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Inferences Drawn from Two Decades of Alinement Array Measurements of Creep on Faults in the San Francisco Bay Region

P.O. Box 140
Twain, California 95984
galehouse{at}snowcrest.net
(J.S.G.)
U.S. Geological Survey 977
Menlo Park, California 94025
jlienk{at}usgs.gov
(J.J.L.)

View larger version (68K): [in a new window]   Figure 1. Holocene faults of the greater San Francisco Bay region shown as black lines. Locations of regular measurement arrays shown as triangles, annual Hayward fault arrays as dots. Numbers indicate abbreviated site names as referred to in text; for example, the full name of site 1 is "SF-1." Stars and rectangles labeled LPEQ, MHEQ, and CLEQ indicate respectively the epicenters and rupture extents of the 1989 Loma Prieta, 1984 Morgan Hill, and 1979 Coyote Lake earthquakes (Lisowski et al., 1990; Oppenheimer et al., 1990). Inset, typical alinement array, discussed in methods section of text. NR, Nyland Ranch, creepmeter; P, Point Pinole.

View larger version (25K): [in a new window]   Figure 2. Right-lateral movement on arrays on the San Andreas fault. Time of LPEQ, M 7, Loma Prieta earthquake of 17 October 1989 is shown as vertical line. Full name of site, SF-18, referred to as "site 18" in text.

View larger version (32K): [in a new window]   Figure 3. Right-lateral creep on regular arrays of the Hayward fault. Distances are given in kilometers southeast of Point Pinole (P, Fig. 1). Additional data are available in Lienkaemper et al. (2001).

View larger version (18K): [in a new window]   Figure 4. Right-lateral creep on faults of the Hayward fault subsystem in the North Bay: Maacama and Rodgers Creek faults.

View larger version (28K): [in a new window]   Figure 5. Right-lateral creep on faults of the Calaveras fault subsystem: Green Valley, Concord, and Calaveras faults. CLEQ, 1979 Coyote Lake earthquake; MHEQ, 1984 Morgan Hill earthquake; LPEQ, 1989 Loma Prieta earthquake.

View larger version (32K): [in a new window]   Figure 6. Right-lateral creep observed at site 33 (Coyote Ranch near Coyote Lake). Model (heavy black line) shows slip calculated by offset fit associated with the 1979 Coyote Lake and 1984 Morgan Hill earthquakes, ignoring the smaller effect of the 1989 Loma Prieta earthquake.

View larger version (29K): [in a new window]   Figure 7. Right-lateral movement measurements on the San Gregorio, West Napa, and Antioch faults.

View larger version (26K): [in a new window]   Figure 8. Map of Holocene faults within the San Andreas fault system projected approximately parallel to the Pacific-North America plate boundary near Hollister. Distances in kilometers southeast of the Mendocino triple junction (MTJ), set to 0 km where the downgoing slab intersects the Hayward fault subsystem. Alinement arrays (large triangles): numbered, this study; B1, B2,..., sites of Burford and Harsh (1980). Geodetic arrays or fault-crossing lines (squares): A, Sleepy-San Pablo (Lienkaemper et al., 1991); B, Covelo-Poonkinney (Lisowski and Prescott, 1989); C, Slide-View (Freymueller et al., 1999); D, Camp Parks (Prescott and Lisowski, 1983); E, Veras (Prescott et al., 1981); F, Calaveras Reservoir (Prescott et al., 1981); G, Grant Ranch (Oppenheimer et al., 1990); H, San Felipe (Lisowski and Prescott, 1981 [LP81]); I, Tres Pinos (LP81); J, Thomas Road (Harsh and Pavoni, 1978); K, Pionne (LP81); L, Chase Ranch (Prescott and Burford, 1976).

View larger version (43K): [in a new window]   Figure 9. Long-term creep rate shown versus distance from the MTJ as described in Figure 8. Solid numbered triangles are arrays in this study; inverted solid triangles are arrays of Burford and Harsh (1980); open triangle A, span of Sleepy-San Pablo line; circles B through K, geodetic data referenced in caption of Figure 8. For the Hayward fault the mean ±2 lines show uncertainty range of best-fit polynomials that express the along-strike variation in long-term creep rate derived from the much larger data set for this fault (Lienkaemper et al., 2001). Data for sites 24 and 27 reflect pre-LPEQ rates of Lienkaemper and Galehouse (1997) and Harsh and Burford (1982). Dashed lines are interpolated between available data; queries indicate areas where major gaps in the data exist. For the Paicines-San Benito section of the Calaveras subsystem, two dashed lines illustrate possible alternative interpretations of existing data.

View larger version (28K): [in a new window]   Figure 10. Long-term dextral creep rates (mm/yr) along the San Andreas fault system shown in bold type. Measured rates of movement (dextral, positive) are shown for noncreeping sites too (e.g., < 1 mm/yr). Map grid and data locations as in Figure 8. Large dashed rectangle shows limits of the WGCEP (2003) study area; fault segment name codes and long-term slip rates of WGCEP (2003) are shown in square brackets. Small rectangles show uncertainty in location of fault segment boundaries. Segment names: CN = Northern Calaveras; CC = central Calaveras; CS = southern Calaveras; CON = Concord; GN = northern Greenville; GS = southern Greenville; GVN = northern Green Valley; GVS = southern Green Valley; HN = northern Hayward; HS = southern Hayward; MTD = Mount Diablo thrust; RC = Rodgers Creek; SAN = north coast San Andreas; SAP = peninsula San Andreas; SAS = Santa Cruz Mountains San Andreas; SGN = northern San Gregorio; SGS = southern San Gregorio.

View larger version (44K): [in a new window]   Figure 11. R, the fraction of each fault segment assumed to be locked in the WGCEP (2002) model, is assigned three weighted values: a central value (most likely, shown as triangles) and upper and lower bounds (arrows). R = 1, totally locked segment; R = 0, totally creeping. Segment name codes as in Figure 10. Most segments are assigned a weight of 0.6 for the central value and 0.2 for the bounds, approximating a Gaussian distribution. For highly uncertain cases, indicated by an asterisk (e.g., CON, GVS, GVN), a weight of one-third was assigned to each central value and bounds. In one case, indicated by two asterisks, evidence suggests using weights skewed toward totally creeping behavior.