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.

中文翻译：

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使用折射层析成像和S波型和Fϕ模式导波的San Andreas断层勘探

1906年4月18日旧金山M.7.9地震的地表破裂分布在加利福尼亚旧金山半岛San Andreas湖的约35米宽区域（舒斯勒，1906年）。自1906年以来，地表破裂大部分被水覆盖，但是在2008-2011年水位处于近历史低位时，我们观察到1906年的地表破裂不再可见。作为故障成像测试，我们在2011年获得了1906年表面破裂的折射层析成像和导波数据。我们发现，可以根据离散的低速带确定舒斯勒（1906）绘制的单个断层迹线。 （⁠VS和VP⁠，分别减少了约40％和〜34％⁠）和高振幅导波。传统上，导波在断层的宽泛区域（几百米到几千米）以大振幅波形被观察到，但是我们通过对导波（包括瑞利/洛夫和洛夫/洛维奇和普维特-SV型）进行评估来证明这一点。速度（PGV），即使在地表破裂后甚至超过100年，也可以精确定位断层带内的各个近地表断层迹线。这种精确的勘探可以用来集中古地震的挖掘工作，并确定或排除特定地点的断层。我们评估了S波型和Fϕ型导波的PGV，发现这两种波型均可用于识别地下断层迹线。在圣安德烈亚斯湖（主要断层），S波型导波的传播速度比S体波慢18％，和Fϕ模式导波的传播速度比P体波慢60％，但比S体波快15％。我们发现导波振幅随频率而变化，但比体波（包括S波）高出五倍。我们的数据与大地震中导波可能会引起强烈震动的概念相一致。