Newly recognized active blind thrust faults in the northwest Los Angeles basin

Gabrielle Ursin, John H. Shaw, & Andreas Plesch

Submitted August 30, 2026, SCEC Contribution #15233, 2026 SCEC Annual Meeting Poster #TBD

Our research characterizes previously undocumented blind thrust faults in the northwestern Los Angeles basin that deform Pleistocene sediments and represent a potential unaddressed seismic hazard. Imbricate thrust faults pose some of the most significant earthquake hazards on the planet but are challenging to study due to their concealed nature, complex architecture, and rupture behaviors. The Los Angeles basin, a transpressional region influenced by the San Andreas fault restraining bend, offers an exceptional setting to study such systems. The basin contains a dense network of blind thrust and strike-slip faults that have sourced both historic earthquakes and those defined in the paleoseismic record. This study characterizes the area extending from Larchmont (near the Salt Lake oil field) to Westwood (near the Sawtelle oil field). Seismicity here is abundant but inadequately explained by structures in the SCEC CFM v7.

We integrate seismicity, surface traces, well data, seismic reflection profiles, cross sections, maps, and water well records using CFM workflows in MOVE and SKUA to investigate faults in the region. Fault-propagation folding theories (Suppe et al., 1990; Allmendinger, 1998; Shaw et al., 2005) are applied to derive fault geometries consistent with stratigraphic and structural data. The resulting subsurface model is expressed as balanced 2D cross sections and 3D fault surfaces. Results reveal at least two thrust faults not represented in the CFM, tentatively named the Sawtelle and Rancho faults. The Sawtelle fault is a newly defined blind thrust system, while our work refines the geometry and extent of the Rancho fault previously described by Tsutsumi et al. (2001). Both faults dip north at moderate angles (≈50°), with displacements that form fault-propagation folds that define the southern margins of the Sawtelle and Cheviot Hills fields. While the Late Pleistocene to Holocene activity of the faults remains undefined, these surfaces provide candidate sources for earthquakes currently unattributed in the CFM v7 and a basis for assessing the potential for future events, including multi-fault rupture scenarios.

Our fault interpretations can guide paleoseismic investigations to constrain recent fault activity, slip rate, and rupture history as well as inform dynamic rupture and strong ground-motion modeling of future scenarios. These results will support probabilistic and deterministic hazard assessments and inform updates to regional models.

Key Words
community fault model, seismic hazards, blind thrusting

Citation
Ursin, G., Shaw, J. H., & Plesch, A. (2026, 08). Newly recognized active blind thrust faults in the northwest Los Angeles basin. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)