New Maps Assessing the Potential Offshore Seismic Hazards Using High-Resolution Geophysical and Stratigraphic Records of the Santa Maria and Santa Barbara Basins, Offshore California

J. M. Saenz¹, K. Zaiger², T. J. O'Neil³, D. Vann¹, F. Denison⁴, and G. Simila^5
1US Department of the Navy, Naval Facilities Engineering Service Center, EV 411, Port Hueneme, CA, [email protected], [email protected]
²US Department of the Navy, Naval Facilities Engineering Service Center, OF 51, Port Hueneme, CA, [email protected]
³Oxnard College, Oxnard, CA, [email protected]
⁴Frank Denison Engineering Geology, [email protected]
⁵California State University, Northridge, CA, [email protected]

New shallow hazard maps were constructed using marine high-resolution seismic reflection data, well logs, and piston cores. These new shallow hazard maps were used to investigate geologic structure, stratigraphy, shallow-gas horizons, seismology, and mass movement deposits of an active part of an offshore fault system, the Hosgri, the Purisima, the Lompoc, and the North Channel fault systems of the Santa Maria and Santa Barbara Basins, Offshore California. The main active structure is the Hosgri fault, an active right-lateral strike-slip fault with a reverse component of movement of Pliocene to Recent age. In the study area, the fault is 50 km long, exhibits shallow crustal seismicity, is associated with shallow-gas horizons, numerous seafloor features (gas-vent craters, pockmarks, tar mounds, buried channels, seafloor rubble and hummocks), en-echelon anticlinal folds, and a seafloor scarp. Shallow structures of fault segments vary in length up to 30 km, and are delineated using geometric, structural, and stratigraphic information.

We postulate that earthquake energy is amplified in faults and folds, interpreted from using geophysical data, evidenced by seismic activity, and fluid migration in seafloor features. Maximum credible earthquake magnitudes, based upon seismic moment calculations, surface rupture lengths, and vertical surface offsets, suggest that historic earthquakes (e.g., November 4, 1927 M_W 7.3 Lompoc Earthquake) formed the present seafloor surface trace. Seismically induced bedrock acceleration rates are estimated to range between 0.1 g and 0.2 g with a 100-year return period, and 0.6 g for a 2500-year return.

Gas plumes deeply sourced in the Monterey Formation migrate vertically upward along faults, anticlinal folds, and steeply dipping beds into near-surface sediment, and escape through the seafloor, forming gas-vent craters. Linked to high geothermal gradients and controlled by active tectonics, a relationship between sedimentation rates, fluid migration, and amplification of seismic energy within geologic structures affects the distribution and size of seafloor features.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009