By R.W. Simpson, R.W. Graymer, R.C. Jachens, D.A. Ponce, C.M. Wentworth1
Open-File Report 2004-1083
2004
Any use of trade names is for descriptive purposes only and does not imply endorsement by the Federal government.
U.S GEOLOGICAL SURVEY
U.S. DEPARTMENT OF INTERIOR
1U.S. Geological Survey, Menlo Park, California 94025
Location map showing the locations of cross-section profiles spaced 2.5 km apart and selected faults that appear on the cross-sections and maps. Green lines represent Alquist-Priolo fault zones, red lines represent selected fault zones of intererest in the region containing the cross-sections.
New methods for relocating earthquakes have yielded, in many cases, a greatly refined picture of the structures on which these earthquakes have occurred. In the San Francisco Bay region, recent studies have focused on parts of the San Andreas fault (Rubin and Gillard, 2000), the Hayward fault (Waldhauser and Ellsworth, 2002), the Calaveras fault (Schaff and others, 2002), and the Mt. Lewis seismicity trend (Kilb and Rubin, 2002).
Relocated events on the Hayward and Calaveras faults in particular, have been especially revealing and suggest that at seismogenic depth the Hayward fault is the major northward extension of the Central Calaveras fault (Manaker, Michaels, and Burgmann, 2003), rather than the Northern Calaveras fault.
Although 3D viewers can be used with some success to examine seismicity, it's difficult to get the big picture using 3D viewers, and especially difficult to construct fault surfaces through clusters of earthquakes. This will probably become easier as such viewers become more versatile, but we fell back on a time-honored technique of plotting cross-section profiles to illuminate the 3-dimensional relations of seismicity and to construct digital representations of the fault surfaces. By using the advantages of HTML, these cross-sections can be linked and made more accessible for viewing than they would be if printed on paper. Links to 3D views using the LiveGraphics3D applet written by Martin Kraus are provided on each cross-section page.
An ellipsoidal transverse mercator projection was used to project the faults and earthquake epicenters. Using the convention of Lienkaemper (1992), the origin of the coordinate system was positioned at the northernmost onshore point of the Hayward fault at Point Pinole (Lat = 38.00302N, Lon = -122.36572W), and the coordinate system was rotated 35-degrees so that the Hayward fault is approximately parallel to one axis. Thus, the onshore creeping trace of the Hayward fault extends from 0 km at Point Pinole to approximately -70 km in southern Fremont. The cross-section profiles occupy a rectangular solid volume 40 km wide by 200 km long by 20 km deep shown in the location map above.
The earthquakes plotted on these cross-sections, maps, and 3D views came from the catalog of double-difference relocated earthquake described by Ellsworth and others (2000). The double-difference technique has been described in detail by Waldhauser and Ellsworth (2000). In brief, the technique does a good job of locating nearby earthquakes relative to each other, although as the events get farther apart, their relative locations become less certain, so that over short distances the locations are expected to be good, but at longer distances there may be biases. There may also be some artifacts introduced by velocity heterogeneities associated with geologic bodies of various compositions.
The depths of events are subject to several sources of uncertainty. Event depths tend to be more poorly determined than horizontal locations, especially for events in areas of sparse station coverage. (Location programs do best with event depths when there are seismometers located closely above the hypocenter.) Location programs typically give depths that are relative to the average elevation of the seismometers used to detect the event. For events in the San Francisco Bay region, this bias could amount to hundreds of meters for some events. No attempt has been made to correct for this bias.
The double-difference approach does not work for isolated events, and in addition, the original catalog was screened for location quality before double differencing so that not all events recorded during the interval 1984-2000 appear in the double-difference catalog. The sub-catalog used to prepare the plots shown here consisted of approximately 43,706 events, compared with a total of about 54,550 events in the Northern California Seismic Network (NCSN) catalog for the same region and same time interval, so that approximately 80 percent of the recorded events are in the double-difference catalog. The NCSN data were obtained from the Northern California Earthquake Data Center (NCEDC).
A discussion and evaluation of various relocation techniques can be found in Wolfe (2002). Zhang and Thurber (2003) have proposed a new approach which yields both a velocity model and a catalog of relocated events, which should minimize artifacts that might be caused by velocity changes.
Faults are shown on the maps as red or green lines and on the cross sections as red or green dots at the top where the cross section intercepts a fault trace. The green lines represent Alquist-Priolo Earthquake Faults mapped as a result of the 1972 State of California Alquist-Priolo Earthquake Fault Zoning Act. The red lines represent selected fault traces (Lienkaemper, 1992; Graymer and others, 1994, 1995, 1996; Wentworth and others, 1998; Graymer, 2000) in the study area labelled on the location map. The Silver Creek fault of Jachens and others (2003) shown on maps is defined as the bounding fault to the Evergreen Basin. This basin is concident with a sizable gravity low, and the bounding fault is located, especially at the northwest end, with the help of gravity gradients.
We thank Bill Ellsworth and his coauthors (Ellworth and others, 2000) for allowing us to use and distribute their catalog of double-difference relocated earthquakes. Felix Waldhauser, David Schaff, and Haijaing Zhang kindly provided us with their relocated catalogs for comparison with the relocated catalog used in this report. We are also grateful to Martin Kraus for creating his terrific LiveGraphics3D Java applet. Scott Haefner and Jeanne Hardebeck provided useful and constructive reviews.
