scholarly journals Two-dimensional seismic image of the San Andreas Fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone

1997 ◽  
Vol 24 (13) ◽  
pp. 1591-1594 ◽  
Author(s):  
C. Thurber ◽  
S. Roecker ◽  
W. Ellsworth ◽  
Y. Chen ◽  
W. Lutter ◽  
...  
Keyword(s):  
Fault Zone ◽  
San Andreas Fault ◽  
Two Dimensional ◽  
Central California ◽  
San Andreas ◽  
Seismic Image

scholarly journals Arkosic rocks from the San Andreas Fault Observatory at Depth (SAFOD) borehole, central California: Implications for the structure and tectonics of the San Andreas fault zone

Lithosphere ◽  
10.1130/l13.1 ◽  
2009 ◽  
Vol 1 (4) ◽  
pp. 206-226 ◽  
Author(s):  
S. D. Springer ◽  
J. P. Evans ◽  
J. I. Garver ◽  
D. Kirschner ◽  
S. U. Janecke
Keyword(s):  
Fault Zone ◽  
San Andreas Fault ◽  
Central California ◽  
San Andreas

Attenuation of shear waves along the San Andreas fault zone in central California

1975 ◽  
Vol 65 (1) ◽  
pp. 277-292 ◽  
Author(s):  
Tuneto Kurita
Keyword(s):  
Fault Zone ◽  
San Andreas Fault ◽  
Ground Motions ◽  
Shear Waves ◽  
Spectral Ratio ◽  
Ratio Method ◽  
Sh Waves ◽  
Central California ◽  
San Andreas ◽  
Crustal Structures

abstract Ground motions at distances of 20 to 30 km from two moderate earthquakes, which occurred along the San Andreas fault zone in central California on February 24, 1972 (M = 5.0) and on September 4, 1972 (M = 4.6), were well recorded at two stations on quite different crustal structures astride the fault. The similarity of the focal mechanisms and the observed spectra at each station, for both earthquakes, makes it possible to apply the spectral ratio method for evaluating Q for direct shear waves propagating along and adjacent to the fault zone. The resultant linear relations between the average values of Q of SH waves for the wave paths from both earthquakes to each station, together with a few reasonable assumptions, suggest the following properties of the variation of Q in the upper crust: In the fracture zone of Tertiary and Cretaceous sedimentary rocks along the fault to the northeast of the Gabilan range, the average Q in the upper 6 km is as low as 20. Although less certain, under the northeastern part of the Gabilan range composed of Mesozoic granites southwest of the fault, the average Q in the upper 6 km is about 100 or more and the intrinsic Q begins to decrease at a depth of several kilometers. This depth corresponds to the bottom of the well-defined zone of aftershock occurrence following moderate earthquakes.

scholarly journals Velocity contrast across the San Andreas fault in central California: Small-scale variations from P-wave nodal plane distortion

1977 ◽  
Vol 67 (6) ◽  
pp. 1565-1576
Author(s):  
Karen C. McNally ◽  
Thomas V. McEvilly
Keyword(s):  
Fault Zone ◽  
Fault Plane ◽  
San Andreas Fault ◽  
P Wave ◽  
Wave Radiation ◽  
Small Scale ◽  
Depth Range ◽  
Radiation Patterns ◽  
Central California ◽  
San Andreas

abstract Systematic variations in P-wave radiation patterns, evident in a data set of 400 central California earthquakes, have been analyzed for variations in velocity contrast across the San Andreas fault zone. Vertical strike-slip faulting characterizes the region, with radiation patterns well constrained by the dense local seismographic station network. A discontinuity in crustal velocity occurs across the San Andreas fault. The distribution of systematically inconsistent first motions indicates that first arrivals observed along the fault plane within the northeastern block have followed refracted paths through the higher velocity crustal rocks to the southwest, retaining P-wave polarities characteristic of the quadrant of origin, and thus appearing reversed. A simple geometrical interpretation, with P waves refracted at the fault plane near the focus, yields the velocity contrast across the fault zone; the distribution of hypocenters allows its mapping in time and space. The velocity contrast so determined ranges up to 15 per cent, for a depth range of 1 to 10 km. The observed pattern of contrast values is coherent, with the greatest contrast related apparently in space, and possibly in time, to the larger earthquakes occurring on the fault. We suggest the phenomenon reflects changes in stress state at the fault and, by virtue of its ease of measurement, offers a new and valuable technique in earthquake studies.

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