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DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF SOUTHERN CALIFORNIA, LOS ANGELES, CALIFORNIA 90089-0740
Abstract
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 ts/tp 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
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 Qß 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 Qß, because they are limited to a narrow band between 5 and 10 Hz and affected by the near-surface amplification. A Qc 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 (fc 10 Hz) at 1500 m depth than at (fc 7 Hz) 420 and 0 m depth. Similarly, fmax significantly decreases as the waveforms travel toward the earth's surface, indicating that fmax is affected by near-surface attenuation. Beyond fc, 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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