Seismometer Array : Site Response , Low Q Values , and /

A three-level downhole array is being operated in a 1500-m-deep borehole within the seismically active Newport-lnglewood 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 Pand 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 . / tp between 1500 and 420 m depth is 1.7 and between 420 and 0 m is 3.4. A nearsurface 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 Q~ 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~, 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, fm.x significantly decreases as the waveforms travel toward the earth's surface, indicating that fm,= 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. INTRODUCTION A three-level downhole seismometer array with seismometer sondes at the surface, and at 420 and 1500 m depth has been installed in a deep abandoned oilwell in the Baldwin Hills, Southern California. The Baldwin Hills are cross-cut by the NewportInglewood fault zone, a major earthquake fault in the Los Angeles basin (Yerkes et al., 1965; Hauksson, 1987; Figure 1). The downhole seismometer array is designed to record seismic waves from local earthquakes to study near-surface attenuation and site response such as amplification of seismic waves. Numerous investigators have suggested that surface recordings of local earthquakes and hence the source parameter determinations are contaminated by attenuation of high frequencies and/or dominated by a narrow band of frequencies amplified through reverberation close to the earth's surface (e.g., Frankel, 1982; Hanks, 1982; Archuleta et al., 1982; Cranswick et al., 1985). Downhole seismometer arrays should be able to provide data to quantify such effects. In addition to 1883 1 8 8 4 EGILL HAUKSSON, TA-LIANG TENG, AND THOMAS L. HENYEY 25.0 ' 22.0 ' 21.0' I ". I Oownhole i : , I " . . . . I I Seismometer ~ ~'_" ". . I Array i ' l ' " ~';:. " ...... • , ' . . " ' . . o,' i -q,' ,, ' / t. I "~ l I " . , i ........ . . .,,. ,.,, ~ . , -~ .. . . , ~ 3 4 o 0 0 ' ' ' ' , " , I A ' . ' , ~ ' / i July 31, 1986 M : 2.8 / \ lce~tral%~(ll / X ~ Gr~ben~\~,z ;"