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30 Years of Landscape Response Following the 1992 Landers, CA, Earthquake

Description: 

The landscape record of earthquakes is a primary tool to interpret the location, displacement, and recency of active faulting. Landscapes evolve after earthquake rupture from both natural and anthropogenic causes (e.g., precipitation, gravity, dirt bikes), causing erosion and modification of vertical fault scarps and laterally offset features. The 1992 M7.3 Landers, California, earthquake offers an opportunity to quantify the degradation of lateral and vertical displacements that were well documented post-earthquake (Arrowsmith & Rhodes 1994, McGill & Rubin 1999). We focus on an ~500-m-long section of the Emerson fault scarp near Galway Lake Road that had up to 5 m right-lateral and 1.75 m vertical displacement with areas of localized and distributed deformation. We document landscape change over 30 years using fault-parallel and fault-normal trenches and vertical differencing of topography derived from 1992 USGS aerial photographs taken two days after the earthquake and 2022 drone photogrammetry. The 1992 colluvial wedge in the fault-normal trench is 30-50 cm thick at the base of the vertical scarp, diminishing to 10-15 cm, 1.5 m down-trench from the base of the scarp. This wedge overlies a basal rock line, interpreted to be the ground surface prior to the 1992 earthquake, followed by chaotic debris deposits, and topped with finely bedded slope wash, burying ~1/3 of the original free face. The fault-parallel trench exposes a new channel incised since 1992, a channel offset in the 1992 earthquake, and a broad wash containing a thalweg possibly offset in the penultimate event. Vertical differencing and field observations document extensive rills across the scarp (some with apparent left-lateral offset due to flow patterns), headward-eroding channels deeply incised into what were broad washes in 1992, and the formation of post-1992 channels on the downthrown block. Evidence of most distributed deformation (cracks and minor faults) is erased in 30 years. The results of this work will inform how landscapes are interpreted for evidence of active faults, with implications for fault displacement hazard models.

Session: The Landscape Record of Earthquakes and Faulting [Poster]

Type: Poster

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Nadine

Student Presenter: No

Invited Presentation: 

Poster Number: 11

Authors