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Examining Local and Regional Heterogeneous Deformation Using a 3-D Geologic Map of the Peninsula Segment of the San Andreas Fault: Eric Horsman

Project Title: Examining Local and Regional Heterogeneous Deformation Using a 3-D Geologic Map of the Peninsula Segment of the San Andreas Fault
Mendenhall Fellow: Eric Horsman
Duty Station: Menlo Park, CA
Start Date: January 7, 2008
Education: Ph.D. (Geology), University of Wisconsin–Madison, 2006
Research Advisors: Russell Graymer, (650)329-4988,; Robert Jachens, (650)329-5300,; Robert Simpson, (650)329-4865,; Thomas Brocher, (650)329-4737,

Project Description: At the latitude of San Francisco Bay, the San Andreas Fault zone is one part of a broad, deforming plate boundary zone that is more than 100 km wide.  The San Andreas, Hayward, and Calaveras faults, the deforming blocks they surround, and other structures together accommodate ~38-40 mm/yr of fault system-parallel displacement and ~4 mm/yr of fault system-perpendicular displacement.  This project will address the fact that we do not yet fully understand how this regional deformation is accommodated by the various structures in the plate boundary zone.  In particular, I am studying the role of the Peninsula segment of the San Andreas Fault system in regional deformation. 

The heart of the project is construction of a 3-D geologic map of the fault segment and adjacent rock bodies at seismogenic depths in the crust.  This task requires integration of a wide range of geological, geophysical, and geodetic datasets to provide constraints (including uncertainties) on the subsurface structure of geological bodies and the faults in the region.  Essentially this task involves extending cross-sectional models of the region (for example, McLaughlin and others, 2004, fig. 1) into three dimensions to produce a model involving both faults and rock bodies.  Once the 3-D map is constructed, several questions can be addressed:

    1. What is the geometry of the Peninsula segment of the San Andreas Fault at depth?  How are adjacent subsurface rock bodies related to this geometry? 
    2. Are spatial patterns of seismicity associated with particular rock types abutting faults? 
    3. How has deformation in the region been partitioned into discrete slip on faults versus distributed folding of blocks?
    4. How does the deformation recorded by the Peninsula segment of the San Andreas Fault relate to the history of deformation across the entire plate boundary?  Has the role played by the San Andreas Fault changed over time? 
    5. How have the geometry of the faults and rock bodies evolved over time? 
Figure 1

3-D Mapmaking

One of the primary goals of the USGS 3-D Geologic Maps and Visualization Project is construction of a fully 3-d geologic map of the San Francisco Bay region.  Maps of the Santa Clara valley region and the Hayward Fault region (Graymer and others , 2005, fig. 2 ) have already been completed.  This project will complement these existing maps by extending the coverage area to include the Peninsula segment region of the San Andreas Fault west of San Francisco Bay.

Figure 2

Construction of fully 3-D maps is not a straightforward task.  Numerous data sets (geology, gravity, magnetics, well data, seismicity, and so on) must be integrated.  A traditional geologic map can be extrapolated to predict subsurface geometry of rock bodies and structures (for example, construction of a cross section) but provides no direct evidence of the subsurface geology.  Mapping can be supplemented with well data, where available, to provide additional direct constraints.  Geophysical methods can be used to provide indirect constraints on subsurface geology.  Simultaneous inversion of gravity and magnetic data sets provides a way to cross-check results between the data sets.  Comparison of these direct and indirect constraints on subsurface geometries with earthquake hypocenter data allows for refinement of fault locations and orientations, as well as improved estimates of boundaries between rock bodies. 

References Cited:

Graymer, R.W., Ponce, D.A., Jachens, R.C., Simpson, R.W., Phelps, G.A., and Wentworth, C.M., 2005, Three-dimensional geologic map of the Hayward Fault, northern California: Correlation of rock units with variations in seismicity, creep rate, and fault dip: Geology, v. 33, p. 521–524.

McLaughlin, R.J., Clark, J.C., Brabb, E.E., Helley, E.J., and Wentworth, C.M., 2004, Geologic Map of the Loma Prieta Region, California: U.S. Geological Survey Professional Paper 1550-E, scale 1:50000.

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