TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 54 (No. 11), pp. 1005-1010, 2002

Deformation in the lower crust and downward extent of the San Andreas Fault as revealed by teleseismic waveforms

Lupei Zhu

Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, Missouri 63103, U.S.A.

(Received January 14, 2002; Revised August 5, 2002; Accepted August 9, 2002)

Abstract: High resolution images of crustal structure across the San Andreas Fault (SAF) were obtained by using the common conversion point stacking of teleseismic P-to-S converted waves recorded during the Los Angeles Region Seismic Experiments (LARSE-I and II). In the upper crust, several sedimentary basins were delineated in the images, including the San Fernando and the Santa Clarita Basins. The San Fernando Basin reaches a depth of 8 km under the northern edge of the San Fernando Valley. On the LARSE-I profile, the downward projection of the SAF truncates several lower crustal interfaces including the Moho on both sides. The Moho is vertically offset by as much as 8 km. Along the LARSE-II profile, the impedance contrast and slope of the Moho are seen to change across the fault. These results indicate that the fault penetrates into the lower crust and probably uppermost mantle as a narrow (<10 km) feature. The Moho beneath the San Gabriel Mountains is shallower (~26 km) than under the San Gabriel Valley to the south and the Mojave Desert to the north, suggesting that the mountain ranges were lifted en masse by horizontal compression. On the northeast side of the SAF, the Mojave Desert has a sharp and essentially flat Moho at a depth of ~32 km. The lower crustal structure beneath the San Fernando and Santa Clarita Valleys along the LARSE-II profile south of the SAF is complicated as indicated by the large undulation and low impedance contrast of the Moho. These observations suggest that the deformation in the lower crust is localized and often concentrates near boundaries of crustal blocks or beneath those places which have experienced intensive faulting and deformation in the upper crust.

Corresponding author E-mail: lupei@eas.slu.edu

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