Strain Partitioning Along The Onshore Palos Verdes Fault Zone

High-res 2D seismic reflection data were acquired along the LACSD’s proposed JWPCP Effluent Outfall Tunnel alignment to image the Palos Verdes fault zone (PVFZ) in the eastern Palos Verdes Peninsula (PVP).

23 Apr 2015
Josh Goodman, Dean Ostenaa, Phil Hogan, Dan O'Connell, Jamey Turner

Abstract

Reflection data were used with aerial photography and DEMs of early 20th-century topography to refine the location, geometry and kinematics of shallow faulting and folding along the alignment. In the eastern PVP, the PVFZ is approximately 2,000 m wide and consists of three principal strands: the southern, middle and northern splays (PFVS, PVFM and PVFN, respectively). Collectively, these faults define a steep, asymmetric flower structure. The dip and style of faulting changes progressively to the northeast: PVFS is a near vertical strike-slip fault; PVFM is a right-oblique reverse fault with an 80- to 85-deg southwest dip; and PVFN is a blind reverse fault with a 75-deg southwest dip. Our preferred estimates of right-lateral offset of an abandoned channel of the LA River that crosses the PVFZ are 109 m (PVFS) and 250 m (PVFM). No discernible lateral offset is observed where the abandoned channel crosses PVFN. The abandoned channel is incised into the OIS 5e marine terrace (120 ka). Assuming a ~5,000-year lag between the 5e highstand and subsequent channel incision, our preferred post-Late-Pleistocene slip rates are 0.91 mm/yr (PVFS) and 2.17 mm/yr (PVFM), though uncertainties in age and offset amounts permit a range of 2 to 5.6 mm/yr for the entire PVFZ. Topographic profiles of the OIS 5e abrasion platform and thalweg of the abandoned channel show approximately 115 m of uplift across the PVFZ since 120 ka, and >15 m since channel abandonment, ~35 ka. The northern edge of the broad uplift pattern has a sharp hinge above the tipline of PVFN and north of the PVFM; it then tapers off across and south of PVFS. Distribution and patterns of deformation across the PVFZ may reflect partitioning of oblique slip in the upper crust due to a change in fault strike at the PVP.

 

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