LOCATING 1906 SAN FRANCISCO EARTHQUAKE SURFACE FAULT RUPTURES USING PGV OF GUIDED WAVES

Schussler (1906) showed that surface ruptures from the 18 April 1906 M ~7.9 San Francisco earthquake were distributed over an ~35-m-wide zone at San Andreas Lake on the San Francisco Peninsula in California. Since ~1906, the surface ruptures were 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 at the surface. We acquired refraction tomography and guided-wave data across the zone of 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 Vs (~40%) and Vp (~ 34%) zones and high-amplitude guided waves. Guided waves have previously been used to locate faults, which appear as high-amplitude waveforms over wide (hundreds of meters to kilometers) zones, 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 years 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 Ff-mode-type guided waves and found that both wave types can be used to identify subsurface fault traces. At San Andreas Lake (main fault), the S-wave-type guided waves travel up to 18% slower than S body waves, and Ff-mode guided waves travel ~ 60% slower than P body waves but ~15% faster than the 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.