INVESTIGATING THE ROLE OF UPPER-PLATE FAULTS IN SOUTHERN CASCADIA: A PALEOSEISMIC INVESTIGATION OF THE LITTLE SALMON FAULT AND GOOSE LAKE FAULT, HYDESVILLE, HUMBOLDT COUNTY, CALIFORNIA

Along Southern Cascadia Subduction Zone (SCSZ) coupling between the subducting Gorda slab and the overriding (North American) upper-plate drives strain accumulation. However, the role of upper-plate faults in accommodating strain along the SCSZ is uncertain. To test competing hypotheses—whether upper-plate faults rupture coseismically with or independently of the SCSZ—we conducted a paleoseismic study within the central segment of the Little Salmon fault (LSF), the principal upper-plate structure within the SCSZ; and the Goose Lake fault (GLF), situated approximately 1.5 km south of the LSF. Previous paleoseismic studies on the western segment of the LSF document large slip per event (>3.5m) and at least three earthquakes within the last 1,700 years, allowing the possibility the LSF ruptures coseismically with the SCSZ. Here, we present results from two fault-normal paleoseismic trenches across the LSF and GLF. The Quail trench was excavated across a prominent 5-meter-high south-facing scarp exposing the LSF as a series of low angle (8^o to 11^o) north-dipping thrust faults. Event chronology based on preliminary ¹⁴C-dating implies the most recent earthquake (MRE) occurred within the last ~2,000 yrs BP and the penultimate event is younger than ~ 6,700 yrs BP. Additional ¹⁴C and optical stimulated luminescence analyses are pending to further constrain the MRE. Excavation of the Goose trench revealed the GLF as a 2-meter-wide subvertical fault zone characterized by a vertically oriented imbricated shear fabric with fissure fills recording surface fault ruptures. Laterally discontinuous gravel units combined with apparent right-laterally displaced fluvial terraces suggest the GLF is an oblique-slip fault with a sizable, but unknown, lateral component. Based on our results to date, we infer a model wherein the GLF partitions strain with the LSF to the north. We will evaluate our earthquake chronology in light of the extant SCSZ earthquake history in order to assess the possibility the LSF ruptures coseismically with SCSZ. Our results, consistent with previous studies, indicate the central segment of the LSF is capable of large magnitude surface ruptures (~4.5m) during the late Holocene.