin, probably on the order of 5-6 mm/yr or greater, consistent with published rates. High stream power is also detected in the southeast Puente Hills, consistent with an erosion rate (a proxy for uplift rate) approaching 2 to 3 mm/yr. This region of potentially high uplift rate could not have been detected by other methods because Quaternary rocks are not well-preserved in the southeast Puente Hills. Distribution of stream power data in the Santa Ynez range indicates a clear decrease in uplift rate from east to west along the range crest. A lack of calibration sites for Paleogene strata presently limits an absolute determination of these uplift rates. Further work on this methodology, to be completed this winter, will include calibration of stream power rates to uplift rates for a variety of lithologic types, as well as inclusion of additional channel width measurements from the field. This project has shown that there is a strong correlation between stream power of small-order catchments and uplift rate of folds and faults, making such analyses a potentially useful tool for determining the seismic hazard of blind structures.

Vertical Motion Database and Map - The vertical motion database for Southern California is a compilation of geologic data, reference frame information, and processing tools to determine vertical crustal motions at 104 – 106 year time-scales. All original data, reference frames, and processing information are encapsulated within a PostgreSQL object-relational database. Querying data proceeds interactively with the database through three steps: (1) select data points, optionally filtered by location and data type, (2) select appropriate reference frame for each data type in selected set, and (3) process the data points into vertical motion rates. Correlations between data points are preserved in the data set and are followed via recursive queries (implemented in PL/pgSQL) to produce vertical motion results. Data compilation efforts are now largely complete for marine terraces from central California to the border with Mexico. The majority of these data are for terraces formed 80 – 120 ka near the present coastline, with a few older points inland. Thermochronology data available for the Transverse Ranges have been compiled to provide exhumation rates (a proxy for uplift rates) at million-year time scales (Figure III.3). Aquifer elevations that map basin subsidence rates, provided by K. Mueller, have also been incorporated into the database. River terrace, Quaternary stratigraphy, and river gradient indices are in the proto-type stage of database development. Significant challenges remain in defining appropriate reference frames for these interior data sets. Through collaboration with the SCEC Community Modeling Environment program, the vertical motion database will soon be available alongside other SCEC spatial data through a geographical data server located at scecdata.usc.edu.