scholarly journals Crustal structure and seismic anisotropy near the San Andreas Fault at Parkfield, California

2009 ◽  
Vol 178 (2) ◽  
pp. 1098-1104 ◽  
Author(s):  
A. Arda Ozacar ◽  
George Zandt
Keyword(s):  
Crustal Structure ◽  
Seismic Anisotropy ◽  
San Andreas Fault ◽  
San Andreas

Crustal structure southwest of the San Andreas fault from quarry blasts

1964 ◽  
Vol 54 (1) ◽  
pp. 67-77
Author(s):  
Robert M. Hamilton ◽  
Alan Ryall ◽  
Eduard Berg
Keyword(s):  
San Francisco ◽  
Crustal Structure ◽  
San Andreas Fault ◽  
P Wave ◽  
Geological Survey ◽  
San Andreas ◽  
Crustal Model ◽  
Crystalline Crust ◽  
The U.S

abstract To determine a crustal model for the southwest side of the San Andreas fault, six large quarry blasts near Salinas, California, were recorded at 27 seismographic stations in the region around Salinas, and along a line northwest of the quarry toward San Francisco. Data from these explosions are compared with results of explosion-seismic studies carried out by the U.S. Geological Survey on a profile along the coast of California from San Francisco to Camp Roberts. The velocity of Pg, the P wave refracted through the crystalline crust, in the Salinas region is 6.2 km/sec and the velocity of Pn is about 8.0 km/sec. Velocities of the direct P wave in near-sur-face rocks vary from one place to another, and appear to correlate well with gross geologic features. The thickness of the crust in the region southwest of the San Andreas fault from Salinas to San Francisco is about 22 kilometers.

A multiscale study of the mechanisms controlling shear velocity anisotropy in the San Andreas Fault Observatory at Depth

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. F131-F146 ◽  
Author(s):  
Naomi L. Boness ◽  
Mark D. Zoback
Keyword(s):  
Seismic Anisotropy ◽  
San Andreas Fault ◽  
Shear Waves ◽  
Shear Velocity ◽  
Granitic Rocks ◽  
Velocity Anisotropy ◽  
Bedding Planes ◽  
San Andreas ◽  
State Of Stress ◽  
Sonic Logs

We present an analysis of shear velocity anisotropy using data in and near the San Andreas Fault Observatory at Depth (SAFOD) to investigate the physical mechanisms controlling velocity anisotropy and the effects of frequency and scale. We analyze data from borehole dipole sonic logs and present the results from a shear-wave-splitting analysis performed on waveforms from microearthquakes recorded on a downhole seismic array. We show how seismic anisotropy is linked either to structures such as sedimentary bedding planes or to the state of stress, depending on the physical properties of the formation. For an arbitrarily oriented wellbore, we model the apparent fast direction that is measured with dipole sonic logs if the shear waves are polarized by arbitrarily dipping transversely isotropic (TI) structural planes (bedding/fractures). Our results indicate that the contemporary state of stress is the dominant mechanism governing shear velocity anisotropy in both highly fractured granitic rocks and well-bedded arkosic sandstones. In contrast, within the finely laminated shales, anisotropy is a result of the structural alignment of clays along the sedimentary bedding planes. By analyzing shear velocity anisotropy at sonic wavelengths over scales of meters and at seismic frequencies over scales of several kilometers, we show that the polarization of the shear waves and the amount of anisotropy recorded are strongly dependent on the frequency and scale of investigation. The shear anisotropy data provide constraints on the orientation of the maximum horizontal compressive stress [Formula: see text] and suggest that, at a distance of only [Formula: see text] from the San Andreas fault (SAF), [Formula: see text] is at an angle of approximately 70° to the strike of the fault. This observation is consistent with the hypothesis that the SAF is a weak fault slipping at low levels of shear stress.

