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San Francisco Bay Area 3-D Velocity Model
Entry ID: USGS_BAVM
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Summary
Abstract: Overview of the 3-D Velocity Model

The USGS is developing a 3-D seismic velocity model for the San Francisco Bay
Area primarily for the calculation of realistic strong-ground-motion synthetic
seismograms for local earthquakes. The model, gridded at 100-m intervals,
extends from the ground surface to the Moho, from the coast to the Great
Valley, and from the Geysers (38.8°) to Watsonville (36.9° ). The model is a
hybrid of geological and geophysical components; the Cenozoic sedimentary cover
is defined primarily by geologic data and the pre-Cenozoic units primarily by
geophysical data. The model boundaries include: (1) elevation and bathymetry,
(2) major strike-slip faults, (3) contacts between surficial geologic
materials, (4) base of the Holocene Bay Mud, (5) base of Quaternary alluvium,
(6) top of Franciscan or Salinian basement inferred from an inversion of
gravity data and constrained by outcrop, well control, and seismic profiles,
(7) top of lower crust, and (8) the Moho. S- and P-wave velocities for fine,
medium, and coarse-grained Quaternary alluvium have been compiled for >120
shallow boreholes in the South Bay. Fifty sonic and density logs from oil test
wells have been compiled to determine typical P-wave velocities and densities
of the Cenozoic sedimentary units. A P-wave tomographic velocity inversion is
used to constrain the velocity model between the top of Franciscan or Salinian
basement and the top of the lower crust, at about 16 to 18 km. Velocity models
from more than 20 existing deep-crustal seismic refraction lines have been
compiled to determine the geometries and velocities of the lower crust and
Moho. Important features of the model with implications for strong ground
motion include the presence of several deep (>3 km) sedimentary basins which
may locally enhance ground motions and the regional presence of a strong S- and
P-wave reflector at the top of the lower crust producing prominent peaks in
ground motions at ranges between 40 and 60 km.

Compilation of Oil Test Well Sonic and Density Log Data

Several relatively thick (>3 km deep) Cenozoic basins, including the Cupertino,
Evergreen, Livermore, and San Pablo basins, may locally enhance strong ground
motions in the San Francisco Bay area, California. As part of a crustal-scale,
three-dimensional seismic velocity and density model for the Bay area, we have
compiled data from sonic and density logs from oil test wells in the Bay area
to better understand strong motion resonances generated by these basins. We
have compiled the velocities and densities of sediments and rocks within these
Cenozoic basins using 59 sonic and density logs from 51 oil test wells. The
well data are primarily from the Livermore, Concord, and Los Medanos oil
fields, and the Sacramento-San Joaquin delta, and provide measurements from the
surface to as much as 5.3 km subsurface. Only a few logs from the South Bay are
included in this compilation. The logs were hand digitized at non-uniform
intervals between 3 and 30 m to capture the significant variations of the logs
with depth for frequencies up to 2 Hz. Linear regression through 41 sonic logs
yields Vp (km/s) = 2.24 + 0.599Z, where Z is depth in km. Shallow borehole
data, generally from the South Bay, and from less than 30 m deep, indicate that
the average surficial P-wave velocity at 10 holes in weathered Tertiary
sedimentary units ranges from 2.21 and 2.32 km/s and is in close agreement with
extrapolated P-wave velocities inferred from the oil test wells. A sonic log
for Eocene sediments from Butano Ridge in San Mateo County shows that at a
given depth, velocities are approximately 0.5 km/s higher than those near
Livermore. The higher P-wave velocities for the Tertiary sedimentary rocks at
Butano Ridge probably result from a combination of dense volcanic clasts in
conglomerates plus very tight compaction of the sandstones. Density logs in
Cenozoic sedimentary rocks show higher scatter. Linear regression of 18 density
logs yield r (g/cm3) = 2.25 + 0.065Z. Average densities of weathered Tertiary
sedimentary rocks measured on core samples from 5 shallow boreholes in the
South Bay lie between 2.20 and 2.25 g/cm3, in close agreement with the
surficial density inferred from linear regression of oil well data. This report
presents the locations, elevations, depths, stratigraphic and other information
about the oil test wells, provides plots showing the density and sonic
velocities as a function of depth for each well log, and compiles all data to
better understand the velocities and densities of Cenozoic sedimentary rocks in
the Bay area.
[Summary provided by the USGS.]

