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Results from a 1500 m deep, three-level downhole seismometer array: Site response, low Q values, and f_(max)

Hauksson, Egill and Teng, Ta-Liang and Henyey, Thomas L. (1987) Results from a 1500 m deep, three-level downhole seismometer array: Site response, low Q values, and f_(max). Bulletin of the Seismological Society of America, 77 (6). pp. 1883-1904. ISSN 0037-1106.

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A three-level downhole array is being operated in a 1500-m-deep borehole within the seismically active Newport-Inglewood fault zone, Los Angeles basin. The array consists of three three-component 4.5 Hz seismometers deployed at the surface, and at 420 and 1500 m depth. An M = 2.8 earthquake that occurred 0.9 km away from the array at a depth of 5.3 km on 31 July 1986 generated rays traveling almost vertically up the downhole array. The P- and S-wave pulse shapes show increasing pulse rise time with decreasing depth, and the initial pulse slope is less steep at the surface than at 1500 m. The average value of t_s/t_p between 1500 and 420 m depth is 1.7 and between 420 and 0 m is 3.4. A near-surface site response results in amplification on the P wave by a factor of four and S waves by a factor of nine. These data indicate a near-surface Q_α of 44 ± 13 for rays traveling almost vertically. In the case of S waves, most of the high frequency content of the waveform beyond ∼ 10 Hz observed at 1500 m depth is lost through attenuation before the waveform reaches 420 m depth. The average Q_β is 25 ± 10 between 1500 and 420 m depth and 108 ± 36 between 420 and 0 m depth. The spectra of the S waves observed at 420 and 0 m of the downward reflected S phases may overestimate Q_β, because they are limited to a narrow band between 5 and 10 Hz and affected by the near-surface amplification. A Q_c of 160 ± 30 at 6 Hz was determined from the decay rate of the coda waves at all three depths. The corner frequency as determined from displacement spectra may be higher (f_c ∼ 10 Hz) at 1500 m depth than at (f_c ∼ 7 Hz) 420 and 0 m depth. Similarly, f_(max) significantly decreases as the waveforms travel toward the earth's surface, indicating that f_(max) is affected by near-surface attenuation. Beyond f_c, the average slopes of the spectra falloff of P-wave spectra is ∼f^(−2) at 1500 m depth and ∼ f^(−3) at the surface.

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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1987 Seismological Society of America. Manuscript received 12 January 1987. M. Roberston, D. Manov, and J. Scott helped design and install the downhole array and performed necessary technical maintenance. We thank K. Aki, an anonymous reviewer, and D. M. Boore for comments, and L. M. Jones, A Frankel, and P. E. Malin for reviews. This research was supported by U.S. Geological Survey Grant 14-08-0001-G1158, U.S. Geological Survey Contract 14-08-0001-A0264, and NSF Grants EAR-8407830 and EAR-8608997. The figures were drafted by J. Dodds and the manuscript typed by D. Moton and S. Turnbow.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
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Issue or Number:6
Record Number:CaltechAUTHORS:20130306-112015290
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Official Citation:Egill Hauksson, Ta-liang Teng, and Thomas L. Henyey Results from a 1500 m deep, three-level downhole seismometer array: Site response, low Q values, and fmax Bulletin of the Seismological Society of America December 1987 77:1883-1904
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37342
Deposited By: Ruth Sustaita
Deposited On:06 Mar 2013 19:41
Last Modified:03 Oct 2019 04:46

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