3-D Geologic and Seismic Velocity Models of the San Francisco Bay Region

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Previous work by the USGS and others has shown that the 3-D structure of the earth has a significant impact on how strongly an earthquake is felt at different locations and on the duration of the shaking. Because seismic waves propagate through different rock types with differing speeds that depend on the rock properties, the waves can be reflected and refracted as they travel through various rock types. These effects need to be understood in order to predict the shaking in future large earthquakes.

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The 3-D models combine 100 years of surface geologic mapping by the USGS, the California Geological Survey, and many other institutions together with decades of research into the seismic properties of the rocks in the Bay Area. They also include information from boreholes and variations in Earth's gravity and magnetic fields. Traditional two-dimensional geologic maps show only the distribution of rock units at Earth's surface.

The geologic model is a fault block model - that is, the upper 45 km (28 miles) of Earth's crust has been broken up into irregular shaped blocks, bounded by faults. The model also includes the subsurface shape of basins that underlie the Santa Clara Valley, Livermore Valley, and Santa Rosa Plain. The soft sediments in these basins trap seismic energy and greatly enhance shaking levels relative to surrounding regions.

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Seismic velocities of rocks and sediments at shallow depths determine how strongly an area will shake. By assigning velocities to rock types in the 3-D geologic model, geologists can gain an understanding of the extent of areas of low shear velocity that are most likely to experience localized strong shaking and future earthquake damage. These models have been used in computer simulations of the 1906 San Francisco and 1989 Loma Prieta earthquakes.

Some additional important applications of these new models include:

• forecasting strong ground motions that may damage buildings and essential infrastructure or destroy levees in the Sacramento Delta
• predicting where destructive liquefaction of the ground may occur 
• locating earthquakes more accurately
• defining the extent of groundwater aquifers and modeling of subsurface transport of contaminants.

For additional information on 3-D Geologic Models see:

• Compressional and Shear Wave Velocity Versus Depth in the San Francisco Bay Area, California: Rules for USGS Bay Area Velocity Model 05.0.0 (USGS Open-File Report 2005-1317 by Thomas M. Brocher)
• Three-Dimensional Geologic Map of the Hayward Fault Zone, San Francisco Bay Region, California (Scientific Investigations Map 3045 by G.A. Phelps, R.W. Graymer, R.C. Jachens, D.A. Ponce, R.W. Simpson, and C.M. Wentworth)
• Three-dimensional geologic map of the Hayward fault, northern California: Correlation of rock units with variations in seismicity, creep rate, and fault dip (15 MB pdf file) is a recent paper on the Hayward Fault Model by Russell Graymer and others.

Geologic Block Model

• Detailed model
  • Zone and fault surfaces USGSBayAreaBM-05.1.0_allsurfs.tgz (42.3 MB)

    Tarball of tsurf files for all zone and fault surfaces for the detailed geologic block model.

  • Fault surfaces only USGSBayAreaBM-05.1.0_faultsurfs.tgz (8.0 MB)

    Tarball of tsurf files for fault surfaces for the detailed geologic block model.

• Regional model
  • Zone and fault surfaces USGSBayAreaBMExt-05.1.0_allsurfs.tgz (2.8 MB)

    Tarball of tsurf files for mantle, lower crust, and topography/bathymetry zone surfaces and the San Andreas fault surface for the regional geologic block model.

Seismic Velocity Model

• Detailed model USGSBayAreaVM-08.3.0.etree.gz (1.6 GB)

  Etree database file for inner, fine resolution model covering the San Francisco Bay area

• Regional model USGSBayAreaVMExt-08.3.0.etree.gz(1.1 GB)

  Etree database file for outer, coarser resolution model. The model fits around the detailed model and extends the region covered by the model. If you are only interested in the volume covered by the detailed model, you do not need to download this model.

• Checksums for database files

  For use with md5sum software to verify the integrity of downloaded files.

