Geohazard at Velero Basin, Outer California Borderland: mapping seafloor mass wasting using new high-resolution marine geophysics

Andrea Fabbrizzi, Jillian M. Maloney, & Bradley Keith

Published September 8, 2024, SCEC Contribution #13890, 2024 SCEC Annual Meeting Poster #102 (PDF)

Most seafloor slopes are shaped by a gravity sediment transport process known as submarine mass wasting, frequently associated with seismic activity on tectonically active margins. Although poorly understood and difficult to reach, offshore active fault systems and seafloor mass wasting pose a risk to infrastructure and coastal populations. Thus, mapping and describing the dynamics of mass wasting is important for geohazard evaluations and tsunami modeling. The mass wasting that occurs on active margins, like the California Continental Borderland, is typically located along strike-slip fault scarps that have vertical components of slip, causing oversteepening of basin slopes and failure initiation during rupture. In the Outer California Borderland (OCB), the Ferrelo fault is a continuous right-lateral strike-slip fault extending ~400 km across several basins (e.g., from north to south, Santa Cruz, San Nicolas, Tanner, Cortes, and Velero basins). In the southern OCB, approximately 190 km west of San Diego, we mapped an ~8 km3 by volume mass-transport complex (MTC) in Velero Basin with multibeam bathymetry and high-resolution sub-bottom. The Velero Basin is one of the most distal semi-enclosed, fault-bounded basins in the Borderland, and it is bounded at west by the southern prolongation of the Ferrelo fault, running parallel to the basin margin at the upper slope. Within the basin, we identified a southern MTC composed of multiple channel-fans and debris fans on the basin floor, locally imbricated, and with stratigraphic overlap; this MTC funnels sediment downslope from an upper slope source. The northern MTC is characterized by coalescent MTDs along-strike on the lower slope and basin floor, and associated with the failure of discrete slope sections. At least two distinct slope collapses are defined by multiple evacuation scars, which suggests progressive downslope progradation of the failure. The differences between the southern and northern MTCs may be related to a change of strike in the southern Ferrelo fault zone that is suggestive of a left-stepping restraining stepover. The mass wasting could then be related to fault steepening, which contributes to slope instability and collapse. The mass wasting could also be triggered by earthquakes on the fault zone, but better age constraints are needed to evaluate the paleoseismic history of the basin.

Key Words
Mass wasting, Fault-related submarine landslides, Slope failure, Southern California Borderland, Velero Basin

Citation
Fabbrizzi, A., Maloney, J. M., & Keith, B. (2024, 09). Geohazard at Velero Basin, Outer California Borderland: mapping seafloor mass wasting using new high-resolution marine geophysics. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology