Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California

1995 ◽  
Vol 85 (6) ◽  
pp. 1805-1820
Author(s):  
Denis Jongmans ◽  
Peter E. Malin
Keyword(s):  
Fault Zone ◽  
Guided Waves ◽  
Velocity Structure ◽  
San Andreas Fault ◽  
High Gain ◽  
S Wave ◽  
Lateral Velocity ◽  
Wave Amplification ◽  
S Waves ◽  
San Andreas

Abstract High-gain three-component seismometers from 0- to 1-km deep along the Varian A-1 well at Parkfield, California, were used to record the waveforms of nearby microearthquakes. Despite being in the thick Tertiary sediments of the Parkfield Syncline, the S-wave amplification at this site is only about a factor of 3. The spectral content and spectral ratios of S waves along the well show that the average Qs in the top 1 km at this site is 37, with the Qs in different subintervals varying between 8 and 65. Based on initial S-wave polarizations, a complex S-wave velocity structure must exist at and below the Varian site. This structure appears to include position-dependent anisotropy as well as steep lateral velocity gradients. At a depth of 1 km, S-wave splitting parallel and normal to the San Andreas fault zone is consistently observed. This splitting scales at roughly 0.01 sec/km. Subsequent to the split S waves, the particle motion seems to be controlled by event focal mechanism. Above 1 km, the upgoing S waves attenuate and change directions of polarization, with a new splitting rate of 0.1 sec/km. Uniquely, for some events on the San Andreas fault immediately below the Varian site, large, post-S-wave signals with normal dispersion are present. We propose that these phases are fault-zone guided waves channeled from the San Andreas fault to the Varian site along the Gold Hill fault.

Interpretation of seismic reflection profiling data for the structure of the San Andreas fault zone

1983 ◽  
Vol 73 (6A) ◽  
pp. 1701-1720
Author(s):  
R. Feng ◽  
T. V. McEvilly
Keyword(s):  
Fault Zone ◽  
Seismic Reflection ◽  
San Andreas Fault ◽  
Velocity Model ◽  
Seismic Reflection Profile ◽  
Lateral Velocity ◽  
Zone Structure ◽  
Ray Method ◽  
Velocity Change ◽  
San Andreas

Abstract A seismic reflection profile crossing the San Andreas fault zone in central California was conducted in 1978. Results are complicated by the extreme lateral heterogeneity and low velocities in the fault zone. Other evidence for severe lateral velocity change across the fault zone lies in hypocenter bias and nodal plane distortion for earthquakes on the fault. Conventional interpretation and processing methods for reflection data are hard-pressed in this situation. Using the inverse ray method of May and Covey (1981), with an initial model derived from a variety of data and the impedance contrasts inferred from the preserved amplitude stacked section, an iterative inversion process yields a velocity model which, while clearly nonunique, is consistent with the various lines of evidence on the fault zone structure.

Shear-wave anisotropy in the Parkfield Varian well VSP

1990 ◽  
Vol 80 (4) ◽  
pp. 857-869 ◽  
Author(s):  
T. M. Daley ◽  
T. V. McEvilly
Keyword(s):  
Shear Wave ◽  
San Andreas Fault ◽  
Monitoring Program ◽  
Seismic Profile ◽  
S Wave ◽  
S Waves ◽  
Velocity Anisotropy ◽  
San Andreas ◽  
The Difference ◽  
Fault Trace

Abstract A vertical seismic profile (VSP) survey was run to 1334 m depth in the instrumented Varian well, 1.4 km from the San Andreas fault trace at Parkfield, California, to test the sensor string shortly after its permanent installation. The cable subsequently failed near the 1000 m level, so the test survey represents the deepest data acquired in the study. A shear-wave vibrator source was used at three ofsets and two orthogonal orientations, and the data have been processed for P- and S-wave velocities and for S-wave velocity anisotropy. Velocities are well-determined (3.3 and 1.9 km/sec, respectively, at the deeper levels), and the S waves are seen clearly to be split by anisotropy below about 400 m. Some 8 per cent velocity difference is apparent between polarizations parallel to and perpendicular to the San Andreas fault (faster and slower, respectively), and the difference seems to decrease with distance from the fault, suggesting that the cause may be the fabric of the fault zone. Repeated surveys at the 1000 m depth are being conducted as part of the Parkfield monitoring program.

