W. H. Prescott

The velocity field along the San Andreas Fault in central and southern California

The velocity field within a 100-km-broad zone centered on the San Andreas fault between the Mexican border and San Francisco Bay has been inferred from repeated surveys of trilateration networks in the 1973–1989 interval. The velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the San Andreas fault in the Transverse Ranges of southern California. Across-strike profiles of the fault-parallel component of velocity exhibit the expected sigmoidal shape, whereas across-strike profiles of the fault-normal component of velocity are flat and featureless. No significant convergence upon the fault is observed even along the big bend sector of the fault. Simple dislocation models can explain most of the features of the observed velocity field, but those explanations are not unique. About 35 mm/yr of relative plate motion is accounted for within the span of the trilateration networks. Geologic studies indicate that the secular slip rate on the San Andreas fault is about 35 mm/yr. The agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes. The relative motion observed across the San Andreas fault (35 mm/yr) plus that observed across the Eastern California shear zone (8 mm/yr) accounts for most (43 mm/yr) of the observed North America-Pacific relative plate motion (47 mm/yr).

Inversion of GPS data for spatially variable slip‐rate on the San Andreas Fault near Parkfield, CA

We analyze GPS data collected from 1991-1998 at 35 sites near the Parkeld segment of the San Andreas Fault. Inverting the resultant site velocities for the distri- bution of interseismic slip-rate on the San Andreas reveals an area of low slip-rate on thefault extendingfrom between Middle Mountain and Carr Hill to southeast of Gold Hill. This slip-rate patternis similar to thatfoundbyHarris and Segall(1987) using trilateration data collected between 1966 and 1984. We infer a deep slip-rate (33 mm/yr) and depth of the transition between seismogenic and non-seismogenic slip(14km)thatagree betterwithindependentgeologic ev- idence than those found in the 1987 study. In contrast to Harris and Segall(1987), wendnoevidenceoffault-normal contraction.

Strain accumulation in western Washington

The Juan de Fuca plate is subducted beneath the North American plate off the coast of Washington at a rate of about 40 mm/yr N68°E. The average principal strain rates (extension reckoned positive) measured in northwestern Washington are as follows: Olympic peninsula 25 km south of Port Angeles from 1982 through 1990, e˙1=0.011±0.027 μstrain/yr N31°E±6.6° and e˙2=−0.092±0.028 μstrain/yr N59°W±6.6° and near Seattle from 1972 through 1985, e˙1=0.027±0.019 μstrain/yr N22°E±6.4° and e˙2=‐0.036±0.013 μstrain/yr N68°W±6.4°. Both strain measurements are consistent with uniaxial contraction in the direction of plate convergence. Uplift rates inferred from tide gage recordings are about 4 mm/yr on the Pacific coast and near 0 mm/yr farther inland near Seattle. These deformation rates are consistent with a model of the Cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward. The limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth (∼20 km) of the isotherm (∼450°C) that defines the brittle-ductile transition. Small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths. The principal strain rates measured from 1972 through 1983 in the back arc region near Richland, Washington, are e˙1=‐0.016±0.013 μstrain/yr N03°W±34° and e˙2=‐0.024±0.013 μstrain/yr N87°E±34°.

Geodetic estimates of fault slip rates in the San Francisco Bay area

Bourne et al. [1998] have suggested that the interseismic velocity profile at the surface across a transform plate boundary is a replica of the secular velocity profile at depth in the plastosphere. On the other hand, in the viscoelastic coupling model the shape of the interseismic surface velocity profile is a consequence of plastosphere relaxation following the previous rupture of the faults that make up the plate boundary and is not directly related to the secular flow in the plastosphere. The two models appear to be incompatible. If the plate boundary is composed of several subparallel faults and the interseismic surface velocity profile across the boundary known, each model predicts the secular slip rates on the faults which make up the boundary. As suggested by Bourne et al., the models can then be tested by comparing the predicted secular slip rates to those estimated from long-term offsets inferred from geology. Here we apply that test to the secular slip rates predicted for the principal faults (San Andreas, San Gregorio, Hayward, Calaveras, Rodgers Creek, Green Valley and Greenville faults) in the San Andreas fault system in the San Francisco Bay area. The estimates from the two models generally agree with one another and to a lesser extent with the geologic estimate. Because the viscoelastic coupling model has been equally successful in estimating secular slip rates on the various fault strands at a diffuse plate boundary, the success of the model of Bourne et al. [1998] in doing the same thing should not be taken as proof that the interseismic velocity profile across the plate boundary at the surface is a replica of the velocity profile at depth in the plastosphere.

