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Seafloor expression of active transpressional faulting offshore Southern California

Mark R. Legg, Simon L. Klemperer, Christopher M. Castillo, Marie-Helene Cormier, Michael Brennan, Katy Croff Bell, Dwight Coleman, Chris Goldfinger, & Jason Chaytor

Published August 15, 2016, SCEC Contribution #7043, 2016 SCEC Annual Meeting Poster #113

Recent observations using Remotely Operated Vehicles (ROV) of the seafloor along active transpressional fault zones offshore southern California reveal the morphology of active fault ruptures in the deep marine environment. Pressure ridges were first identified using high-resolution multibeam bathymetry and subsequently investigated using high resolution MCS surveys conducted by Stanford and Oregon State university scientists. These MCS data were used to guide ROV Hercules operated from the Ocean Exploration Trust Exploration Vessel Nautilus. These pressure ridges were located within long, narrow, hillside valleys on the flanks of major transpressional uplifts including the Santa Cruz-Catalina Ridge in the Inner Borderland and Southwest Bank in the Outer Borderland. In general, the pressure ridge is expressed in the bathymetry as an elongate and somewhat sinuous ridge with relatively minor seafloor relief. Although located within the sedimentary fill of significant hillside valleys, these ridges are covered with angular blocks of bedrock, inferred to be brecciated and squeezed upward during large earthquake ruptures along the fault. In a sense, these may represent the long term expression of mole tracks, common to large strike-slip earthquake ruptures in alluvial basins onshore, but better preserved over multiple earthquake cycles in the deep marine environment where erosion is subdued and sedimentation is slow. Along the flanks of the hillside valley are steep to vertical scarps in bedrock outcrops between sediment cover with angular blocks in talus slopes below. Scarps exposed near the crest of large restraining bend pop-up structures show complex fracturing and fault trends consistent with dextral shear in the hanging wall of a moderate-to steep-dipping primary fault. These seafloor observations are consistent with large-scale transpressional structure identified in geophysical data (seismic reflection surveys). The lateral scale of the features, >10 km, and vertical relief 10-1000 meters (or greater) may represent infrequent large earthquakes (M>7) and potential for local tsunami generation. As yet, fault slip rates are unknown but the new data may provide first-order estimates of deformation rates for these prominent offshore structures.

Key Words
transpression, seafloor fault rupture, strike-slip, California Continental Borderlad

Legg, M. R., Klemperer, S. L., Castillo, C. M., Cormier, M., Brennan, M., Bell, K., Coleman, D., Goldfinger, C., & Chaytor, J. (2016, 08). Seafloor expression of active transpressional faulting offshore Southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology