B.E. Tucker’s research while affiliated with Massachusetts Institute of Technology and other places

A closely spaced microearthquake array was operated in the Garm region of Central Asia during a 4 week period in 1978. In all, 142 events were well recorded during the investigation, 86 of which are considered to be located to an accuracy of better than 2km. Fault plane solutions for individual events are not well constrained, but first motions are sufficiently consistent to construct 5 composite solutions. The earthquakes can be divided into 3 regions: a southern region beneath the Peter I range, a central region beneath the Vakhsh thrust, and a northern region beneath the S Tien Shan. Each of these regions displays a distinct style of deformation.-from Authors

The response to earthquake motion of a site in one sediment-filled valley was observed to depend strongly on frequency and the position of the site within the valley and weakly on the input signal's azimuth and incidence angles. The observed input motion was weak, varying between 10−5 and 10−3g. Measurements of ground velocity were made on profiles across and along the valley and simultaneously on two nearby rock sites. The valley was 400 m wide and 700 m long, with a maximum sediment thickness of 60 m. The average seismic impedance contrast between the sediments and underlying basement rock was about 6:1. Ratios of Fourier spectra from soil sites to spectra from nearby rock sites showed apparent site amplifications of as much as a factor of ten, depending strongly on frequency and the distance of the site from the valley edge. The scatter in spectral ratios for earthquakes with different azimuths and angles of incidence was about a factor of two; thus local site effects that changed the amplitude of incident motion by a factor of two or more could be identified in the records of most earthquakes. Ground motion at valley-edge and mid-valley sites, separated by less than 100 m, differed by as much as a factor of five. This difference could cause large differential motion in structures spanning these sites. A theoretical, flat layer model of sediment response predicts the average behavior of the middle of the valley but not that of the valley edge. The lowest frequency responance apparently involves the sediments across the width of the valley, not just the vertical column of soil beneath each site.

Differences were measured in seismograms recorded at eight proximate hardrock sites: five in tunnels and three on outcrops on hills. Tunnel sites affected the amplitude of incident signals by as much as a factor of three, at frequencies inversely proportional to the depth of the site into the tunnel. For example, the maximum effect of a site 12 m deep was at 25 Hz while that of a site 85 m deep was at 11 Hz. Most of the observed tunnel effects could be explained in terms of interference between the incident and surface-reflected waves. Rock outcrop sites on hills affected the amplitude of incident signals by as much as a factor of eight, at frequencies inversely proportional to the dimension of the hill, and more for horizontally than for vertically polarized signals. Hard-rock site effects typically varied by no more than a factor of three for input signals from different earthquakes, having widely different azimuth and incident angles.

For three sediment-filled valleys, average site responses were experimentally determined and found to apply, to within ±0.25 log units, to earthquakes at any azimuth and to S waves and coda waves; for one of these valleys, ground accelerations were measured over the range 10−5 to 0.2 g, and no significant differences in site responses were apparent. In two valleys with sediment-basement impedance contrasts of 5:1 and 6:1, average site amplification is as much as a factor of five in certain frequency bands. In another valley, with a sediment-basement impedance contrast of 3:1, sediment amplification is about a factor of two. The duration of motion on the valley sediments, in narrow frequency bands, is equal to that on the bordering rock, implying that the observed amplification is not due to a high-Q resonance of the sediments.

Digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the Hindu Kush region of Afghanistan were used to determine the attenuation factors of the S-wave coda (Qc) and primary S waves (Qβ). An anomalously rapid decay of the coda shortly after the S-wave arrival, observed also in a study of coda in central Asia by Rautian and Khalturin (1978), seems to be due primarily to depth-dependent variations in Qc. In particular, we deduce the average Qc in the crust and uppermost mantle (<100-km depth) is approximately four times lower than the deeper mantle (<400-km depth) over a wide frequency range (0.4 to 24 Hz). Further, while Qc generally increases with frequency at any depth, the degree of frequency dependence of Qc depends on depth. Except at the highest frequency studied here (∼48 Hz), the magnitude of Qc at a particular frequency increases with depth while its frequency dependence decreases. For similar depths, determinations of Qβ and Qc agree, suggesting a common wave composition and attenuation mechanism for S waves and codas. Comparison of these determinations of Qc in Afghanistan with those in other parts of the world shows that the degree of frequency dependence of Qc correlates with the expected regional heterogeneity. Such a correlation supports the prejudice that Qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments.