Imaging the San Andreas Fault

We aim to image subsurface geologic structures across the San Andreas fault system in South Central California. Old industry seismic reflection data are reprocessed and imaged with advanced prestack migration techniques in order to produce new seismic images down to the lower crust.

Leitung:

Mitarbeiter/innen:

We reprocessed old industry seismic reflection data across the apparently locked segment of the San Andreas fault near Cholame and imaged the middle and lower crust in this highly active non-volcanic tremor region.

Fresnel Volume migration was applied to produce new seismic images along the profile line that runs from the Pacific Coast at Morro Bay up to the foothills of the Sierra Nevada.

In order to obtain the reflectivity structure of the middle and lower crust we recorrelated the original field data to 26 seconds two-way traveltime. A 3D tomographic velocity model derived from local earthquake data was used and the imaging technique was implemented in 3D in order to take account of the true shot and receiver positions along the crooked profile line.

The most prominent features in the resulting images are strong northeast and southwest dipping reflectors in the lower crust southwest of the San Andreas fault indicating heterogenous material within and below the Salinian Block.

East to the San Andreas fault the imaged structures show thrusts and folds beneath the Kettleman Hills and the San Joaquin Valley. The San Andreas fault appears as a near vertical non-reflective zone that is characterized by the termination of coherent reflectors.

The interpretation of the imaging results is performed under consideration of recent magnetotelluric studies, microseismic and non-volcanic tremor event locations as well as previous results of studies from the same data set.

References

  • Buske, S., S. Gutjahr, C. Sick (2009), Fresnel Volume Migration of single-component seismic data, Geophysics, Vol.74, No.6, WCA47--WCA55.
  • Nadeau, R. M., and A. Guilhem (2009), Nonvolcanic tremor evolution and the San Simeon and Parkfield, California earthquakes, Science, 325, 191-193, doi:10.1126/science.1174155.
  • Thurber, C., H. Zhang, F. Waldhauser, J. Hardebeck, A. Michael, D. Eberhart-Phillips (2006), Three-dimensional compressional wavespeed model, earthquake relocations, and focal mechanisms for the Parkfield, California region, Bull. Seism. Soc. Am., 96, S38--S49.
  • Trehu, A., and W. H. Wheeler (1987), Possible evidence in the seismic data of profile SJ-6 for subducted sediments beneath the Coast Ranges of California, USA, Interpretations of the SJ-6 seismic Reflection/Refraction Profile, South Central California, Open-File Report 87-73.
  • Waldhauser, F., and D.P. Schaff (2008), Large-scale relocation of two decades of Northern California seismicity using cross-correlation and double-difference methods, J. Geophys. Res., 113, B08311, doi: 10.1029/2007JB005479.