scholarly journals Three‐dimensional imaging of steeply dipping structure near the San Andreas fault, Parkfield, California

Geophysics ◽  
1988 ◽  
Vol 53 (2) ◽  
pp. 176-185 ◽  
Author(s):  
John N. Louie ◽  
Robert W. Clayton ◽  
Ronan J. LeBras
Keyword(s):  
Seismic Reflection ◽  
San Andreas Fault ◽  
Three Dimensional ◽  
Summation Method ◽  
Ultramafic Rocks ◽  
Back Projection ◽  
Lateral Velocity ◽  
Full Wave ◽  
San Andreas ◽  
Vertical Fault

Shot gathers from the Parkfield, California, deep crustal seismic reflection line, recorded in 1977 by COCORP, reveal coherent events having horizontal to reverse moveouts. These events were migrated using a multioffset three‐dimensional Kirchhoff summation method. This method is a ray‐equation back projection inversion of the acoustic wave field, which is valid under the Born, WKBJ, and far‐field assumptions. Migration of full‐wave acoustic synthetics, having the same limitations in geometric coverage as the COCORP survey, demonstrates the utility of the imaging process. The images obtained from back projection of the survey data suggest that the Gold Hill fault carries ultramafic rocks from the surface to 3 km depth at a dip greater than 45 degrees, where it joins the San Andreas fault, which may cut through more homogeneous materials at shallow depths. To the southwest, a 2 km Tertiary sedimentary section appears to terminate against a near‐vertical fault. The zone between this fault and the San Andreas may be floored at 3 km by flat‐lying ultramafics. Lateral velocity inhomogeneities are not accounted for in the migration but, in this case, do not seriously hinder the reconstruction of reflectors.

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.

On: “Three‐dimensional imaging of steeply dipping structure near the San Andreas fault, Parkfield, California” by John N. Louie, Robert W. Clayton, and Ronan J. LeBras, (GEOPHYSICS, 53, 176–185, February 1988).

Geophysics ◽  
1988 ◽  
Vol 53 (10) ◽  
pp. 1364-1365 ◽  
Author(s):  
J. H. McBride
Keyword(s):  
Survey Data ◽  
Fault Zone ◽  
Complex Terrain ◽  
San Andreas Fault ◽  
Three Dimensional ◽  
Migration Process ◽  
Prestack Migration ◽  
San Andreas ◽  
Survey Line ◽  
Lateral Heterogeneities

Louie et al. (1988) apply to COCORP survey data a prestack migration process, which they describe, to image better reflections associated with structure in the upper 5 km of the San Andreas fault zone near Parkfield, California. They demonstrate the usefulness of this approach in an area along the survey where, as they point out, the CMP-stacking process may be particularly troublesome. While the authors were sensitive to the extreme lateral heterogeneities in and about the fault zone, the crooked survey line, and the complex terrain in which the survey was mounted (McBride and Brown, 1986), I suspect they were nevertheless a little too zealous in discounting, in this case, the value of conventional stacking applied and interpreted judiciously. Moreover, Louie et al. imply that their approach yields previously unobtained results; however, this is not the case.

scholarly journals Stratigraphic evidence for seven meters of dextral slip on the San Andreas fault during the 1857 earthquake in the Carrizo Plain

1993 ◽  
Vol 83 (3) ◽  
pp. 619-635 ◽  
Author(s):  
Lisa B. Grant ◽  
Kerry Sieh
Keyword(s):  
San Andreas Fault ◽  
Alternative Hypothesis ◽  
Three Dimensional ◽  
Large Earthquake ◽  
Alluvial Deposits ◽  
Landers Earthquake ◽  
San Andreas

Abstract The smallest geomorphic offsets along a 35 km section of the San Andreas fault in the Carrizo Plain vary from 7 to 10 m. Our three-dimensional excavation of alluvial deposits a few km southeast of Wallace Creek confirms that at least 6.6 to 6.9 m of dextral slip occurred there during the latest large earthquake, in 1857. Dates on detrital charcoal suggest that the last event prior to the 1857 earthquake occurred before a date within the range A.D. 1305 to 1623. The 3-m range in smallest offsets along this portion of the fault may reflect either a 3-m variation in slip along the San Andreas in 1857, or 2 to 3 m of slip during an event prior to 1857. Observations made after the recent Landers earthquake are compatible with the hypothesis of large, local variations in slip during a single earthquake, but do not explain the cause of such variations. Off-fault dextral rotations would be one plausible explanation. However, paleoseismic data in the Carrizo Plain are too sparse to allow rejection of the alternative hypothesis that slip in the event prior to 1857 was only 2 to 3 m, an amount of slip which would be several times too small to fit a time-predictable model.

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.

Three-Dimensional Simulation of a Magnitude 7.75 Earthquake on the San Andreas Fault

Science ◽  
1995 ◽  
Vol 270 (5242) ◽  
pp. 1628-1632 ◽  
Author(s):  
K. B. Olsen ◽  
R. J. Archuleta ◽  
J. R. Matarese
Keyword(s):  
San Andreas Fault ◽  
Three Dimensional ◽  
San Andreas ◽  
Dimensional Simulation

Three-dimensional simulations of ground motions in the San Bernardino Valley, California, for hypothetical earthquakes on the San Andreas fault

1993 ◽  
Vol 83 (4) ◽  
pp. 1020-1041 ◽  
Author(s):  
Arthur Frankel
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
San Andreas Fault ◽  
Three Dimensional ◽  
Rupture Propagation ◽  
Earthquake Simulation ◽  
S Wave ◽  
Wave Maps ◽  
San Andreas ◽  
San Bernardino

Abstract Three-dimensional finite difference simulations of elastic waves in the San Bernardino Valley were performed for two hypothetical earthquakes on the San Andreas fault: a point source with moment magnitude M5 and an extended rupture with M6.5. A method is presented for incorporating a source with arbitrary focal mechanism in the grid. Synthetics from the 3-D simulations are compared with those derived from 2-D (vertical cross section) and 1-D (flat-layered) models. The synthetic seismograms from the 3-D and 2-D simulations exhibit large surface waves produced by conversion of incident S waves at the edge of the basin. Seismograms from the flat-layered model do not contain these converted surface waves and underestimate the duration of shaking. The seismograms from the 3-D simulations have larger amplitude coda than do the seismograms from the 2-D case because of the presence of off-azimuth surface wave arrivals in the 3-D simulations that are not included in the 2-D simulations. Snapshots of the wavefield of the 3-D simulation show that these off-azimuth arrivals represent surface waves reflected from the edges of the basin. The anelastic attenuation of the sediments is a key parameter controlling the overall duration of motion. Some of the coda energy at rock sites near the basin edges represents leakage of surface wave energy out of the basin. For the M6.5 earthquake simulation, the largest ground velocities occur where surface waves reflected from the edge of the basin interfere constructively with the trapped waves that follow the direct S-wave. Maps of maximum ground velocity are produced for two directions of rupture propagation. The largest velocities occur in localized portions of the basin. The location of the largest velocities changes with the rupture propagation direction. Contours of maximum shaking are also dependent on asperity positions and radiation pattern.

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