| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
UNITED STATES GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
Abstract
To investigate near-surface site effects in granite rock, we drilled 300-m-deep boreholes at two sites which are collocated with stations from the digital array at Anza, California. The first borehole was sited at station KNW (Keenwild fire station), which is located along a ridge line about 8.7 km east of the San Jacinto Fault zone. Station PFO (Piñon Flat Observatory), chosen for the second site, is another 6 km further to the east of station KNW and is located on a gently sloping hillside. We logged each borehole for P- and S-wave velocities, as well as for crack density and orientation.
P waves were generated by striking a plate with a hammer at the surface. A tool consisting of weighted anvils driven by compressed air against end plates along a 3.5-m beam was used to generate shear waves. Signals were recorded downhole with a three-component sensor package at 2.5-m intervals from the surface to 50 m depth, and at 5-m intervals from 50 m depth to the bottom of the hole. Velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth. Travel times were computed for the first breaks at shallow depths, however, below about 100 m depth, times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable. The KNW site yielded a shear velocity of 1.9 km/sec by only 30 m in depth and reached close to 2.6 km/sec at the bottom of the hole. P-wave velocities at KNW were also high at 5.4 km/sec starting at 120 m depth. The PFO site had similar but slightly higher shear-wave velocities. The bottom-hole shear-wave velocity reached 3.0 km/sec, and the P-wave velocity was 5.4 km/sec.
Shear-wave attenuation was computed using both the pulse rise time and spectral ratio methods. At station KNW, attenuation was significant only in an interval between 17.5 and approximately 40 m in depth. Over the top 50 m, attenuation corresponding to a Q of about 8 was obtained. A total T* of 0.004 sec was measured for this interval. Pulse rise times also increased rapidly in this zone. The spectral ratio data for station PFO yields two peaks in attenuation above 50 m. Similar to the attenuation found for station KNW, the peak in attenuation corresponds to a Q of about 11, averaged over the top 50 m. Spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 Hz.
Significant motion perpendicular to the polarizations of the first shear-wave arrival was recorded within a few meters of the surface. Apparently, the rock structure is sufficiently complicated that body waves are being converted (SH to SV at oblique incidence) very close to the surface. The presence of these elliptical particle motions within a mere few m of the pure shear-wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not, by itself, a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth's crust.
Crack densities and orientations were determined from televiewer records. These records showed cracks with a preferred direction at station KNW and of a greater density than at station PFO. At station PFO, crack densities were smaller and more diffuse in orientation.
This article has been cited by other articles:
|
|
 |
|
  K. W. Campbell Estimates of Shear-Wave Q and {kappa}0 for Unconsolidated and Semiconsolidated Sediments in Eastern North America Bulletin of the Seismological Society of America, August 1, 2009; 99(4): 2365 - 2392. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Kinoshita and M. Ohike Coherency Characteristics of Body Waves Traveling in a Sedimentary Layer-Basement System in the Kanto Region, Japan Bulletin of the Seismological Society of America, February 1, 2006; 96(1): 165 - 175. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Viscoelastic Wave Simulation in Basins by a Variable-Grid Finite-Difference Method Bulletin of the Seismological Society of America, December 1, 2001; 91(6): 1741 - 1749.
|
|
|
|
|
|
Empirical Green's Function Analysis of Recent Moderate Events in California Bulletin of the Seismological Society of America, June 1, 2001; 91(3): 456 - 467.
