3D Geometry and Slip Distribution in the Long Beach Earthquake Gate, Newport–Inglewood Fault, Los Angeles, California

Abstract

ABSTRACT We present a new, 3D representation of the Long Beach restraining bend system along the Newport–Inglewood fault (NIF), Los Angeles, California. The NIF is an active strike-slip system that cuts over 60 km through densely populated metropolitan Los Angeles and poses one of the greatest deterministic seismic hazards in the United States (California Division of Mines and Geology, 1988). Part of the NIF sourced the 1933 M 6.4 Long Beach earthquake, which claimed ∼120 lives and remains one of the deadliest events in California history (Barrows, 1974; Hauksson and Gross, 1991). The event is thought to have arrested at Signal Hill within the Long Beach restraining bend, which is formed by a left step in the NIF (Hough and Graves, 2020). Events that rupture through Signal Hill could generate larger (M ≈7) events that pose a significant hazard to urban Los Angeles. Our analysis integrates a diverse range of datasets, including over 4200 fault and horizon penetrations from 243 wells, 2D seismic reflection surveys, field maps, machine-learning-based tomography studies, and the U.S. Geological Survey QFaults surface traces. We show that the fault system in Long Beach has three main strike-slip segments connected by orthogonal reverse faults. The strike-slip faults are nonvertical and nonplanar, merge at depth, and extend through the seismogenic crust. The Long Beach restraining bend system presents numerous rupture pathways and arrest points that NIF earthquakes may follow. We apply a novel, map-based restoration to quantify how much total slip has passed through each of the fault segments. About 375 m of total slip is partitioned into the three main fault strands, including the Reservoir Hill fault (≈75 m), the Northeast Flank fault (≈120 m), and the Cherry Hill fault (≈209 m). This slip partitioning informs our understanding of the tendency of ruptures to involve different fault segments or arrest at specific junctures.