Dynamic models of branching faults and surface rupture in the Signal Hill Stepover on the Newport-Inglewood Fault, Southern California

David D. Oglesby, Natasha Toghramadjian, Andreas Plesch, John H. Shaw, & Wenqiang Zhang

Submitted September 7, 2025, SCEC Contribution #14780, 2025 SCEC Annual Meeting Poster #TBD

The right-lateral Newport-Inglewood Fault (NIF) system cuts across the highly populated Los Angeles (LA) metropolitan area. A segment of the fault sourced the highly destructive 1933 MW 6.4 Long Beach Earthquake, and the system poses significant seismic hazard to Southern California. Throughout the LA area the fault is highly segmented in its surface expression. At Long Beach, it manifests as a complex system of splay faults and linking reverse faults, leading to local uplift of Signal Hill and Reservoir Hill. Oil industry logs and other data sets help to precisely define the fault geometry in this area, while offset geological markers and topographic evidence help to determine the slip history across multiple splays of this fault system [Toghramadjian and Shaw, 2024].

To understand the past behavior and future earthquake potential of this region, we use the 3D discontinuous Galerkin method [Zhang et al., 2023] to model the dynamics of potential earthquakes on the NIF system in the Signal Hill region. Our main goal is to determine which factors determine the complex rupture paths implied by observational data. We are also exploring what factors may cause ruptures to terminate at the restraining bend system, as is thought to have occurred in the 1933 earthquake [Hough and Graves, 2020]. We find a strong directional dependence in the ability of earthquakes to propagate to the splay and reverse faults: earthquakes propagating from southeast to northwest can activate the splay faults, resulting in vertical uplift of the local hills, in agreement with observations. In contrast, earthquakes that propagate from northwest to southeast approach the branches in the reverse direction, for which propagation to the splays is dynamically-inhibited backwards branching [e.g., Kame et al., 2003]. In such cases, there is essentially no uplift of the hills in the region. A lack of geometrically continuous, through-going fault segments to the northwest of the Signal Hill region may help to explain a lack of rupture propagation from that direction, and consequently a bias of ruptures arriving from the southeast, leading to the formation of the hills in the region. The results have implications for the surface rupture and ground motion hazard in this densely populated region, as well as for complex strike-slip/reverse-faulting systems worldwide.

Key Words
Earthquake Dynamics, Earthquake Modeling, Fault Geometry, Slip History

Citation
Oglesby, D. D., Toghramadjian, N., Plesch, A., Shaw, J. H., & Zhang, W. (2025, 09). Dynamic models of branching faults and surface rupture in the Signal Hill Stepover on the Newport-Inglewood Fault, Southern California. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)