Designing Shots for the 2010 Seismic Refraction and Reflection Survey in the Salton Trough, Southern California

The NSF and the USGS have funded a large seismic refraction and reflection survey of the Salton Trough in southern California. The goals of the project are to study earthquake hazards on the San Andreas and Imperial Faults as well as rifting processes in the northernmost Gulf of California extensional province. Seismic velocity models and reflection images of the basins, whole crust, and upper mantle will constrain the structure and petrology of the rift valley, adjacent ranges and bounding faults. Fault and sedimentary basin geometry and 3-D seismic velocity will aid in the construction of seismic hazard models in an area considered at high risk. The obliquely rifted continent appears to be producing new lithosphere in the central trough by magmatic underplating and sedimentation. Interactions between extension, magmatism, sedimentation, and faulting will be constrained by the seismic images. Fieldwork is tentatively scheduled to take place in January 2010. To prepare for the upcoming fieldwork, shot design was investigated based on data from a 1979 USGS seismic survey, when 41 shots of varying sizes were detonated at 9 locations throughout the Imperial Valley. Picking errors in the first arrival travel times were correlated with source-receiver offset, shot size, and shot-hole geology. As expected, shots in the irrigated valley farmland in lake sediments produced much better shot coupling than those located in unsorted desert stream wash. This is likely due to better packing of the lake sediments and/or drilling through a shallower water table. The goals of the 2010 survey include two 250-km whole-crust refraction profiles. This would require shot sizes of 1000-1500 kg to record the entire length of these lines. Several shorter upper-crust refraction lines would require ~500-kg shots to record to distances greater than 50 km. Dense refraction and low-fold reflection shots spaced at 2-4 km in the valley floor need 100-200 kg to image both whole-crust low-fold reflectivity and refraction to distances greater than 20 km.