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We used high-resolution topography to quantify the spatial distribution of scarps, linear valleys, topographic sinks, and oversteepened stream channels formed along an extensional step over on the San Andreas Fault (SAF) at Mustang Ridge, California. This location provides detail of both creeping fault landform development and complex fault zone kinematics. Here, the SAF creeps 10-14 mm/yr slower than at locations ~20 km along the fault in either direction. This spatial change in creep rate is coincident with a series of en echelon oblique-normal faults that strike obliquely to the SAF and may accommodate the missing deformation. This study presents a suite of analyses that are helpful for proper mapping of faults in locations where high-resolution topographic data are available. Furthermore, our analyses indicate that two large subsidiary faults near the center of the step over zone appear to carry significant distributed deformation based on their large apparent vertical offsets, the presence of associated sag ponds and fluvial knickpoints, and the observation that they are rotating a segment of the main SAF. Several subsidiary faults in the southeastern portion of Mustang Ridge are likely less active; they have few associated sag ponds and have older scarp morphologic ages and subdued channel knickpoints. Several faults in the northwestern part of Mustang Ridge, though relatively small, are likely also actively accommodating active fault slip based on their young morphologic ages and the presence of associated sag ponds. doi: 10.1029/2010TC002673.

Surface ruptures from the 18 April 1906 M~7.9 San Francisco earthquake were distributed over an ~35-meter-wide zone at San Andreas Lake on the San Francisco Peninsula in California (Schussler, 1906). Since ~1906, the surface ruptures have been largely covered by water, but with water levels at near-historic low levels in 2008-2011, we observed that the 1906 surface ruptures were no longer visible. As a fault imaging test, we acquired refraction tomography and guided-wave data across the 1906 surface ruptures in 2011. We found that individual fault traces, as mapped by Schussler (1906), can be identified on the basis of discrete low-velocity zones (VS and VP, reduced ~40% and ~34%, respectively) and high-amplitude guided waves. Guided waves have traditionally been observed as large-amplitude waveforms over wide (hundreds of meters to kilometers) zones of faulting, but we demonstrate that by evaluating guided waves (including Rayleigh/Love- and P/SV-types) in terms of peak ground velocity (PGV), individual near-surface fault traces within a fault zone can be precisely located, even more than 100 yr after the surface ruptures. Such precise exploration can be used to focus paleoseismic trenching efforts and to identify or exclude faulting at specific sites. We evaluated PGV of both S-wave-type and Fϕ-mode-type guided waves and found that both wave types can be used to identify subsurface fault traces. At San Andreas Lake (main fault), S-wave-type guided waves travel up to 18% slower than S body waves, and Fϕ-mode guided waves travel ~60% slower than P body waves but ~15% faster than S body waves. We found that guided-wave amplitudes vary with frequency but are up to five times higher than those of body waves, including the S wave. Our data are consistent with the concept that guided waves can be a strong-shaking hazard during large-magnitude earthquakes. [ABSTRACT FROM AUTHOR]

Surface fracturing occurred along the San Andreas fault, the subparallel Southwest Fracture Zone, and six secondary faults in association with the 28 September 2004 (M 6.0) Parkfield earthquake. Fractures formed discontinuous breaks along a 32-km-long stretch of the San Andreas fault. Sense of slip was right lateral; only locally was there a minor (1-11 mm) vertical component of slip. Right-lateral slip in the first few weeks alter the event, early in its afterslip period, ranged from 1 to 44 mm. Our observations in the weeks following the earthquake indicated that the highest slip values are in the Middle Mountain area, northwest of the mainshock epicenter (creepmeter measurements indicate a similar distribution of slip). Surface slip along the San Andreas fault developed soon after the mainshock; field checks in the area near Parkfield and about 5 km to the southeast indicated that surface slip developed more than 1 hr but generally less than 1 day after the event. Slip along the Southwest Fracture Zone developed coseismically and extended about 8 km. Sense of slip was fight lateral; locally there was a minor to moderate (1-29 mm) vertical component of slip. Right-lateral slip ranged from 1 to 41 mm. Surface slip along secondary faults was right lateral; the fight-lateral component of slip ranged from 3 to 5 mm. Surface slip in the 1966 and 2004 events occurred along both the San Andreas fault and the Southwest Fracture Zone. In 1966 the length of ground breakage along the San Andreas fault extended 5 km longer than that mapped in 2004. In contrast, the length of ground breakage along the Southwest Fracture Zone was the same in both events, yet the surface fractures were more continuous in 2004. Surface slip on secondary faults in 2004 indicated previously unmapped structural connections between the San Andreas fault and the Southwest Fracture Zone, further revealing aspects of the structural setting and fault interactions in the Parkfield area. [ABSTRACT FROM AUTHOR]