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Stream Channel Offset and Late Holocene Slip Rate of the San Andreas Fault at the Van Matre Ranch Site, Carrizo Plain, California

¹ University of California, Irvine
Department of Environmental Health, Science and Policy
Irvine, California 92697-7070
 (G.R.N., L.B.G.)

² Arizona State University
Department of Geological Sciences
Tempe, Arizona 85287-1404
 (J R.A., J.J.Y.)

View larger version (145K): [in a new window]   Figure 1. (A) Major historic ruptures along the SAF (modified from Allen, 1968) showing location of VMR site in northwestern portion of SAF that ruptured in the 1857 earthquake. (B) Aerial photograph mosaic of Carrizo Plain showing active fault trace observations from Vedder and Wallace (1970) as well as locations of Bidart Fan (BF), Phelan Fan (PF), Wallace Creek (WC), and Van Matre Ranch (VMR) paleoseismic sites. Polygon at VMR shows the location of aerial photography in Figure 3.

View larger version (92K): [in a new window]   Figure 2. (A) Shaded relief, active faults (modified from Quaternary Fault and Fold Database for the United States, http://qfaults.cr.usgs.gov/) of southern California and selected site velocities from the Southern California Earthquake Center Crustal Motion Map (CMM3; Shen et al., 2003). White triangles show all CMM3 sites in the region, and those we selected from the group have velocities shown by arrows (note the velocity scale at the bottom). The maximum velocity uncertainty is 4.6 mm/yr, but the maximum in the N40°W direction is 1.6 mm/yr. The clear velocity gradient across the SAF is shown in (B). The location of the VMR site is indicated by the star. (B) The N40°W parallel component of the selected CMM3 velocities is plotted with respect to distance from the SAF and relative to 0 mm/yr at the SAF. The two curves show modeled velocities for steady interseismic slip below 12 km and bracket most of the observed motion. At left, the slip-rate estimate from our study of 35.4 ± 8.8 mm/yr with uncertainties indicated by the white boxes is drawn for comparison.

View larger version (104K): [in a new window]   Figure 3. (A) Rectified aerial photograph (9 March 1991; original scale 1:3,000) showing the clear traces of the SAF at the VMR site (see Fig. 1 for location) with numerous small offsets (those studied by Sieh [1978] are shown with corresponding numbers). In this study, we focused on channel 44 (the outline of Fig. 5 is shown by the black rectangle). (B) Oblique view to the east-southeast of aerial photo draped over photogrammetrically produced and digitized elevation data.

View larger version (126K): [in a new window]   Figure 4. (A) Overview to the west of the VMR site. The arrow shows the view direction of B. (B) View southeast along the SAF showing the upstream channel segment at the study site (behind person on left in B) leaving the upland watershed, slightly dammed by the shutter ridge to the right, and then the offset channel segment traversing the view under the nearer person and turning southwest just outside the view to the right. Trench T2 was dug parallel to the SAF and across the upstream channel mouth approximately where the person on the left crouches, and T3 was dug perpendicular to T2 up onto the shutter ridge. Compare with Figure 5.

View larger version (72K): [in a new window]   Figure 5. Topographic map of the study site with 1-m contour interval. Channels once connected to the main channel (southeast) have been offset as the three beheaded channels to the northwest. Our trench excavations exposed offset channels in T2, T3, T8, T6', and T7. The T7 channel has subsequently captured the small drainage immediately above it. The cross section below shows the topographic profile through the beheaded channels (dashed line on map) and the projected channel cross sections exposed and surveyed in the excavations. Note that the offsets determined by matching the channels defined in the trenches are different than those determined just from the offset geomorphology. Trench locations (closed polygons) are mapped from survey data.

View larger version (197K): [in a new window]   Figure 6. (a) Lithologic log of northeast wall of trench T2 (see Fig. 5 for location). Several nested channels are cut into the local Paso Robles bedrock unit. The channels are filled with clast- supported gravel that grade outward or are cut into and overlain by colluvial units or channel deposits that have been pedogenically altered. The lowermost channel thalweg (base of unit 210) is the match for the offset thalwegs exposed in T6', T8, and T7. The explanation indicates the patterns, line types, and labels used in all of the logs. (b) Lithologic log of the northwest wall of trench T3 (see Fig. 5 for location). This trench exposed three main shear zones that progressively cut up section from southwest to northeast. Gray-green unit 300 is correlative with unit 200 from trench T2 (Fig. 6a) and is juxtaposed against highly sheared and deformed tan Paso Robles or older Quaternary units to the southwest. Units 370 and 375 represent the channel fill sequence. The two channel fill packages are separated by a fissure fill unit that probably formed in the 1857 earthquake. See the trench log explanation in Figure 6a. (c) Lithologic log of the southwest wall of trench T6 (see supplemental Fig. 1a, available in the electronic supplement to this article, for the northeast wall and Fig. 5 for location). Clast-supported sand and gravel fluvial deposits fill nested channels cut at the base into local Plio-Quaternary bedrock (units 610 and 630). The base of unit 640 is the thalweg offset from the exposures in trench T2. Importantly, 14C sample T6'-4 dates the colluvium into which the channel was cut, providing a maximum channel age. See the trench log explanation in Figure 6a. (d) Lithologic log of the northeast wall of trench T8 (see supplemental Fig. 7b, available in the electronic supplement to this article, for the southwest wall and Fig. 5 for location). Clast-supported sand and gravel fluvial deposits fill nested channels cut at the base into local Plio-Quaternary bedrock (units 810 and 830). The base of unit 840 is the thalweg offset from the exposures in trench T2. Most of the 14C samples in this study come from the top of unit 860 and provide a minimum channel age. See the trench log explanation in Figure 6a. (e) Lithologic log of the northeast wall of trench T7 (see supplemental Fig. 7c, available in the electronic supplement to this article, for the southwest wall and Fig. 5 for location). Because of its older age, the stratigraphic relationships are more obscured than the younger trenches by pedogenesis. Nonetheless, unit 730 has eroded into the local bedrock (a Quaternary fan deposit) and its base represents the thalweg that we correlate with that exposed in trenches T2, T6', and T8. This channel thalweg is offset about 48.8 m. See the trench log explanation in Figure 6a.