Strain accumulation and rotation in western Nevada, 1993–2000

[1]xa0The positions of 44 GPS monuments in an array extending from the Sierra Nevada at the latitude of Reno to near Austin, Nevada, have been measured several times in the 1993–2000 interval. The western half of the array spans the Walker Lane belt, whereas the eastern half spans the central Nevada seismic zone (CNSZ). The principal strain rates in the Walker Lane belt are 29.6 ± 5.3 nstrain yr−1 N88.4°E ± 5.4° and −12.8 ± 6.0 nanostrain yr−1 N01.6°W ± 5.4°, extension reckoned positive, and the clockwise (as seen from above the Earth) rotation rate about a vertical axis is 13.6 ± 4.0 nrad yr−1. The quoted uncertainties are standard deviations. The motion in the Walker Lane belt can then be represented by a zone striking N35°W subject to 16.8 ± 4.9 nstrain yr−1 extension perpendicular to it and 19.5 ± 4.0 nstrain yr−1 right-lateral, simple shear across it. The N35°W strike of the zone is the same as the direction of the local tangent to the small circle drawn about the Pacific-North America pole of rotation. The principal strain rates for the CNSZ are 46.2 ± 11.0 nstrain yr−1 N49.9°W ± 6.0° and −13.6 ± 6.1 nstrain yr−1 N40.1°E ± 6.0°, and the clockwise rotation rate about a vertical axis is 20.3 ± 6.3 nrad yr−1. The motion across the CNSZ can then be represented by a zone striking N12°E subject to 32.6 ± 11.0 nstrain yr−1 extension perpendicular to it and 25.1 ± 6.3 nstrain yr−1 right-lateral, simple shear across it. The N12°E strike of the zone is similar to the strikes of the faults (Rainbow Mountain, Fairview Peak, and Dixie Valley) within it.

Deformation across the forearc of the Cascadia subduction zone at Cape Blanco, Oregon

Over the interval 1992–1999 the U.S. Geological Survey measured the deformation of a geodetic array extending N80°E (approximate direction of plate convergence) from Cape Blanco on the Oregon coast to the volcanic arc near Newberry Crater (55 and 350 km, respectively, from the deformation front). Within about 150 km from the deformation front, the forearc is being compressed arcward (N80°E) by coupling to the subducting Juan de Fuca plate. Dislocation modeling of the observed N80°E compression suggests that the main thrust zone (the locked portion of the Juan de Fuca-forearc interface) is about 40 km wide in the downdip direction. The transverse (N10°W) velocity component of the forearc measured with respect to the fixed interior of North America decreases with distance from the deformation front at a rate of about 0.03 mm yr−1 km−1. That gradient appears to be a consequence of rigid rotation of the forearc block relative to fixed interior North America (Euler vector of 43.4°±0.1°N, 120.0°±0.4°W, and −1.67±0.17 ° (m.y.)−1; quoted uncertainties are standard deviations). The rotation rate is similar to the paleomagnetically measured rotation rate (−1.0±0.2 °(m.y.)−1) of the 15 Ma lava flows along the Columbia River 250 km farther north. The back arc does not appear to participate in this rotation but rather is migrating at a rate of about 3.6 mm yr−1 northward with respect to fixed North America. That migration could be partly an artifact of an imperfect tie of our reference coordinate system to the interior of North America.

