MISINTERPRETATION OF LATERAL ACOUSTIC VARIATIONS ON HIGH-RESOLUTION SEISMIC-REFLECTION PROFILES AS FAULT OFFSETS OF HOLOCENE BAY MUD BENEATH THE SOUTHERN PART OF SAN-FRANCISCO BAY, CALIFORNIA

We collected high-resolution seismic reflection profiles in the southe rn part of San Francisco Bay in 1992 and 1993 to investigate possible Holocene faulting along postulated transbay bedrock fault zones. The i nitial analog records show apparent offsets of reflection packages alo ng sharp vertical boundaries. These records were originally interprete d as showing a complex series of faults along closely spaced, sharp ve rtical boundaries in the upper 10 m (0.013 s two-way travel time) of H olocene bay mud. A subsequent survey in 1994 was run with a different seismic reflection system, which utilized a higher power source. This second system generated records with deeper penetration (max. 20 m, 0. 026 s two-way travel time) and demonstrated that the reflections origi nally interpreted as fault offsets by faulting 19 were actually latera lly continuous reflection horizons. The pitfall in the original interp retations was caused by lateral variations in the amplitude brightness of reflection events, coupled with a long (greater than 15 ms) source signature of the low-power system. These effects combined to show app arent offsets of reflection packages along sharp vertical boundaries. These boundaries, as shown by the second system, in fact occur where t he reflection amplitude diminishes abruptly on laterally continuous re flection events. This striking lateral variation in reflection amplitu de is attributable to the localized presence of biogenic(?) gas.