Dr. Carl Tape is awarded an NSF Earth Science Postdoctoral Fellowship to carry out a program of research and education at Harvard University. He will construct a three-dimensional structure model of the San Joaquin basin and adjacent regions. The basin will be described in terms of seismic velocity variations, density variations, and geometries of the major geologic interfaces, such as basin boundaries and faults. The investigation will consist of three stages. First, a basin model will be constructed using industry well logs and seismic reflection data. Second, numerical simulations of previously recorded earthquakes in the region will be performed using supercomputing clusters. Third, the basin model will be iteratively improved using the results of the numerical simulations in a procedure known as adjoint tomography. The resultant three-dimensional structure will be interpreted in the context of the local active tectonics, such as the delamination of the adjacent Sierra Nevada. Results from the earthquake simulations will be used to assess the seismic hazard in the San Joaquin basin.
The Great Valley lies west of the Sierra Nevada and east of the Coast Ranges. It is the largest agricultural producer in the United States, a major oil producer, and home to more than 6.5 million people. The southern portion of the valley, the San Joaquin basin, is filled with several kilometers of sediment that makes the valley prone to enhanced and prolonged shaking during earthquakes. The region is surrounded by active faults, including the San Andreas fault to the west and the White Wolf and Garlock faults to the south. The basin is one of the most prominent tectonic features in southern California, yet its structure and seismic response are not well known. The findings of both the seismic hazard and the active tectonics will be conveyed to the communities of the San Joaquin basin, with emphasis on Bakersfield, which has the largest population and greatest seismic risk.
The Great Valley of central California, comprising the Sacramento basin to the north and the San Joaquin basin to the south, is a dominant producer of agriculture and oil in the United States. The San Joaquin basin contains several kilometers of sedimentary deposits and is therefore prone to amplified and long-lasting shaking during earthquakes. Large earthquakes may originate from the west on the San Andreas fault and from the south on the White Wolf fault, and thus an accurate three-dimensional representation of the basin is needed in order to accurately model ground motions. The primary objective of this project was to develop a three-dimensional model of the San Joaquin basin, and to interpret the model in the context of on-going Earth deformation (or "active tectonics") within the region, as well as to better quantify the seismic hazard for communities in the region. We have constructed a three-dimensional model for the San Joaquin basin using oil well data, seismic reflection survey data, and published geological cross sections. The San Joaquin basin model supplements a revised, detailed three-dimensional model of the southern California crust. Numerical simulations of relatively small (M < 5.5) earthquakes have helped validate the crustal model and illuminate regions for future improvement. Our effort has emphasized the importance of the San Joaquin basin in understanding the seismic hazard and active tectonics in central California. The southernmost end of the basin is marked by an 8-km-thick layer of sediments that produces strong resonant shaking during earthquakes. Seismic waves may laterally reflect off of the southern boundary and remain within the basin. Our model for the San Joaquin basin has been distributed within the Southern California Earthquake Center's Community Velocity Model, which is available to scientists for research and includes on-line documentation for the interested public. The project contributes to our understanding of the profound effects of major sedimentary basin on earthquake ground motion.
