S-Wave Triggering of Tremor beneath the Parkfield, California, Section of the San Andreas Fault by the 2011 Tohoku, Japan, Earthquake: Observations and Theory

D. P. Hill; Z. Peng; D. R. Shelly; C. Aiken

doi:10.1785/0120120114

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0850061805 ◽

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.

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0830041020 ◽

1993 ◽

Vol 83 (4) ◽

pp. 1020-1041 ◽

Cited By ~ 14

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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Shear-wave anisotropy in the Parkfield Varian well VSP

Bulletin of the Seismological Society of America ◽

10.1785/bssa0800040857 ◽

1990 ◽

Vol 80 (4) ◽

pp. 857-869 ◽

Cited By ~ 2

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.

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S-wave observations in the Franciscan terrane, central California

Bulletin of the Seismological Society of America ◽

10.1785/bssa0710061863 ◽

1981 ◽

Vol 71 (6) ◽

pp. 1863-1874

Author(s):

Alan R. Levander ◽

Robert L. Kovach

Keyword(s):

Wave Velocity ◽

Upper Mantle ◽

Poisson’S Ratio ◽

San Andreas Fault ◽

Poisson's Ratio ◽

S Wave ◽

Central California ◽

Wave Velocities ◽

San Andreas ◽

Pn Velocity

Abstract We have examined S-wave arrivals from local earthquakes at a three-station seismograph array in the Franciscan terrane of the Diablo Range, California. A single crustal S-wave phase is observed with a velocity of 3.30 km/sec. Poisson's ratio calculated for the crust from a composite Wadati diagram is 0.27. Beyond epicentral distances of 90 km we have tentatively identified an Sn phase with a velocity of 4.35 km/sec. Other investigators have reported a Pn velocity of 8.0 km/sec corresponding to an upper mantle Poisson's ratio of 0.29. The 3.30 km/sec crustal S-wave velocity is intermediate in value between crustal S-wave velocities measured in similar terranes 75 km north at Berkeley and 90 km south at Bear Valley, suggesting a NW-SE crustal S-wave velocity gradient east of the San Andreas fault in the Franciscan terrane. This may be indicative of an increase in crustal rigidity from southeast to northwest, possibly associated with the differing levels of seismic activity observed along portions of the San Andreas fault.

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

Scientific Drilling ◽

10.5194/sd-11-14-2011 ◽

2011 ◽

Vol 11 ◽

pp. 14-28 ◽

Cited By ~ 77

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 2–3 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 – earthquake nucleation, propagation, and arrest – 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>

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