Field Guide to Exhumed Major Faults in Southern California

Joseph Jacobs, Samuel Howard, David H. Forand, Ory Dor, & James P. Evans

Published September 10, 2006, SCEC Contribution #7245

This field guide provides an overview of exposures and provides a field trip guide to localities of exhumed faults in southern California. We focus on exposures of faults that are documented or inferred to be exhumed from seismogenic depths. The goal of this guidebook is to provide geoscientists who are interested in fault zone mechanics and earthquake processes a summary of the results of the work on these sites. Fault growth, evolution, and fault mechaics are of much interest to the seismological community because it is related to topics of earthquake mechanics, such as the width of the fault surface that slips during an earthquake (Kanamori and Heaton, 2000), fault zone structure (Rubin et al., 1999), and rupture propagation processes (Heaton, 1990; Andrews and Ben-Zion, 1997; Harris and Day, 1999). Characterization of fault zone structure and composition provides insight into these processes of fault nucleation,
propagation, and termination. The study of exhumed faults has been previously shown by such workers as Chester and Logan (1986, 1987), Chester et al. (1993), Schulz and Evans (2000), Pachell and Evans (2002), and Wilson et al. (2003) as a means to investigate active and inactive faults at the earth’s surface that are representative of faults at depth. The direct study of faults at depth poses both logistical and economic challenges, although it certainly may be accomplished (i.e., the San Andreas Fault Observatory at Depth – SAFOD). Thus, studies of exhumed faults provide insight into aspects of fault composition and structure, and we can somtimes establish the history of the fault zone over time. This field guide presents the initial effort to catalog fault exposures in southern California that are significant in understanding earthquake source and propagation processes. This work was conducted in a fault and rock mechanics (FARM) type of approach in order examine the rocks in a seismological context. The exhumed fault data sets provide interpretations on fault zone structure from depths of ~ 1 km to > 6 km predominantly in the San Gabriel and San Bernardino mountains (Figure 1). This field guide summarizes the following (when available) at each site:
• Location of site (UTM coordinates in meters in WGS 84 projection), including clear directions
• Basic geological and seismological context of site; rock type, exhumation amount, and fault/seismic history
• Current understanding of fault geometry and segmentation
• Summary of observations from the site, including:
–Physical characteristics of the fault zone (e.g., density of fractures, presence of subsidiary faults, and thicknesses of the fault core and damage zone) –Chemical characterization, such as mineralogy, whole-rock geochemistry, and isotopic analyses
–Particle size distribution
• Seismological implications
This guidebook is the result of the efforts of Joe Jacobs, David Forand, and Sam Howard. Joe worked on this during part of his M.S. degree work at Utah State University, and Dave and Sam were SCEC summer interns. Most of the descriptions of how to get to sites, summaries of the work, and formatting of the guidebook are the results of their efforts. This is a beta version of the guidebook. Suggestions and comments are welcome. We have established a format for this field guide, and we welcome contributions from other workers. We will edit for format, but will not change the content of any contribution, and we will acknowledge all other scientific
works. We will work with SCEC to make this available online in the near future.

Jacobs, J., Howard, S., Forand, D. H., Dor, O., & Evans, J. P. (2006, 09). Field Guide to Exhumed Major Faults in Southern California. Oral Presentation at SCEC 2005 Meeting Field trip. .

Related Projects & Working Groups
Fault and Rupture Mechanics