Characterization of the Continental Slope and Headscarp of the Palos Verdes Debris Avalanche Based on New High-Resolution Seismic Reflection Data

R. D. Dame and P. J. Hogan
Fugro West Inc., Ventura, CA, [email protected], [email protected]

In 2004, 2006, and 2008 Fugro West collected high-resolution subbottom profiler and multichannel seismic reflection data on the Palos Verdes (PV) and San Pedro (SP) Shelves along several offshore alignment routes under consideration for a proposed ocean outfall tunnel for the Sanitation Districts of Los Angeles County. Subsurface characterization of the PV Shelf was of specific interest to the Districts, as the alignment corridor proposed within this area would be substantially shorter and less expensive than the alternate SP Shelf route.

Integration of seismic and geotechnical data was key for subsurface characterization within the project area. Stratigraphic units mapped by Fugro (2004, 2006, 2007) on the SP Shelf include the Monterey Formation, Malaga Mudstone, and Fernando Formation. These units were correlated to the PV Shelf based on seismic character and reflection patterns previously observed in seismic data (Bohannon et al., 2004). Late Pleistocene and Holocene sediments on the PV Shelf were interpreted from CPT and Vibracore explorations, depositional geometry and orientation relative to underlying strata, and seismic reflection data characteristics observed by Fugro (2007), Bohannon et al. (2004), and Normark et al. (2004).

The multichannel seismic lines were recorded across the shelfbreak and downslope across the San Pedro Sea Valley to assist in characterizing the Palos Verdes Debris Avalanche and potential slope stability implications for the PV Shelf alignment route. The interpreted data provided direct imaging of the near surface structure and stratigraphy, and correlated well with Normark et al.’s (2004) interpretation of a major slope failure event in this area approximately 7500 yBP. A thick accumulation of stacked mass movement deposits at the base of the slope and rapid seismic signal attenuation through chaotic near-surface material zones suggest multiple slope failures on a variety of scales have occurred episodically in this area throughout the late Quaternary.

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