Sequence-Stratigraphic Evidence for Late Quaternary Faulting and Folding in the Long Beach and Torrance Plains, Los Angeles Basin, California

Ponti, Daniel J.¹ (1) U.S. Geological Survey, Menlo Park, CA

A recently defined sequence-stratigraphic framework for the Quaternary of the southwestern Los Angeles basin, derived from core hole and high-resolution geophysical data, reveals a late Quaternary zone of deformation beneath the Long Beach and Torrance plains. The zone consists of broad folding with W-NW and N-NE-trending near surface high-angle faults – the Pacific Coast Highway (PCH) fault zone – that likely reflects a ~1 km-wide zone of right-lateral shearing and N-NE shortening. Deformation approximately parallels, but is several kilometers north of, the Wilmington anticlinorium trend and extends for at least 16 km from Naples through downtown Long Beach, and into the City of Torrance.

Fault displacement is inferred to occur 17 m subsurface and below, marked by a change in sedimentary facies that mostly expresses down-to-the-north vertical separation of up to 200 m. Faulting began about 600-700 ka and was most active about 500 ka during deposition of the type Silverado aquifer. Subtle folding of the surfaces of the Late Pleistocene-age Long Beach and Torrance plains indicates that deformation continued at least until 30 ka. Holocene deformation is not evident, but cannot be entirely ruled out.

While the current seismic hazard from the PCH fault is uncertain, the fault zone may serve to accommodate slip from the Newport-Inglewood fault as it approaches and terminates against the Transverse Ranges. Similar kinds of structures, associated with other major strike-slip faults, may therefore lie beneath the LA area and may factor into the regional seismic hazard.