  • Compressed files
  • Uncompressed files
• Model query software

  This software library is required to query the seismic velocity model for physical properties. The software and its documentation are available at github.com/usgs/earthquake-cencalvm. 

Bounding boxes of the models

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The detailed model is surrounded by the regional (extended) model to permit capturing large length scale features of seismic wave propagation along the northern San Andreas fault outside the domain of the detailed model. In the regional model details away from the San Andreas fault are grossly generalized in places. The regional model fits around the detailed model so it does not include the region covered by the detailed model.

The detailed and regional velocity models are rectangular volumes in a Transverse Mercator (TM) projection with the following parameters:

• Scale factor for central meridian: 0.99960000
• Longitude of central meridian: -123.00000
• Latitude of origin: 35.00000
• False easting: 0.00000
• False northing: 0.00000
• Azimuths of sides: 53.638, 323.638

KML file with bounding boxes.

Coordinates are WGS84 longitude/latitude. Models extend from sea level or top of ground surface down to 45 km below sea level. The model includes the material properties of water for the Pacific Ocean.

Geologic Block Model

Introduction

The detailed and regional (extended) 3-D geologic models of Central California, centered on the San Francisco Bay region, were made using the Dynamic Graphics EarthVision(TM) software. This program builds 3-D models starting with a hierarchy of fault surfaces that divide the volume of interest into Fault Blocks. These fault blocks are further subdivided by Zone Surfaces that most commonly represent contacts between rock units within a fault block.

Fault and zone surfaces were exported from the EarthVision (EV) model using an EV utility named "ev_ev2gocad", which outputs a file in the TSurf format used by the GOCAD(TM) software. The TSurf format represents a surface by specifying the locations a number of triangles which approximate the surface.

Various options for viewing TSurf files may be found under "Tools" on the SCEC Community Fault Model website. There are also scripts available in the tarball to project the tsurfs and to convert the projected versions to the .vtp format for viewing in the ParaView 3-D visualization software.

Documentation

• Slices through the detailed model
• Fault block hierarchy
• Text file of detailed model block and zone labels
• Text file of regional model block and zone labels
• Version history

Seismic Velocity Model

Introduction

The velocity model is stored in Etree database files. We provide a library to query the database from C++, C, and Fortran 77. The query software is called cencalvm (current version is 0.6.5) and is generally updated independently of the velocity model. In order to use the seismic velocity model, you need to download the Etree database file for the detailed model and install the query software. Downloading the Etree database file for the regional (extended) model is optional.

Documentation

• Query software
• Version history

Resolution

Detailed model

Transition from detailed model to extended model. This is a region 3.2km in width surrounding the detailed model. It is part of the extended model.

Extended model

¹Elevation is given with respect to mean sea level, so -400m corresponds to 400m below mean sea level.

Geologic Block Model Releases

Version 05.1.0

This release of the geologic block model includes both a detailed model and an extended (regional) model. The extended model extends the San Andreas fault and major zone surfaces (top of the mantle, bathymetry/topography, and the top of the lower crust) to approximately 650 km x 330 km x 45 km.

Version 05.0.0

Initial release of the geologic block model.

Seismic Velocity Model Releases

Version 08.3.0

• Switched from Olsen et al model for Qp and Qs to Brocher (2008) model for Qp and Qs that is based on Graves's model.
• Reduced Vp and Vs in granites, Francsican, gabrro, lower crust, and upper mantle.
• Increased Vp and Vs by 10% in the La Honda basin. More work is still needed to constrain the seismic velocities in this basin.

Version 05.1.0

This release of the USGS Bay Area Velocity Model includes both a detailed model and an extended (regional) model. The extended model extends the velocity model to approximately 650 km x 330 km x 45 km. Note that the extended model contains much less detail than the detailed model, hence the names. Additionally, the detailed model has been extended down to a depth of 45 km.

Version 05.0.0

Initial release of the USGS Bay Area Velocity Model. It fills the volume covered by the detailed 3-D geologic model.