The Application of LiDaR Imaging in Techtonic Geomorphology a Study of the 1857 Offset in the Carizzo Plain

Light detection and ranging (LiDaR) has emerged as one of the most promising new tools for the geosciences, with applications ranging from the environmental sciences to planetary geology. The "B4" LiDaR data set, used in this study was recorded last year by aircraft mounted laser scanner. The LiDaR point cloud covers three hundred square kilometers of the southern San Andreas and consists of over ten billion X, Y and Z coordinate sets. Data sets of this size are not easily processed. We have been working in cooperation with the San Diego Supercomputing Center (SDSC) and geongrid.org to establish and test access protocol and processing methods with the existing infrastructure we are able to select and process LiDaR data sets as large as 800,000 points covering up to 250,000 m².

Our study serves as a test of the accessibility and versatility of this new technology, specifically its applications in tectonic geomorphology and paleoseismology. We focus on a well documented section of the San Andreas Fault which last ruptured in 1857. We began by measuring and photographing geomorphic offsets, first documented by Dr. Kerry Sieh in 1978, from Wallace creek to the SE end of the Carrizo plain. These offsets were then identified and measured within the gridded LiDaR data using Arcmap. The offset measurements taken from the LiDaR data were similar to those taken in the field; however they were more easily made and numerous other features were identified to further characterize the 1857 offset in the Carrizo plain.

Interactive modeling and visualization of deformation due to slip on geometrically complex fault surfaces

We built a suite of software tools for accurately and interactively modeling the elastic deformation associated with slip on geometrically complex fault surfaces. The interface allows a user to display and manipulate triangulated models of fault system geometry, invert geodetic data for fault slip distributions, and visualize and resolve new stresses and displacements at any point. The software includes tools to coarsen a fault mesh to improve computation speed, a smoothing function for estimating fault slip, and tools that allow the resolution of stress components, including Coulomb failure stresses, onto the surface of faults. We tested sensitivities of slip inversions based on the geometry of faults contained in the Community Fault Model database to variability in imposed smoothing and fault geometry parameterizations. We also began testing the software's utility with the Northridge, Landers, and Hector Mine earthquakes.

Three-Dimensional Crustal Model of the San Bernardino Mountains, Transverse Ranges of Southern California

This study aims to contribute to a three-dimensional crustal model of southern California by using density and magnetic susceptibility measurements of a rock suite from the San Bernardino Mountains. Samples included are varieties of gneiss, granite, diorite, quartzite and amphibolite. Compressional wave velocity (Vp) were derived from sample density based on past standard observations. In the San Bernardino Mountains, Vp generally increases towards the south, with the highest velocities observed in the southeast. Magnetic susceptibility was measured on hand samples and calculated on some samples using FeO content, then plotted on the same scale as Vp. In a similar pattern to Vp, magnetic susceptibility increases toward the northwest and southeast, with a region of relatively low values of susceptibility running northeast to southwest through the range. An exhumation map matches the magnetic susceptibility map with shallow depths to the northeast, which is expected with the abundance of metasedimentary rocks in the location, and increasing depths to the northwest and to the southeast with a region of relatively little exhumation running northeast to southwest. The models indicate the study area is a tilted cross section with higher-velocity igneous and metamorphic rocks to the southwest and shallower, lower-velocity gneiss and metasedimentary rocks to the northeast.

Can Slip on the San Andreas and San Jacinto Fault Systems Drive Regional Deformation in Mechanical Models of Southern California?

The Southern California Earthquake Center's (SCEC) Community Fault Model (CFM) has defined the three-dimensional configuration of active fault surfaces in southern California. Since this database will be used in future geological studies including earthquake predictive models, validation of the CFM geometries is needed. The CFM, when combined with numerical modeling software, can be used to determine both slip rates and recurrence rates, which can be compared to rates calculated in past studies. The focus of this particular project was to expand the existing three-dimensional boundary element method model of the Los Angeles region to include faults within Southern California's Mojave area (faults east of the San Andreas). Unlike past models, which were driven using remote strain, we drive deformation in southern California by prescribing slip onto the San Andreas and San Jacinto faults. It is believed that the San Andreas fault drives deformation and slip along surrounding faults. We tested this hypothesis by prescribing slip on the San Andreas and San Jacinto fault systems and evaluated the rate and direction in which the remaining faults were slipping. Our results indicate that unless remote strains are also applied, many faults slip in the opposite sense and are not within past slip rate ranges. For example, in our model, contrary to past geologic studies, the Garlock fault is slipping in a right- lateral sense and many other known right-lateral faults are slipping in a left-lateral sense. The zero remote strain boundary condition causes these faults to counter- act the prescribed slip along the San Andreas and San Jacinto faults. The slip sense discrepancies may be alleviated by prescribing both slip along the San Andreas and San Jacinto faults as well as remote strain corresponding to the overall plate motion displacement field.

Predicting NO Earthquake: Regional Seismicity Along the Wasatch Fault, Utah

Many large earthquakes are preceded by an increase in regional seismicity known as "accelerating moment release" (AMR). It has been suggested that the observation of ongoing AMR can be interpreted as a signal that a fault is near the end of its seismic cycle. We present a preliminary attempt to search for AMR along the Wasatch Fault, a 240-mile long normal fault in north-central Utah. Scenario events are based on the segmentation model of the 2002 National Seismic Hazard Maps, and include multi-segment events. A backslip dislocation model is used to calculate an approximate geologically-constrained loading model that defines the regions of precursory AMR for each scenario. Seismicity data from the ANSS Composite Catalog do not show any significant ongoing AMR for any scenario event along the Wasatch Fault, suggesting that a major (M> 6.5) earthquake on the fault is not likely in the next 5-10 years.

Initial Results from the 2006 Bidart Fan Site Excavations

Past paleoseismic research at the Bidart Fan site along the San Andreas Fault (SAF) in the Carrizo Plain has produced many stratigraphic indicators which provided evidence of surface rupture associated with large earthquakes. In 2005, two 11-foot deep trenches perpendicular to the SAF (BDT5 and BDT6) were excavated in order to refine existing rupture data. In 2006, two new trenches (BDT7 and BDT8) were excavated adjacent to the original two. Also, a third trench (BDT9) parallel to the SAF was excavated to connect BDT5, BDT7, and BDT8. The trenches revealed significant fissures with overlying deposition and a preserved sag pond. Enough reliable data was obtained to create a chronology of large earthquakes in the Carrizo Plain. The results indicate that large earthquakes occur more frequently then was once believed. As a SCEC intern, I assisted with logging portions of each excavated trench and I learned how to differentiate important stratigraphic contacts using colored nails as markers to tie the record of earthquakes in BDT7 and BDT8 to last year's excavated trench, BDT5. By marking these important contacts, a more accurate paleoseismic representation of fractures, fissure fills and stratigraphic offset between layers can be easily viewed for interpretation. I also assisted in a process called "Trenchomatic," which involved photographing each exposure of the trench walls and the overlying sedimentary layers between the shores into a 1:20 scale panel image which was printed out and used for logging. I compiled these panels using Adobe Illustrator and created a 1:5 scale version of the trenches adding color to each unit marker as well as detailing the contacts, fractures, faults, charcoal sample areas and animal burrows within. Additionally, we collected detrital charcoal samples for radiocarbon dating. Analysis of radiocarbon samples has enabled us to date a series of five distinguishable earthquakes between 1400 AD up to 1857 AD.