Developing a long (~3000 yr) paleoearthquake record for the west-central Denali fault at the Nenana River, Alaska

The understanding of earthquake behavior and the associated seismic hazards for crustal faults in Alaska is strongly inhibited by a lack of substantial paleoearthquake chronologies for known active faults. As one of the most prominent of these faults due to its length, topographic prominence, and historic earthquakes, the Denali fault system has been the target of more paleoseismic investigations than any other regional crustal fault and these investigations have established a first-order understanding of the recent earthquake history. Preliminary results of these studies document anywhere from 1 to 5 paleoearthquakes at 16 different paleoseismic sites; however, only 2 sites contain evidence for 5 earthquakes and the other 14 sites preserve evidence for 3 or fewer earthquakes. To expand the paleoearthquake record for the Denali fault system and to investigate the connections between earthquakes and the incision history of the Nenana River, we have begun to develop a new paleoseismic on the west-central Denali fault - west of the 2002 Denali fault earthquake rupture. Our new paleoseismic site (the "Dead Mouse site") is situated at the intersection between the north-flowing Nenana River and the ~east-west striking Denali fault on the south margin of the Alaska Range. A complex history of Pleistocene glacial erosion and subsequent fluvial incision and aggradation has created a number of landforms that preserve the active geomorphic trace of the Denali fault. Two paleoseismic trenches across a portion of the geomorphic scarp, excavated by hand during July 2012, exposes up to 2.5 m or bedded silt and sand deposits. Our initial interpretation of the exposed trench stratigraphy and deformation documents evidence for up to 4 paleoearthquakes and the presence of a fine-grained sedimentary section spanning at least 3000 years. These trenches did not exposed the full width of the surface expression of the Denali fault, nor the full depth of the fine-grained sedimentary section, thus it is clear the additional evidence for paleoearthquakes is preserved outside of our 2012 trenches. In order to fully develop the paleoearthquake record for the Dead Mouse site, we seek support to expand our trenching across the full width of the surface expression of the Denali fault and resolve the complete stratigraphic section available at this site. Completing these investigations at this site is critical because, 1. The Denali fault is the highest slip rate fault in the region (~7-9 mm/yr along this section), 2. This paleoseismic site is the most accessible on the entire Denali fault system, situated less than 400 m from the Parks Highway (800 m by trail), 3. We expect to constrain the timing of 5-7 paleoearthquakes over the past 3000+ years, 4. A longer paleoearthquake record provide a critical test of earthquake behavior models derived from the existing, mostly short, earthquake records , 5. Speculative connections exist between the timing of major earthquakes on this section of the Denali fault, and major landslides that span the Nenana River valley, 6. The Nenana River/Parks Highway corridor provides a critical infrastructural path through the Alaska Range, currently hosting the highway, the Alaska Railroad, an electrical intertie, and is the proposed path of a major natural gas pipeline, 7. This proposed research directly addresses NEHRP program element I, National and regional earthquake hazards assessments, and specifically addresses the priorities in the Pacific Northwest and Alaska Research Area to, "improve the understanding of active faulting and the paleoseismic record of large earthquake on major crustal faults in Alaska, including the Denali, …)