SEISMIC REFRACTION AT TYSON'S LAGOON, SOUTHERN HAYWARD FAULT

We recorded three seismic refraction lines near the southern end of the active trace of the Hayward fault at Tyson's Lagoon in Fremont, California. Tyson's Lagoon is a sag pond situated at a stepover between two parallel traces of the Hayward fault, an active right-lateral strike-slip fault that runs along the eastern side of San Francisco Bay. We used a 24-channel seismograph with a sledgehammer source to record three intersecting refraction lines: one 146 m long line crossing the western trace of the fault, and two ~60 m long lines between the two traces of the fault. The refraction data indicate a strong velocity contrast at 1-10 m depth, with seismic P-wave velocity (V_P) changing from V_P ~300 m/s in the top layer to V_P ~1900 m/s in the second layer. This velocity contrast occurs at ~10 m depth in the central portion of Tyson's Lagoon, between the fault traces, and it increases to ~1 m depth outside of the pond, on the west side of the western fault trace. We also recorded two GPR (Ground-Penetrating Radar) lines at this site that appear to image the same surface detected by seismic refraction. Prior trenching at the site has precisely located the western trace of the fault, and borehole logs and Cone Penetrometer Test (CPT) data indicate a transition from Holocene deposits and fill to Pleistocene gravels at ~3 m depth on the west and ~15 m on the east side of the west fault trace. The inferred Holocene-Pleistocene surface is deeper on the east side than the west, and the GPR lines indicate that this surface is dipping to the northwest. In this study, seismic refraction and GPR are complementary techniques: seismic refraction provides good depth control and seismic velocities, while the GPR provides a more continuous subsurface image. One of the goals of this work is to develop “virtual trenching” methods that use geophysics to laterally extend real trenches beyond their actual dimensions and/or to aid in optimal location of proposed trenches. We continue to refine our processes for employing these two geophysical methods together at other sites where direct subsurface sampling methods are not facilitated.