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THE SAN ANDREAS FAULT IN SOUTHERN CALIFORNIA IS ALMOST NOWHERE VERTICAL—IMPLICATIONS FOR TECTONICS
The San Andreas Fault (SAF) in southern California is in most places non-vertical, based on seismic-imaging, potential-field, earthquake-aftershock, and selected microseismicity studies of the crust. The dip on the SAF changes from SW (55-75 deg.) near the Big Bend to NE (10-70 deg.) southeastward of the eastern San Gabriel Mountains, describing a crude propeller shape (see below).
A P-wave tomographic image of the mantle in southern California suggests, in cross sections across the SAF, that the plate boundary extends into the mantle with a dip similar to that of the SAF the crust. Mantle velocities southwest of this projected plate boundary, within the Pacific plate, are relatively high and constitute the well documented upper-mantle high-velocity body of the Transverse Ranges. This relationship is similar to that between the Alpine fault of New Zealand and its underlying mantle, and suggests that in both California and New Zealand, Pacific-plate lithospheric mantle is downwelling along the plate boundary.
The dip of the SAF is important for estimating shaking potential for scenario major earthquakes, and for calculating geodetic deformation.
LOCATION ALONG SAF (NORTHWEST TO SOUTHEAST)
| DIP (DEG); [uncertainty ~ 5-10 deg.]
| MAXIMUM DEPTH OF CONSTRAINT (KM)
| DATA SET
|
20 km northwest of junction of SAF and Garlock fault
| 55 SW
| 5
| gravity
|
30 km southeast of junction of SAF and Garlock fault
| 75 SW
| 8
| magnetic
|
45 km southeast of junction of SAF and Garlock fault
| 90
| 35
| seismic-imaging (LARSE line 2)
|
10 km northwest of Wrightwood
| 85 NE
| 20
| seismic-imaging (LARSE line 1)
|
| 90
| 15
| gravity
|
Central San Bernardino
| 37 NE
| 15
| magnetic
|
San Gorgonio Pass (Banning strand)
| 10 N
| 2
| gravity
|
North Palm Springs (Banning or Garnet Hill strand)
| 45-50 NE
| 14
| MW 6.1 1986 North Palm Springs earthquake
|
Desert Hot Springs (Banning strand)
| 60-70 NE
| 12
| relocated ML 6.3 1948 Desert Hot Springs earthquake
|
Indio
| 70 NE
| 6
| gravity and magnetic
|
Northeast side of Salton Sea
| 55-60 NE
| 10
| microseismicity
|