Quaternary Mapping and Paleoseismic Assessment of the Warm Springs Valley Fault in the Northern Walker Lane, Nevada

Abstract

The northern Walker Lane, in western Nevada and northern California, is a region of distributed deformation that accommodates ~15% of the motion between the North American plate and the Pacific plate along northwest-oriented right-lateral strike-slip faults, east-west oriented left lateral strike slip faults and, north-oriented normal faults. A series of left-stepping en echelon right lateral strike slip faults including the Honey Lake, the Warm Springs Valley (WSVF), and the Pyramid Lake faults are capable of large surface ruptures in close proximity to the metropolitan area of Reno. This thesis presents results of a multi-faceted study of the WSVF combining Quaternary mapping, a paleoseismic trenching investigation, and a slip rate analysis of displaced pluvial lake shorelines. Mapping observations show fault scarps in young alluvial material across a minimum length of 70 km from the Warm Springs Valley to the Honey Lake basin. The fault is expressed as (1) a single trace in Warm Springs Valley, (2) multiple parallel stepping and anastomosing fault strands, uphill and downhill facing scarps, pop up features, and sag ponds between the Dogskin and Virginia mountains, and (3) a range front fault along the Fort Sage mountains that steps to the east and cuts pluvial lake shorelines in the Honey Lake basin. Fault scarps cutting young surficial deposits are generally 0.5-3 meters in height, are short and discontinuous along strike and are at a range of orientations suggesting a distributed rupture pattern. The distributed pattern of faulting within the WSVF is consistent with the youthful development of the northern Walker Lane, which has been compared to the initial stages of deformation in clay models. Therefore, the distributed nature of faulting within the Walker Lane may be self-similar between the regional and fault scales. A trenching investigation across the northern section of the fault reveals evidence for three and up to four surface rupturing earthquakes since the latest Pleistocene with an average recurrence interval of 2500-3100 yrs. In the Honey Lake basin, the fault displaces pluvial lake shorelines at 1231 meters in elevation which corresponds to the Younger Dryas climactic period at 12,000-12,500 cal. B.P. when pluvial lakes connected Honey, Pyramid and Winnemucca basins at a maximum highstand elevation of 1231m. Evaluation of the displaced shorelines provides a lateral offset estimate of 14-30 m and a vertical separation of 4-5 m. Combined with the maximum age of the Younger Dryas highstand, I infer a Holocene slip rate of 1.1-2.4 mm/yr which is similar to the previously reported late Pleistocene and geodetic rate. The combined event chronology and slip rate estimates suggest single event displacements range between 3.5 m and 10 m. The upper end of this range is larger than displacements estimated by empirical relations between fault length and single event displacement suggesting that 1) the trenching does not capture the full Holocene earthquake history, 2) the fault is distributed across multiple fault strands that rupture independently during earthquakes, or 3) longer fault ruptures may have occurred, possibly associated with fault-to-fault rupture with other adjacent faults in the northern Walker Lane. This study highlights the need for multifaceted approaches to characterize distributed fault zones, and their associated seismic hazards.