A method for determination of the lower crustal structure along the San Andreas fault system in central California

1977 ◽  
Vol 67 (3) ◽  
pp. 793-807
Author(s):  
L. G. Peake ◽  
J. H. Healy
Keyword(s):  
Crustal Structure ◽  
Seismic Waves ◽  
San Andreas Fault ◽  
Fault System ◽  
Near Surface ◽  
Central California ◽  
San Andreas ◽  
Crustal Thinning ◽  
San Andreas Fault System ◽  
Lower Crustal

abstract A method for determining lower crustal structure by comparing residuals of distant events, thereby reducing the effect of near-surface structure on seismic waves, has been tested through a straightforward numerical analysis of teleseismic residuals in central California. The results show a general crustal thinning to the west, with a sharp gradient along the San Andreas fault and a uniform crustal thickness between the San Andreas fault and the Calaveras and Hayward faults.

scholarly journals Crustal structure across the San Andreas Fault at the SAFOD site from potential field and geologic studies

2004 ◽  
Vol 31 (12) ◽  
pp. n/a-n/a ◽  
Author(s):  
Darcy K. McPhee ◽  
Robert C. Jachens ◽  
Carl M. Wentworth
Keyword(s):  
Crustal Structure ◽  
Potential Field ◽  
San Andreas Fault ◽  
San Andreas

Eikonal equation-based P-wave seismic azimuthal anisotropy tomography of the crustal structure beneath northern California

2021 ◽  
Vol 226 (1) ◽  
pp. 287-301
Author(s):  
Yongsheng Liu ◽  
Ping Tong
Keyword(s):  
Eikonal Equation ◽  
Seismic Anisotropy ◽  
San Andreas Fault ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Tectonic Deformation ◽  
Northern Coast ◽  
Northern California ◽  
San Andreas ◽  
Great Valley

SUMMARY Delineating spatial variations of seismic anisotropy in the crust is of great importance for the understanding of structural heterogeneities, regional stress regime and ongoing crustal dynamics. In this study, we present a 3-D anisotropic P-wave velocity model of the crust beneath northern California by using the eikonal equation-based seismic azimuthal anisotropy tomography method. The velocity heterogeneities under different geological units are well resolved. The thickness of the low-velocity sediment at the Great Valley Sequence is estimated to be about 10 km. The high-velocity anomaly underlying Great Valley probably indicates the existence of ophiolite bodies. Strong velocity contrasts are revealed across the Hayward Fault (2–9 km) and San Andreas Fault (2–12 km). In the upper crust (2–9 km), the fast velocity directions (FVDs) are generally fault-parallel in the northern Coast Range, which may be caused by geological structure; while the FVDs are mainly NE–SW in Great Valley and the northern Sierra Nevada possibly due to the regional maximum horizontal compressive stress. In contrast, seismic anisotropy in the mid-lower crust (12–22 km) may be attributed to the alignment of mica schists. The anisotropy contrast across the San Andreas Fault may imply different mechanisms of crustal deformation on the two sides of the fault. Both the strong velocity contrasts and the high angle (∼45° or above) between the FVDs and the strikes of faults suggest that the faults are mechanically weak in the San Francisco bay area (2–6 km). This study suggests that the eikonal equation-based seismic azimuthal anisotropy tomography is a valuable tool to investigate crustal heterogeneities and tectonic deformation.

Source parameters of moderate size earthquakes and the importance of receiver crustal structure in interpreting observations of local earthquakes

1977 ◽  
Vol 67 (2) ◽  
pp. 301-313
Author(s):  
Donald V. Helmberger ◽  
Lane R. Johnson
Keyword(s):  
Crustal Structure ◽  
San Andreas Fault ◽  
Source Parameters ◽  
Accurate Estimate ◽  
Strike Slip ◽  
Wave Shape ◽  
Earthquake Excitation ◽  
Central California ◽  
San Andreas ◽  
Crustal Model

Abstract Broadband observations of three central California earthquakes as recorded on opposite sides of the San Andreas fault zone are studied. The earthquake mechanisms are of the strike-slip type occurring along the fault at epicentral distances between 15 and 30 km. The seismograms obtained at the two sites are distinctly dissimilar in both amplitude and wave shape even though they are at roughly the same azimuth. We suppose that the earthquake excitation is identical for the two sites and that the differences in seismograms are caused by the receiver structure. The problem is idealized by assuming that the first 10 sec of each record can be modeled synthetically with a point shear dislocation embedded in a half-space with a two-layer upper-crustal model appropriate for each site. The results determined by matching the observations indicate that the durations for these events with ML = 4 to 5 are about 0.3 to 0.6 sec. Furthermore, the results demonstrate that accurate estimate of source parameters can only be accomplished after a detailed appreciation of crustal structure.

Sign in / Sign up

Export Citation Format

Share Document