Relative plate motions and surface velocity fields

In the southwestern USA, relative motion between the Pacific and North American plates occurs at a rate of about 48-50 mm a-1 (DeMets & Dixon, 1999; Sella et al., 2002). Geodetic and seismologic data suggest that up to 70% of this motion presently may be accommodated by dextral slip on the San Andreas Fault (Argus & Gordon, 2001). Out of a total of approximately 11001500 km of strike-slip motion, since the Oligocene (Stock & Molnar, 1988), only 300 and 450 km of right lateral slip have accumulated along the southern and northern reaches of the San Andres Fault, respectively (Dillon & Ehlig, 1993; James et al., 1993). The remaining movements, therefore, must be accommodated elsewhere within the diffuse zone of deformation that stretches from the Coast of California to the Basin and Range (Fig. 8.1).

Along its ~1200 km length, the San Andreas Fault is divided into segments that exhibit different short-term mechanical behaviors. Some segments, such as those located north of Los Angeles and north of San Francisco, have ruptured recently, generating large historical earthquakes, and now exhibit little evidence of slip. These segments appear locked at depth and are now accumulating significant nonpermanent (elastic) strains near the surface, making them a major potential earthquake hazard (Section 2.1.5). Between the two locked segments is a 175-km-long fault segment in central California that is characterized by aseismic slip, shallow (<15 km depth) microearthquakes, and few large historical earthquakes. Along this segment, the aseismic slip reflects a relatively steady type of creep that results from frictional properties promoting stable sliding on the fault plane (Scholz, 1998). These different behaviors, and especially the occurrence of aseismic creep on or near faults at the surface, complicate the estimation of horizontal velocity fields (Section 8.5.3).

In northern California, earthquakes reveal the presence of a ~120-km-wide zone of faulting within the Coast Ranges between the Pacific plate on the west and the Great Valley-Sierra Nevada microplate on the east (Fig. 8.18a). In this region, horizontal velocities (Fig. 8.18b) show an approximately uniform distribution of right lateral motion toward N29°W (Savage et al., 2004a). This direction is close to the local strike (N34°W) of the San Andreas Fault and results in dominantly strike-slip motion along the major faults in the area. A velocity profile along a great circle passing through the Pacific-North America pole of rotation (Fig. 8.18c) illustrates this result by showing the components of motion that occur parallel to and perpendicular to the trace of the San Andreas Fault. Slip rates parallel to the fault are highest. Other faults display lower rates. In addition, the westward movement of the Great Valley-Sierra Nevada block relative to the Pacific plate (Dixon et al., 2000; Williams et al., 2006) and the slight obliquity between this motion and the trace of the San Andreas Fault (Prescott et al., 2001; Savage et al., 2004a) produces a small component of contraction across the Coast Ranges. This latter result is supported by both geologic data (Fig. 8.7b, inset) and by earthquake focal mechanisms that show thrust solutions west of the Great Valley (Fig. 7.10). By contrast, little deformation occurs across the Great Valley and within the Sierra Nevada, suggesting that these regions form a coherent, rigid block.

In southern California, the distribution of earthquakes indicates that relative plate motion is accommodated across a zone that is several hundreds of kilometers wide (Fig. 7.8). South of latitude 34°N, some motion is distributed between the San Jacinto and San Andreas faults (Fig. 8.1). Becker et al. (2005) estimated that the former accommodates some 15 mm a-1 of slip and the latter ~23 mm a-1. Within the Transverse Ranges, where crustal shortening and surface uplift accommodate a component of the motion (Fig. 8.8), slip on the San Andreas Fault appears to be significantly slower (Meade & Hager, 2005). Other displacements occur along major sinistral faults, such as the Garlock, Raymond Hill, and Cucamonga faults (Fig. 8.8a), and by the clockwise and anticlockwise rotation of crustal blocks about vertical axes (Savage et al., 2004b; Bos & Spakman, 2005).

Figure 8.19a shows an example of a velocity field for southern California in a local reference frame (Meade & Hager, 2005). Stations on the North American plate move toward the southeast at about half of the relative plate velocity, those on the Pacific plate move toward the northwest. The observed velocities vary smoothly across the San Andreas Fault. Two profiles (Fig. 8.19b) (gray shaded areas) show similar total velocity changes of ~42 mm a-1 but distinctly different velocity gradients. Along the northern profile, the fault-parallel velocity drops by ~30 mm a-1 across the San Andreas Fault and decreases slightly through the San Joaquin Valley before distributing some 12 mm a-1 across the Eastern California Shear Zone. By contrast, the southern profile shows a total velocity drop across a distance that is about 50% that in the northern profile. This reflects the difference in the geometry of the fault system from north to south. The flat portion of the northern profile mirrors the relative stability of the ~200-km-wide Great Valley-Sierra Nevada microplate, with the deforming central segment of the San Andreas Fault to the west and Eastern California Shear Zone to the east. By contrast, the southern profile shows that the 40-km-wide zone between the San Andreas and San Jacinto faults accommodates approximately 80% of the relative plate motion.

