Evidence for water-filled cracks in earthquake source regions

We identify lowered Vp/Vs ratios near earthquake source regions in southern California using observations from a seismic tomography model and high‐resolution local Vp/Vs estimates using waveform cross‐correlation data from within similar event clusters. The median tomographic Vp/Vs ratio is 1.716 ± 0.008 at all the relocated crustal earthquake locations, compared to the background median value of 1.729 ± 0.007 for the tomography model, although the error estimates overlap slightly. The median in situ Vp/Vs ratio of 1.673 ± 0.022 within the similar event clusters suggests that tomographic studies are overestimating Vp/Vs at source regions. Interpretation of Vp/Vs anomalies is complicated by the scatter in values obtained for individual clusters and in comparisons to absolute Vp and Vs velocities in the tomography model. However, the low Vp/Vs ratios measured for the seismicity clusters are hard to explain with known rocks and suggest the presence of water‐filled cracks with several percent porosity in earthquake source regions in southern California, which likely has an effect on faulting and earthquake activity.