GPS Monitoring of Dynamic Behavior of Long‐Period Structures

Global Positioning System (GPS) technology with high sampling rates (∼10 sps) allows scientifically justified and economically feasible dynamic measurements of relative displacements of long-period structures—otherwise difficult to measure directly by other means, such as the most commonly used accelerometers that require post-processing including double integration. We describe an experiment whereby the displacement responses of a simulated tall building are measured clearly and accurately in real-time. Such measurements can be used to assess average drift ratios and changes in dynamic characteristics and therefore can be used by engineers and building owners or managers to assess the building performance during extreme motions caused by earthquakes and strong winds, by establishing threshold displacements or drift ratios and identifying changing dynamic characteristics. Such information can then be used to secure public safety and/or take steps to improve the performance of the building.

Strain accumulation across the Coast Ranges at the latitude of San Francisco, 1994–2000

[1]xa0A 66-monument geodetic array spanning the Coast Ranges near San Francisco has been surveyed more than eight times by GPS between late 1993 and early 2001. The measured horizontal velocities of the monuments are well represented by uniform, right-lateral, simple shear parallel to N29°W. (The local strike of the San Andreas Fault is ∼N34°W.) The observed areal dilatation rate of 6.9 ± 10.0 nstrain yr−1 (quoted uncertainty is one standard deviation and extension is reckoned positive) is not significantly different from zero, which implies that the observed strain accumulation could be released by strike-slip faulting alone. Our results are consistent with the slip rates assigned by the Working Group on California Earthquake Probabilities [2003] to the principal faults (San Gregorio, San Andreas, Hayward-Rodgers Creek, Calaveras-Concord-Green Valley, and Greenville Faults) cutting across the GPS array. The vector sum of those slip rates is 39.8 ± 2.6 mm yr−1 N29.8°W ± 2.8°, whereas the motion across the GPS array (breadth 120 km) inferred from the uniform strain rate approximation is 38.7 ± 1.2 mm yr−1 N29.0°W ± 0.9° right-lateral shear and 0.4 ± 0.9 mm yr−1 N61°E ± 0.9° extension. We interpret the near coincidence of these rates and the absence of significant accumulation of areal dilatation to imply that right-lateral slip on the principal faults can release the accumulating strain; major strain release on reverse faults subparallel to the San Andreas Fault within the Coast Ranges is not required.

Deformation across the Alaska-Aleutian Subduction Zone near Kodiak

The Kodiak-Katmai geodetic array, nine monuments distributed along a profile trending north-northwestward across Kodiak Island and the Alaska Peninsula, was surveyed in 1993, 1995 and 1997 to determine the deformation at the Alaska-Aleutian subduction zone. Velocities on Kodiak island measured relative to the stable North American plate decrease with distance from the Alaska-Aleutian trench (distance range 106 to 250 km), whereas no appreciable deformation was measured on the Alaska Peninsula (distances 250 to 370 km from the trench). The measured deformation is reasonably well predicted by the conventional dislocation representation of subduction with the model parameters determined independently (i.e., not simply by fitting the observations). The deformation of Kodiak Island is in striking contrast to the very minor deformation measured in the similarly situated Shumagin Islands, 450 km southwest of Kodiak along the Alaska-Aleutian trench.

Strain accumulation and rotation in western Oregon and southwestern Washington

[1]xa0Velocities of 75 geodetic monuments in western Oregon and southwestern Washington extending from the coast to more than 300 km inland have been determined from GPS surveys over the interval 1992–2000. The average standard deviation in each of the horizontal velocity components is ∼1 mm yr−1. The observed velocity field is approximated by a combination of rigid rotation (Euler vector relative to interior North America: 43.40°N ± 0.14°, 119.33°W ± 0.28°, and 0.822 ± 0.057° Myr−1 clockwise; quoted uncertainties are standard deviations), uniform regional strain rate (eEE = −7.4 ± 1.8, eEN = −3.4 ± 1.0, and eNN = −5.0 ± 0.8 nstrain yr−1, extension reckoned positive), and a dislocation model representing subduction of the Juan de Fuca plate beneath North America. Subduction south of 44.5°N was represented by a 40-km-wide locked thrust and subduction north of 44.5°N by a 75-km-wide locked thrust.