LONG TERM OBSERVATIONS OF EARTHQUAKE SURFACE RUPTURE: 30 YEARS AFTER THE LANDERS 1992 EARTHQUAKE

The original form and initial modification of earthquake surface rupture are essential information about faulted landscapes. We present data from 30 years of the Emerson fault scarp formed in the 1992 M7.3 Landers, California, earthquake. We combine repeat photography and topographic surveys from 12 site visits (1992-2016) with field investigation in 2022.

In 1992, we did analog photography and total station survey (10³ points measured). By 2008, we used digital photography and terrestrial laser scanning (10⁷ points). In 2012, we began to use photogrammetry (10⁸ points). In 2022, we added more photos, revisited the survey control network with differential Global navigation satellite system (dGNSS), and performed a low-altitude Uncrewed Aerial System survey for photogrammetry (10⁹ points). We also made two excavations in 2022. The scarp-parallel cut exposed fine-grained alluvial fan facies consistent with the current depositional environment. The fault-normal trench exposed a 1-m-wide shear zone juxtaposing indurated silt and sandstone with unconsolidated silt and coarse sand with angular pebbles. 1992 fractures terminate a few cm below the surface. Pre-1992 rupture is suggested by upward-terminating fractures and coarse colluvial material within the downthrown, western side.

The 1992 earthquake caused ~5 m right lateral and 1-2 m vertical offset at the Emerson fault site. It lifted a series of 2000-4000 m² watersheds. Landscape response is controlled by seasonal rainfall, drainage basin form, substrate induration, and scarp morphology. Fault scarps modified without runoff have rounded and collapsed. The incision signal has propagated 50-70 m upstream (70-80% of total channel length) with meter-deep and dm-wide channels cutting the low relief pre-1992 valley floors. From these 30 years of observations, we developed a 3-stage model for post-earthquake landscape response: 1) Reestablish flow paths across simple scarps. Runoff diverts into the fracture network in broadly fractured scarps. 2) Integrate drainage network with small-scale capture events. 3) Extend flowpaths headward via knickzone retreat.