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

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.

[1]  M. Ohtake.  Temporal change of Qp −1 in focal area of 1984 Western Nagano, Japan, Earthquake as derived from pulse width analysis , 1987 .

[2]  E. Hauksson.  Seismotectonics of the Newport-Inglewood fault zone in the Los Angeles basin, southern California , 1987 .

[3]  Ta-Liang Teng,et al.  A seismic telemetry system of large dynamic range , 1986 .

[4]  Keiiti Aki,et al.  Site amplification of coda waves from local earthquakes in central California , 1986 .

[5]  Pierre-Yves Bard,et al.  The seismic response of two-dimensional sedimentary deposits with large vertical velocity gradients , 1986 .

[6]  Edward Cranswick,et al.  High-frequency observations and source parameters of microearthquakes recorded at hard-rock sites , 1985 .

[7]  Peter E. Malin,et al.  Preliminary results from vertical seismic profiling of Oroville microearthquake S‐waves , 1985 .

[8]  John G. Anderson,et al.  A MODEL FOR THE SHAPE OF THE FOURIER AMPLITUDE SPECTRUM OF ACCELERATION AT HIGH FREQUENCIES , 1984 .

[9]  Roger D. Borcherdt,et al.  A comparative ground response study near Los Angeles using recordings of Nevada nuclear tests and the 1971 San Fernando earthquake , 1984 .

[10]  T. G. Barker,et al.  Shallow shear wave velocity and Q structures at the El Centro strong motion accelerograph array , 1983 .

[11]  A. Papageorgiou,et al.  A specific barrier model for the quantitative description of inhomogeneous faulting and the prediction of strong ground motion. I. Description of the model , 1983 .

[12]  James N. Brune,et al.  Spectral attenuation of SH waves along the Imperial fault , 1982 .

[13]  Arthur Frankel,et al.  The effects of attenuation and site response on the spectra of microearthquakes in the northeastern Caribbean , 1982 .

[14]  Charles S. Mueller,et al.  Source parameters of the 1980 Mammoth Lakes, California, earthquake sequence , 1982 .

[15]  J. Boatwright,et al.  A spectral theory for circular seismic sources; simple estimates of source dimension, dynamic stress drop, and radiated seismic energy , 1980 .

[16]  Einar Kjartansson,et al.  Constant Q-wave propagation and attenuation , 1979 .

[17]  M. J. R. Healy,et al.  Multichannel Time Series Analysis with Digital Computer Programs. , 1978 .

[18]  W. B. Joyner,et al.  Analysis of seismograms from a downhole array in sediments near San Francisco Bay , 1976 .

[19]  K. Aki,et al.  Origin of coda waves: Source, attenuation, and scattering effects , 1975 .

[20]  C. Bufe,et al.  Shear-wave attenuation along the San Andreas fault zone in central California , 1975, Bulletin of the Seismological Society of America.

[21]  T. Teng,et al.  Microearthquakes and water flooding in Los Angeles , 1973, Bulletin of the Seismological Society of America.

[22]  P. Newman.  Divergence Effects In A Layered Earth , 1973 .

[23]  J. Brune.  Tectonic stress and the spectra of seismic shear waves from earthquakes , 1970 .

[24]  K. Aki.  Scaling law of seismic spectrum , 1967 .

[25]  David Oppenheimer,et al.  FPFIT, FPPLOT and FPPAGE; Fortran computer programs for calculating and displaying earthquake fault-plane solutions , 1985 .

[26]  Paul G. Richards,et al.  Quantitative Seismology: Theory and Methods , 1980 .

[27]  M. E. O'neill,et al.  Calculation of the frequency response of the USGS telemetered short-period seismic system , 1980 .

[28]  Keiiti Aki,et al.  Attenuation of shear-waves in the lithosphere for frequencies from 0.05 to 25 Hz , 1980 .

[29]  R. F. Yerkes,et al.  Geology of the Los Angeles Basin, California: an introduction , 1965 .