Poster #071, Earthquake Geology

3D geometry and kinematics of the restraining bend at Signal Hill: implications for seismic hazard for the Newport-Inglewood fault, Los Angeles, CA

Natasha Toghramadjian, & John H. Shaw
Poster Image: 

Poster Presentation

2021 SCEC Annual Meeting, Poster #071, SCEC Contribution #11434 VIEW PDF
The Newport-Inglewood fault (NIF) is a complex strike-slip system that cuts over 50 kilometers through metropolitan Los Angeles, forming the southwestern edge of the Los Angeles basin. The fault poses one of the greatest deterministic seismic hazards in the nation.

The last large rupture on the NIF was the 1933 M6.4 Long Beach earthquake. This earthquake—southern California’s deadliest—ruptured only a portion of the fault. The rupture is thought to have arrested at Signal Hill, a large restraining bend formed by a left step in the fault that lies within the Long Beach oil field. The restraining bend at Signal Hill is defined by the Pickler fault, which trends perpendicular to ...
the NIF, dips ~78° SE, and accommodates pure dip-slip motion. The Pickler fault represents a direct physical connection between regional-scale strike-slip strands of the NIF.

Signal Hill provides an excellent opportunity to study the character of restraining bends that may limit the extent of ruptures on strike-slip faults. We leverage an exceptional subsurface dataset to define the subsurface geometry of the restraining bend at Signal Hill and investigate its implications for the deformational history and activity of the strike-slip NIF. We integrate mapped Quaternary active surface fault traces (USGS QFaults), industry field maps of faults and subsurface rock units, hundreds of oil well logs that include direct fault picks, and 2D seismic reflection surveys. We cross-validate these independent constraints to construct a robust, high-resolution 3D fault model of the Signal Hill restraining bend and surrounding Long Beach segment of the NIF.

We further map and analyze numerous stratigraphic horizons of Pliocene through Pleistocene age to constrain the deformational history of the system. From these data, we calculate uplift and slip on the NIF that has been translated through Signal Hill. This, in turn, is used to calculate a minimum slip rate on the fault.

Our new fault model represents an improved understanding of the precise geometry and kinematics of restraining bends that may impact earthquake rupture processes, and this has important implications for the seismic hazards of the NIF and similar fault systems worldwide.