Service Citation
Originators: Thomas M. Brocher
Title: San Francisco Bay Area 3-D Velocity Model
Provider: U.S. Geological Survey
ISO Topic Category
GEOSCIENTIFIC INFORMATION
Access Constraints
Well log data are available in excel4 or excel5 or ascii format. The anonymous
ftp address is: eratos.wr.usgs.gov. Password in required. Change the directory
(cd) to /pub/sfbay/welllogs. The files are named SFBay.sonic.xl4.bin and
SFBay.density.xl4.bin (Excel4) and SFBay.sonic.xl5.bin and
SFBay.density.xl5.bin (Excel5), in Mac Binary II format. Table 1 of this report
in Excel5 format is also in this ftp site, labeled as Table 1.OFR.bin.

Contact Thomas M. Brocher (brocher@andreas.wr.usgs.gov) for password and
access to model.
Service Provider
Northern California, Earthquake Hazards Program, Geological Division, U.S. Geological Survey, U.S. Department of the Interior

Service Provider Personnel
Name: MICHAEL F. DIGGLES
Phone: (650) 329-5404
Email: mdiggles at usgs.gov
Contact Address:
U.S. Geological Survey
345 Middlefield Road
Mail Stop 531
City: Menlo Park
Province or State: CA
Postal Code: 94025
Country: USA
Distribution Media
Distribution_Media: Online FTP
Fees: No fees
Personnel
TYLER B. STEVENS
Role: SERF AUTHOR
Phone: (301) 614-6898
Fax: 301-614-5268
Email: Tyler.B.Stevens at nasa.gov
Contact Address:
NASA Goddard Space Flight Center
Global Change Master Directory
City: Greenbelt
Province or State: MD
Postal Code: 20771
Country: USA

THOMAS M. BROCHER
Role: TECHNICAL CONTACT
Phone: 650-329-4737
Fax: 650-329-5163
Email: brocher at usgs.gov
Contact Address:
U.S. Geological Survey
345 Middlefield Road MS 977
City: Menlo Park
Province or State: CA
Postal Code: 94025-3591
Country: USA
Publications/References
Brocher, T.M., Ruebel, A.L., and Brabb, E.E., 1997, Compilation of 59 sonic and
density logs from 51 oil test wells in the San Francisco Bay area, California,
U.S. Geological Survey Open-File Report 97-xxx, 74 p.

Abramovitz, T., Brocher, T.M., Catchings, R., and Mooney, W.D., 1997,
Geological Implications of P and S-wave Velocity Structure Across the San
Andreas Fault in the San Francisco Bay Area, EOS. Trans. Amer. Geophys. Un., in
press.

Brocher, T.M., E E Brabb, R D Catchings, G S Fuis, T E Fumal, R A Jachens, A S
Jayko, R E Kayen, R J McLaughlin, T Parsons, M J Rymer, R G Stanley, C M
Wentworth, 1997, A Crustal-Scale 3-D Seismic Velocity Model for the San
Francisco Bay Area, California, EOS. Trans. Amer. Geophys. Un., in press.

Ruebel, A.L., T M Brocher, E E Brabb, T E Fumal, R A Jachens, and R G Stanley,
1997, Compilation of Sonic and Density Logs From Oil Test Wells Within Tertiary
Basins in the San Francisco Bay Area, California, EOS. Trans. Amer. Geophys.
Un., in press. Directors Approval 9/11/1997.

Wentworth, C.M., Fumal, T.E., Brocher, T.M., and Brabb, E.E., 1997, The areal
distribution of geologic materials and their seismic velocities, San Francisco
Bay area, California, 1997, EOS. Trans. Amer. Geophys. Un., in press.
Creation and Review Dates
SERF Creation Date: 2005-09-13
SERF Last Revision Date: 2012-09-14
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