Ellsworth, W.L., Beroza, G.C., Julian, B.R., Klein, F., Michael, A.J., Oppenheimer, D.H., Prejean, S.G., Richards-Dinger, K., Ross, S.L., Schaff, D.P., and Waldhauser, F., 2000, Seismicity of the San Andreas Fault system in central California: Application of the double-difference location algorithm on a regional scale: Eos, Transactions, American Geophysical Union, v. 81, p. 919.
Graymer, R.W., 2000, Geologic map and map database of the Oakland metropolitan area, Alameda, Contra Costa, and San Francisco counties, California: U. S. Geological Survey, Miscellaneous Field Studies Map MF-2342, 1 sheet, scale 1:50,000, http://geopubs.wr.usgs.gov/map-mf/mf2342/.
Graymer, R.W., Jones, D.L., Brabb, E.E., 1994, Preliminary geologic map emphasizing bedrock formations in Contra Costa County, California; a digital database U.S. Geological Survey Open-File Report 94-622, 11 pp., http://wrgis.wr.usgs.gov/docs/geologic/ca/california.html.
Graymer, R.W., Jones, D.L., Brabb, E.E., 1995, Geologic map of the Hayward fault zone, Contra Costa, Alameda, and Santa Clara counties, California; a digital database: U.S. Geological Survey Open-File Report 95-597, 8 pp., http://wrgis.wr.usgs.gov/open-file/of95-597/.
Graymer, R.W., Jones, D.L., Brabb, E.E., 1996 Preliminary geologic map emphasizing bedrock formations in Alameda County, California; a digital database: U.S. Geological Survey Open-File Report 96-0252, 14 pp., ftp://wrgis.wr.usgs.gov/pub/geologic/ca/of96-252/al_g1.tar.Z, ftp://wrgis.wr.usgs.gov/pub/geologic/ca/of96-252/alps.tar.Z.
Jachens, R.C., Ponce, D.A., Graymer, R.W., Wentworth, C.M., Hildenbrand, T.G., 2003, The Hayward Fault in the East San Francisco Bay Region, California: A Regional Geophysical and Geological Perspective: Eos Trans. AGU, v. 84, n.46. Fall Meeting Supplement, Abstract T11D-0423.
Kilb, D., and Rubin, A.M., 2002, Implications of diverse fault orientations imaged in relocated aftershocks of the Mount Lewis, ML 5.7, California, earthquake: Journal of Geophysical Research, v. 107, n. B11, 2294, 10.1029/2001JB000149.
Lienkaemper, J.J., 1992, Map of recently active traces of the Hayward fault, Alameda and Contra Costa Counties, California: U.S. Geological Survey, Miscellaneous Field Studies Map MF-2196, scale 1:24,000.
Manaker, D.M., Michaels, A.J., Burgmann, R., 2004, Subsurface structure and mechanics of the Calaveras-Hayward fault stepover from three-dimensional Vp and seismicity, San Francisco Bay Region, California: Bulletin of the Seismological Society of America, submitted.
Rubin, A. M., Gillard, D., 2000, Aftershock asymmetry/rupture directivity among central San Andreas fault microearthquakes: J. Geophys. Res., v. 105, n. B8, p. 19095-19110, 10.1029/2000JB900129.
Schaff, D.P., Bokelmann, G.H.R., Beroza, G.C., Waldhauser, Felix, and Ellsworth, W.L., 2002, High-resolution image of Calaveras Fault seismicity: Journal of Geophysical Research, v. 107, n. B9, 10.1029/2001JB000633.
Waldhauser, Felix, and Ellsworth, W.L., 2000, A double-difference earthquake location algorithm: Method and application to the northern Hayward fault: Bulletin of the Seismological Society of America, v. 90, 1353-1368.
Waldhauser, Felix, and Ellsworth, W.L., 2002, Fault structure and mechanics of the Hayward Fault, California, from double-difference earthquake locations: Journal of Geophysical Research, v. 107, n. B3, 10.1029/2000JB000084.
Waldhauser, Felix, Ellsworth, W.L., and Cole, A., 1999, Slip-parallel seismic lineations along the northern Hayward Fault, California: Geophysical Research Letters, v. 26, p. 3525-3528.
Wentworth, Carl M., Blake, M. Clark, Jr., McLaughlin, Robert J., Graymer, Russell W., 1998, Preliminary geologic map of the San Jose 30 X 60-minute quadrangle, California; a digital database: U.S. Geological Survey Open-File Report 98-795, 14 pp., http://wrgis.wr.usgs.gov/open-file/of98-795/.
Wolfe, C.J., 2002, On the mathematics of using difference operators to relocate earthquakes: Bulletin of the Seismological Society of America, v. 92, 2879-2892.
Zhang, Haijiang, and Thurber, C.H., 2003, Double-difference tomography: The method and its application to the Hayward Fault, California: Bulletin of the Seismological Society of America, v. 93, n. 5, p. 1875-1889.