North of the Garlock Fault, relative motion is deflected east of the Sierra Nevada by deformation in the southern Walker Lane (Figs 7.9, 8.1). This eastward deflection reflects an extensional step-over between the northwest-trending faults of the Eastern California Shear Zone and those located along the eastern margin of the Sierra Nevada (Oldow, 2003). North of the stepover, the zone of deformation broadens into the central and northern Walker Lane and central Nevada seismic belt (Fig. 8.20a). Earthquake focal mechanisms (Fig. 7.10) indicate that displacements in these latter belts involve both strike-slip and normal fault motion on variably oriented faults. Horizontal velocities increase from 2-3 mm a-1 to ~14 mm a-1 from the central Great Basin (Section 7.3) toward the Sierra Nevada (Oldow, 2003). Accompanying this rate increase, the directions of motion rotate clockwise from west-northwest to northwest (Fig. 8.20b), indicating an increase in a component of dextral strike-slip deformation from east to west. Oldow (2003) showed that two distinctive zones of transtension characterize this belt: one that is dominated by extension on the west (domain III) and another that is dominated by strike-slip motion on the east (domain II in Fig. 8.20b). Together with deformation in the central and eastern Basin and Range (Section 7.3), these belts accommodate up to 25% of the relative motion between the Pacific and North American plates (Bennett et al., 1999). This transfer of motion east of

Velocities parallel to the San Andreas Fault (~N34°W)

ocities trans Andreas F

Velocities transverse to the San Andreas Fault (~N56°E)

Coast Ranges

¡Great Valley j Sierra Nevada

Distance NE from the San Andreas Fault along a Great Circle (km)

Figure 8.18 (a) Earthquakes recorded by the Northern California Seismic Network between 1968 and 1999 (images provided by G. Bokelmann and modified from Bokelmann & Beroza, 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union). Over 58,000 seismic events show the seismogenic segments of the greater San Andreas Fault system. Map (b) and (c) profile showing velocities derived from GPS surveys in the San Francisco Bay area (images provided by J. Savage and modified from Savage et al., 2004a, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Error ellipses at ends of velocity arrows in (b) define 95% confidence limits. SAF, San Andreas Fault; HF, Hayward Fault; CF, Calaveras Fault; GF, Greenville Fault. Velocity profile in (c) shows components of motion parallel and perpendicular to the San Andreas Fault. Profile passes through the Pacific-North American pole of rotation and the trajectory shown in (b). Error bars represent two standard deviations on either side of the plotted points.

(c)

Figure 8.19 Results from GPS measurements and block modeling of crustal motion in southern California (images provided by B. Meade and modified from Meade & Hager, 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). (a) Velocities observed during periods between earthquakes (i.e. interseismic velocities), when strain accumulations are elastic and appreciable slip on faults is absent. Confidence ellipses have been removed to reduce clutter. The two shaded swaths show regions in which fault parallel velocities are drawn in two profiles (b). Vertical lines in profiles give uncertainties of one standard deviation. Gray shaded areas show locations of the San Andreas Fault (SAF), San Jacinto Fault (SJF), and the Eastern California Shear Zone (ECSZ). Differences in velocity gradients reflect fault spacing. (c) Block model boundaries (white zones) superimposed on a shaded relief map showing major fault traces. (d) Residual velocities. Gray lines show block boundaries. Note that velocity vectors are drawn at a scale that is five times larger than in part (a).

Figure 8.19 Results from GPS measurements and block modeling of crustal motion in southern California (images provided by B. Meade and modified from Meade & Hager, 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). (a) Velocities observed during periods between earthquakes (i.e. interseismic velocities), when strain accumulations are elastic and appreciable slip on faults is absent. Confidence ellipses have been removed to reduce clutter. The two shaded swaths show regions in which fault parallel velocities are drawn in two profiles (b). Vertical lines in profiles give uncertainties of one standard deviation. Gray shaded areas show locations of the San Andreas Fault (SAF), San Jacinto Fault (SJF), and the Eastern California Shear Zone (ECSZ). Differences in velocity gradients reflect fault spacing. (c) Block model boundaries (white zones) superimposed on a shaded relief map showing major fault traces. (d) Residual velocities. Gray lines show block boundaries. Note that velocity vectors are drawn at a scale that is five times larger than in part (a).

the Sierra Nevada helps to explain the limited amount of slip that is observed on the San Andreas Fault.

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