scholarly journals Scientific Drilling Into the San Andreas Fault Zone – An Overview of SAFOD's First Five Years

2011 ◽  
Vol 11 ◽  
pp. 14-28 ◽  
Author(s):  
M. Zoback ◽  
S. Hickman ◽  
W. Ellsworth ◽  
Keyword(s):  
Fault Zone ◽  
San Andreas Fault ◽  
Scale Up ◽  
Horizontal Stress ◽  
Seismic Velocities ◽  
Fault Creep ◽  
S Wave ◽  
Scientific Drilling ◽  
Pilot Hole ◽  
San Andreas

The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes controlling faulting and earthquake generation along an active, plate-bounding fault at depth. SAFOD is located near Parkfield, California and penetrates a section of the fault that is moving due to a combination of repeating microearthquakes and fault creep. Geophysical logs define the San Andreas Fault Zone to be relatively broad (~200 m), containing several discrete zones only 2&ndash;3 m wide that exhibit very low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3192 m and 3302 m. Cores from both deforming zones contain a pervasively sheared, cohesionless, foliated fault gouge that coincides with casing deformation and explains the observed extremely low seismic velocities and resistivity. These cores are being now extensively tested in laboratories around the world, and their composition, deformation mechanisms, physical properties, and rheological behavior are studied. Downhole measurements show that within 200 m (maximum) of the active fault trace, the direction of maximum horizontal stress remains at a high angle to the San Andreas Fault, consistent with other measurements. The results from the SAFOD Main Hole, together with the stress state determined in the Pilot Hole, are consistent with a strong crust/weak fault model of the San Andreas. Seismic instrumentation has been deployed to study physics of faulting &ndash; earthquake nucleation, propagation, and arrest &ndash; in order to test how laboratory-derived concepts scale up to earthquakes occurring in nature. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.11.02.2011" target="_blank">10.2204/iodp.sd.11.02.2011</a>

Modeling fault‐zone guided waves of microearthquakes in a geothermal reservoir

Geophysics ◽  
1997 ◽  
Vol 62 (4) ◽  
pp. 1278-1284 ◽  
Author(s):  
Min Lou ◽  
José A. Rial ◽  
P. E. Malin
Keyword(s):  
Fault Zone ◽  
Guided Waves ◽  
Velocity Structure ◽  
Geothermal Field ◽  
Guided Wave ◽  
S Wave ◽  
Low Velocity ◽  
Geothermal Fields ◽  
Coso Geothermal Field ◽  
Fault Zone Structures

Fault‐zone guided waves have been identified in microearthquake seismograms recorded at the Coso Geothermal Field, California. The observed guided waves have particle motions and propagation group velocities similar to Rayleigh wave modes. A numerical method has been employed to simulate the guided‐wave propagation through the fault zone. By comparing observed and synthetic waveforms the fault‐zone width and its P‐ and S‐wave velocity structure have been estimated. It is suggested here that the identification and modeling of such guided waves is an effective tool to locate fracture‐induced, low‐velocity fault‐zone structures in geothermal fields.

Water-level fluctuations and earthquakes on the San Andreas fault zone

Geology ◽  
1975 ◽  
Vol 3 (8) ◽  
pp. 437 ◽  
Author(s):  
Robert L. Kovach ◽  
Amos Nur ◽  
Robert L. Wesson ◽  
Russell Robinson
Keyword(s):  
Water Level ◽  
Fault Zone ◽  
San Andreas Fault ◽  
Water Level Fluctuations ◽  
San Andreas
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