Dynamic branching 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 August 30, 2026, SCEC Contribution #15557, 2026 SCEC Annual Meeting Poster #TBD

The right-lateral Newport-Inglewood fault (NIF) system cuts across the densely populated Los Angeles (LA) metropolitan area and sourced the destructive 1933 MW 6.4 Long Beach earthquake, demonstrating its significant seismic hazard. At Long Beach, it manifests as a complex system of splay faults and linking reverse faults, leading to uplift of Signal Hill and Reservoir Hill. Oil industry logs and other data sets help to precisely define the fault geometry. Offset geological markers and topographic evidence aid in determining the slip history across the multiple splays [Toghramadjian and Shaw, 2024].

We use the 3D discontinuous Galerkin method [Zhang et al., 2023] to model the dynamics of potential earthquakes on the NIF in the Signal Hill region. Our goal is to explain the complex rupture paths implied by observational data. We find that the ability of rupture to propagate on the various fault segments depends strongly on the rupture propagation direction and the velocity structure in the region. In particular, velocity structures that have higher wavespeeds at depth (including 1D layer-cake models and the full 3D SCEC CVM) produce qualitatively different rupture branching paths than a homogeneous velocity structure. In addition, we find that rupture through the stepover is facilitated by slip on the splay faults and inhibited when the splay faults do not participate. The latter effect could help explain the termination of the 1933 Long Beach earthquake at this location [Hough and Graves, 2020]. These observations emphasize the importance of subsurface fault connectivity on rupture propagation, particularly in the presence of depth-dependent wavespeeds.

Key Words
Rupture Dynamics, Fault Geometry, Fault Slip, Seismic Wavespeed

Citation
Oglesby, D. D., Toghramadjian, N., Plesch, A., Shaw, J. H., & Zhang, W. (2026, 08). Dynamic branching and surface rupture in the Signal Hill Stepover on the Newport-Inglewood Fault, Southern California. Poster Presentation at 2026 SCEC Annual Meeting.

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