Masanori Horike

Comparison of Site Response Characteristics Inferred from Microtremors and Earthquake Shear Waves

We investigated the validity of seismic site response characteristics es- timated from microtremors by comparing them with those of earthquake motions. For this purpose we observed microtremors as well as earthquake motions using large (5-km diameter) and small (0.5-km diameter) arrays deployed on soft sedi- ments. Specifically, we examined four estimates from microtremors: relative site amplification factors to incident shear waves, site amplification factors by the Naka- mura method, resonance frequency in horizontal-to-vertical spectral ratios, and horizontal-to-vertical spectral ratios. As a result of the comparisons, we obtained the following conclusions. The relative amplification factors can be inferred from horizontal-component ratios of microtremors to a reference site within a small area of several hundred meters. The horizontal-to-vertical spectral ratios inferred by the Nakamura method partly reflect site amplification factors, but do not agree with site amplification factors. A sharp-peak frequency in the horizontal-to-vertical spectral ratios is possibly the resonance frequency. The horizontal-to-vertical spectral ratios of microtremors either agree with those of earthquake motions at some array sites or are slightly smaller at the other sites.

Nonlinear behaviour of scoria soil sediments evaluated from borehole records in eastern Shizuoka prefecture, Japan

We evaluate the non-linear behaviour of soil sediments, analysing five weak and four strong motions observed at depths of 1 m and 28 m, in eastern Shizuoka prefecture, Japan. We identify S-wave velocities and frequency-dependent damping factors by minimizing the residual between observed and theoretical spectral ratios, based on a linear one-dimensional model. We find that S-wave velocities identified from strong motions, whose peak ground acceleration are 440, 210, 176, and 140 cm/s 2 , are significantly smaller than those identified from weak motions. The shear modulus reduction ratios estimated from identified S-wave velocities become clear above an effective shear strain of 10 -4 and agree with laboratory test results below an effective shear strain of 8 x 10 -4 . The differences of damping factors between weak and strong motions are not clear below this effective shear strain, as the laboratory test suggested. The equivalent linear one-dimensional model, with frequency-dependent damping factors, is confirmed to be valid to simulate strong motions at least an effective shear strain of less than 4 x10 -4 .

Inference of Q-Values below 1 Hz from Borehole and Surface Data in the Osaka Basin by Three-Component Waveform Fitting

The quality factor of the shear waves Qs below 1 Hz in the Osaka sedimentary basin, Japan, is inferred from borehole earthquake-motion recordings and surface earthquake-motion recordings by three-component waveform fitting. Assuming quality factor of S waves Qs to be a function of the frequency f and the shear- wave velocity Vs in meters in the form AVs f n , the waveform fitting of borehole data reveals that coefficient A is approximately 0.24, and exponent n is less than or equal to zero. To verify Qs -values from borehole data and, if possible, to confine Qs -values to a narrow range, we infer the Qs -values from surface earthquake-motion recordings by fitting them with earthquake motions simulated by the finite-difference method. It indicates that Qs -values are negatively frequency dependent and that the lower limit of Qs -values is approximately 0.24 Vs f −1.

Possibility of Spatial Variation of High-Frequency Seismic Motions due to Random-Velocity Fluctuation of Sediments

Considering the possibility that random-velocity fluctuation of sediments may affect spatial variation of high-frequency seismic motions in small areas (a few hundred meters in diameter), we examine this possibility experimentally and numerically. By comparing seismic-array data obtained at three geologically different sites (rock, stiff-sediment, and soft-sediment sites), we make two observations related to the effect of random-velocity fluctuation of sediments. The first observation is that above 3 Hz, waveform-spatial variation evaluated in terms of the cross-correlation is larger at the stiff-sediment site than at the rock site. The second is that above 2 Hz, the amplitude-spatial variation evaluated by the spectral ratio shows more observation-station-specific behavior at the soft-sediment site than at the stiff-sediment site. We try to explain these two observations by modeling the sediments as random media. Two-dimensional finite-difference simulations for plane SH - and SV -wave incidences demonstrate that the sediments may affect the seismic waveform variation above 3 Hz if the autocorrelation functions of the velocity fluctuations are in the range derived from well-log velocity data. The velocity structure at the soft-sediment site is modeled as a two-layered random media having a low-velocity surface layer. The finite-difference simulations for this sediment model demonstrate that it is also possible that amplitude variation above 2 Hz is caused by random-velocity fluctuations in the low-velocity surface layer.

Simulation of High-Frequency Strong Vertical Motions using Microtremor Horizontal-to-Vertical Ratios

We examined the possibility for simulation of vertical ground motions using microtremor horizontal-to-vertical spectral ratios. Comparing earthquake and microtremor horizontal-to-vertical spectral ratios, we showed that although microtremor horizontal-to-vertical ratios are smaller than earthquake-motion horizontal-to-vertical ratios, their spectral shapes are similar. Also, we showed that a correction factor to compensate for the difference between the two ratios is almost the same among geologically similar sites. We then tried to reproduce strong vertical ground motions for the 1995 Hyogo-ken Nanbu earthquake using corrected microtremor horizontal-to-vertical ratios. Vertical accelerations simulated were well reproduced in envelope and high-frequency contents. Manuscript received 1 March 2002.

Development of Inference Methods for Rotational Motions on Ground Surface and Application to Microtremors Acquired with a Small-Size Dense Array

In this study, we develop two methods for the inference of rotation vector on ground surface, two rocking rotations and a single torsional rotation. The first, termed nth-order elastic method, is based on the elasticity of the ground surface. The rotation vector is constructed from the first derivative with respect to the space of the ground surface motions. The first derivative is calculated from simultaneous equations by n-th order Taylor expansion obtained by difference motion between multiple observation points. Meanwhile, the second, termed rigid method, is based on the rigidity of ground surface and the rotation.