NEAR-SURFACE GEOPHYSICAL INVESTIGATIONS OF THE HAYWARD AND GREEN VALLEY FAULTS

We conducted seismic refraction and ground-penetrating radar (GPR) surveys at two active right-lateral strike-slip fault zones in the San Francisco Bay Area. Seismic refraction data were recorded using a 24-channel seismograph with sledgehammer source. GPR data were recorded using 50 MHz antennae and 0.5 m trace spacing.

The primary geophysical targets on the Green Valley fault are buried paleochannels that can serve as piercing points for the determination of fault offset. Seismic refraction data indicate three layers in the near-surface zone (as deep as 10 m), with seismic P-wave velocity (V_P) ranging from ~100 m/s to ~1700 m/s. Several GPR lines at this site appear to show paleochannel features at ~2-6 m depths. With auger testing at one suspected paleochannel we found a rounded cobble at 2.7 m depth and smaller pebbles at 3.0 m depth, which would be consistent with remnants of a paleochannel. Cone Penetrometer Test (CPT) holes were logged at several points along seismic and GPR data lines, providing further ground-truth data.

The second site is Tyson's Lagoon, a sag pond between parallel strands of the southern Hayward fault. Detailed trench logs, borehole logs and CPT logs are available. Preliminary refraction data indicate a top layer with V_P ~300 m/s, underlain by material with V_P ~1900 m/s. Two GPR lines appear to image the same surface detected by seismic refraction. Borehole and CPT logs indicate the presence of a surface of significant material contrast at similar depths, inferred to be the contact between Pleistocene gravels and overlying Holocene deposits. We interpret the surface detected through seismic and GPR to be the same contact. None of the trenches in Tyson's Lagoon were deep enough to detect this surface. One of the seismic refraction lines indicates a trough-like feature at depth. This feature may be a cross-fault and could explain the early history of the opening of the sag pond.

We use GPR and seismic refraction as complementary techniques in an attempt to 1) determine optimal locations for proposed trenches, 2) correlate geophysical data with ground-truth data, and ultimately 3) develop “virtual trenching” methods using geophysical techniques to extend real trenches beyond their actual lengths and depths.