Activity and earthquake potential of the Wilmington blind thrust, Los Angeles, CA: The largest earthquake source not on current southern California hazard maps?

Franklin D. Wolfe, James F. Dolan, Andreas Plesch, & John H. Shaw

Published August 15, 2017, SCEC Contribution #7772, 2017 SCEC Annual Meeting Poster #128

The Wilmington blind thrust fault may represent one of the largest deterministic seismic hazards in the United States, in that it extends for more than 30 km along strike beneath the densely populated Los Angeles metropolitan area and the Ports of Los Angeles and Long Beach. The fault has been known for decades (it underlies one of the largest oil fields in southern California), but is not currently included in earthquake hazard assessments. This reflects a long-held view that the fault became tectonically inactive in the late Pliocene. However, offshore 3D seismic reflection data and recent mapping of aquifers in the southwestern Los Angeles basin (Ponti et al., 2007) suggest that Late Quaternary strata are folded and uplifted above this structure.

In this study, we used 2- and 3-D geophysical surveys, well data, and modeling techniques to define the geometry and displacement history of the Wilmington fault. This analysis suggests that the overlying Wilmington anticline is a fault propagation fold with a steep forelimb that is constrained by well picks and dip meter logs. The fault dips ~48-52° NE, and has ~1200-1400m of reverse offset. Footwall ties from the adjacent seismic surveys in the Inner Borderlands and associated wells show an upward decreasing displacement of Miocene and Pliocene units along the fault. Lastly, forward and inverse modeling tools were employed to develop balanced and retro-deformable cross sections that are consistent with these findings and suggest the fault tip does not reach the surface (i.e., the structure is blind).

Mapping of shallow aquifers and axial surfaces on the forelimb of the Wilmington anticline within the Los Angeles Harbor and San Pedro Shelf region clearly demonstrate that folding above the tipline of the Wilmington blind-thrust uplifts and deforms Quaternary strata. The youngest of these strata thin onto the crest of the Wilmington anticline, implying they were deposited syn-tectonically. This suggests that reactivation occurred at ~450ka and has continued in recent times (youngest unit resolvable is ~30ka).

Results of this study will directly contribute to the improvement of regional earthquake hazard assessments and shaking hazard maps by defining how the Wilmington thrust should be considered in such analyses. Representations of the Wilmington blind-thrust fault will also be incorporated into the SCEC CFM, California Reference Fault Parameter Database, and the USGS Fault and Fold database.

Key Words
earthquake, Quaternary, reactivation, hazard, seismic, LA, Los Angeles, southern California

Wolfe, F. D., Dolan, J. F., Plesch, A., & Shaw, J. H. (2017, 08). Activity and earthquake potential of the Wilmington blind thrust, Los Angeles, CA: The largest earthquake source not on current southern California hazard maps?. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology