| Abstract |
Despite a lack of large historical earthquakes, the Surprise Valley fault system in northeastern California is presumed to be seismically active based on the presence of numerous Holocene fault scarps and a fault-controlled geothermal system. The Surprise Valley fault is believed to be similar to historically active basin and range faults such as the Dixie Valley fault in central Nevada. A 5-m-deep paleoseismic trench across the main fault revealed a steeply-dipping (~68 degrees) normal fault, while a vibrator reflection profile located a few kilometers north of the trench imaged a much more shallowly dipping normal fault (~25-30 degrees) at 0.5–1.0 km depth. It is unclear from these data if the steeply dipping fault exposed in the trench soles into the more shallowly dipping fault at depth, or if the currently active fault cuts the fault imaged in the reflection profile. Ongoing geothermal development in Surprise Valley motivates a further understanding of the fault system and thermal controlling structures. In June of 2008, we conducted a 200-m-long high-resolution shallow reflection profile across the fault at the location of the paleoseismic trench, in order to further constrain the geometry of the currently active range-front fault. The survey was conducted with a 48-channel seismograph, a 16-lb sledge, and 48 groups of six 100-Hz phones each, spaced at 2-m intervals. Optimization of P-wave arrival times for a tomographic image of the upper 50 meters shows velocities ranging from 1000 to 2500 m/s. Preliminary reflection processing, together with the optimized velocity section, suggests that the fault dips at an angle of 60 degrees in the upper 40 meters, and may suggest an antithetic fault in the hanging wall. The unprocessed data display strong reflections at depths to 200 m. A detailed understanding of the dip angle of the Surprise Valley fault will impact the ongoing geothermal development and the assessment of seismic hazard in the area, both of which will vary based on the angle of the active fault system. |