|
|
|
|
|
|
L. K. Bear, G. L. Pavlis, and G. H. R. Bokelmann Multi-wavelet analysis of three-component seismic arrays: Application to measure effective anisotropy at Pinon Flats, California Bulletin of the Seismological Society of America, June 1, 1999; 89(3): 693 - 705. [Abstract] [PDF]
|
|
|
|
|
|
F. L. Vernon, G. L. Pavlis, T. J. Owens, D. E. McNamara, and P. N. Anderson Near-surface scattering effects observed with a high-frequency phased array at Pinyon Flats, California Bulletin of the Seismological Society of America, December 1, 1998; 88(6): 1548 - 1560. [Abstract] [PDF]
|
|
|
|
|
|
C. Lomnitz Frequency response of a strainmeter Bulletin of the Seismological Society of America, August 1, 1997; 87(4): 1078 - 1080. [Abstract] [PDF]
|
|
|
|
|
|
D. M. Boore and W. B. Joyner Site amplifications for generic rock sites Bulletin of the Seismological Society of America, April 1, 1997; 87(2): 327 - 341. [Abstract] [PDF]
|
|
|
|
|
|
J. H. Steidl, A. G. Tumarkin, and R. J. Archuleta What is a reference site? Bulletin of the Seismological Society of America, December 1, 1996; 86(6): 1733 - 1748. [Abstract] [PDF]
|
|
|
|
|
|
J. G. Anderson, Y. Lee, Y. Zeng, and S. Day Control of strong motion by the upper 30 meters Bulletin of the Seismological Society of America, December 1, 1996; 86(6): 1749 - 1759. [Abstract] [PDF]
|
|
|
|
|
|
G. Eli Baker, J. Bernard Minster, G. Zandt, and H. Gurrola Constraints on crustal structure and complex Moho topography beneath Pinon Flat, California, from teleseismic receiver functions Bulletin of the Seismological Society of America, December 1, 1996; 86(6): 1830 - 1844. [Abstract] [PDF]
|
|
|
|
|
|
C.-H. Lin and S. W. Roecker P-wave backazimuth anomalies observed by a small-aperture seismic array at Pinyon Flat, southern California: Implications for structure and source location Bulletin of the Seismological Society of America, April 1, 1996; 86(2): 470 - 476. [Abstract] [PDF]
|
|
|
|
|
|
O. Coutant Observation of shallow anisotropy on local earthquake records at the Garner Valley, Southern California, downhole array Bulletin of the Seismological Society of America, April 1, 1996; 86(2): 477 - 488. [Abstract] [PDF]
|
|
|
|
|
|
C. J. Young, E. P. Chael, D. A. Zagar, and J. A. Carter Variations in noise and signal levels in a pair of deep boreholes near Amarillo, Texas Bulletin of the Seismological Society of America, October 1, 1994; 84(5): 1593 - 1607. [Abstract] [PDF]
|
|
|
|
|
|
J. Mori, J. Filson, E. Cranswick, R. Borcherdt, R. Amirbekian, V. Aharonian, and L. Hachverdian Measurements of P and S wave fronts from the dense three-dimensional array at Garni, Armenia Bulletin of the Seismological Society of America, August 1, 1994; 84(4): 1089 - 1096. [Abstract] [PDF]
|
|
|
|
|
|
F. K. Wyatt, D. C. Agnew, and M. Gladwin Continuous measurements of crustal deformation for the 1992 Landers earthquake sequence Bulletin of the Seismological Society of America, June 1, 1994; 84(3): 768 - 779. [Abstract] [PDF]
|
|
|
|
|
|
J. F. Gibbs, D. M. Boore, W. B. Joyner, and T. E. Fumal The attenuation of seismic shear waves in quaternary alluvium in Santa Clara Valley, California Bulletin of the Seismological Society of America, February 1, 1994; 84(1): 76 - 90. [Abstract] [PDF]
|
|
|
|
|
|
R. J. ARCHULETA, S. H. SEALE, P. V. SANGAS, L. M. BAKER, and S. T. SWAIN Garner Valley downhole array of accelerometers: Instrumentation and preliminary data analysis Bulletin of the Seismological Society of America, August 1, 1992; 82(4): 1592 - 1621. [Abstract] [PDF]
|
|
|
|
|
|
J. G. ANDERSON A preliminary descriptive model for the distance dependence of the spectral decay parameter in southern California Bulletin of the Seismological Society of America, December 1, 1991; 81(6): 2186 - 2193. [Abstract] [PDF]
|
|
|
|
|
|
H.-P. LIU, R. E. WARRICK, R. E. WESTERLUND, and J. B. FLETCHER A three-component borehole seismometer for earthquake seismology Bulletin of the Seismological Society of America, December 1, 1991; 81(6): 2458 - 2485. [Abstract] [PDF]
|
|
|
|
|
|
R. C. ASTER and P. M. SHEARER High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations Bulletin of the Seismological Society of America, August 1, 1991; 81(4): 1057 - 1080. [Abstract] [PDF]
|
|
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
