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Group B, Poster #074, Earthquake Geology

Multiple rupture pathways and slip partitioning through the Long Beach restraining bend system of the Newport-Inglewood fault, Los Angeles, CA

Natasha Toghramadjian, & John H. Shaw
Poster Image: 

Poster Presentation

2023 SCEC Annual Meeting, Poster #074, SCEC Contribution #13142 VIEW PDF
We present a new, 3-D 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 kilometers through densely populated metropolitan Los Angeles and poses one of the greatest deterministic seismic hazards in the US. Part of the NIF sourced the 1933 Long Beach (M6.4) 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, a large restraining bend formed by a left step in the Long Beach segment of the NIF (Hough and Graves, 2020). Events ...that rupture through Signal Hill could generate 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 USGS QFaults surface traces. We show that the fault system has three main strike-slip segments connected by orthogonal reverse faults. The strike-slip faults are non-vertical and non-planar, merge into one another both at the surface and at depth, and extend through the seismogenic crust. Our analysis also defines a broad zone of secondary faulting (up to ≈2km wide) that may pose additional hazards.

The complex, multi-stranded Long Beach system presents a diverse range of both rupture pathways and potential arrest points, or “earthquake gates,” that NIF earthquakes propagating through may encounter. To explore how past earthquakes interacted with this fault geometry, we quantify how much total slip has passed through each of the fault segments through a novel, map-based horizon restoration of the pre-growth, Lower Pliocene-age Brown unit. About 375m of total post-Miocene strike-slip is partitioned into the three main fault strands, namely the Reservoir Hill fault (≈75m, ~18% of total slip), the Northeast Flank fault (≈120m, ~27% of total slip), and the Cherry Hill fault (≈209 m, ~55% of total slip). This slip partitioning informs our understanding of the tendency of ruptures to involve different fault segments or arrest at specific junctures. We aim to compare these patterns with dynamic rupture models to assess how fault geometry may control rupture propagation over various timescales.