Tomiichi Uetake

Simulation of Near-Fault Strong-Ground Motion Using Hybrid Green's Functions

The recently proposed hybrid Greens function method is designed to combine the advantages of both deterministic and stochastic approaches to simulating broadband ground motion when records of small events are not available. The method has the flexibility of incorporating complexities in the source, wave path, and local- site effects into strong ground motion simulations. In this article we analyze its effectiveness at simulating near-fault ground motions by comparisons with the em- pirical source time function method, empirical ground-motion-attenuation relations, and recorded near-fault ground motion. We present a simple model for introducing the effect of the radiation pattern to the stochastic Greens functions in the inter- mediate frequency range (1-3 Hz). The numerical test results of the method and the generally good agreement between simulated and recorded ground motion from the 17 January 1995 Kobe earthquake shown in this study indicate that the technique has the capability of reproducing the main characteristics of near-fault ground mo- tion.

Characteristics of Observed Peak Amplitude for Strong Ground Motion from the 1995 Hyogoken Nanbu (Kobe) Earthquake

Over 200 peak amplitudes of strong motion were observed at distances of less than 250 km from the fault during the 1995 Hyogo-ken Nanbu (Kobe) earth- quake. We analyzed the attenuation of the peak-ground acceleration and velocity as a function of distance and geological site conditions. The observed peak amplitudes agree well with those predicted by an empirical attenuation relation that was devel- oped for Japanese earthquakes. This demonstrates that on average the peak amplitude of the ground motion generated by this damaging earthquake did not exceed the level predicted by the empirical attenuation relation. We found a significant effect of the surface geology on the observed ground-motion peak amplitude. In particular for soft-soil sites, located near the fault, the peak-horizontal acceleration decreases rap- idly with distance as a result of the nonlinear response of soils. In order to take into account the effect of the site conditions we introduced correction factors to the ex- isting attenuation relation. This resulted in a significant reduction of the residuals between the predicted and observed peak amplitudes. Based on the attenuation re- lation corrected for the site condition effect we generated a map of horizontal peak- ground acceleration in the Kobe and Osaka area for the Kobe earthquake. The area of simulated large ground motion agrees well with the severe damage zone of inten- sity VII, JMA scale.

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 .

Ground-Motion Attenuation from the 1995 Kobe Earthquake Based on Simulations Using the Hybrid Green's Function Method

Because of the limited number of strong-motion stations in the Kobe area at the time of the 1995 Kobe earthquake, information about the characteristics of the near-fault ground-motion acceleration in bedrock is sparse. In this study we estimated the near-fault ground motion and derived characteristics of its attenuation on rock, using an hybrid broadband technique and a source model that have been validated against data. We found that at high frequencies the near-fault ground motion produced by the Kobe earthquake was of the same level as that predicted by the empirical attenuation relation for Japanese crustal earthquakes. The areas with the largest peak horizontal acceleration are located at the extremities of the fault and include most of the Kobe city.

Assessment of Site Effects on Seismic Motion in Ashigara Valley, Japan

We compared site amplifications at rock sites and sediment sites of Ashigara Valley, Japan, using ground-motion data from five remote (>700 km) large (> M 7) events. The use of remote large events is advantageous to estimating site factors because the source and path effects are considered to be common with a sufficient accuracy and the ground motions will cover a wide-frequency band. Ground motions at both sediment and rock sites were coherent in frequencies lower than 0.1 Hz. This means that the wavelength in these frequencies is longer than the size of the valley (12 km long and 5 km wide). Site amplification factors were determined by taking spectral ratios with reference to one rock outcrop site. The amplification factors of sediment sites deviated 2–10 times with respect to the rock site in the frequency range higher than 0.1 Hz, in which significant peaks at about 1–2 Hz were found at most sites. These dominant amplifications in sedimentary basin are most essential for assessing earthquake hazard in the region. For sediment sites, the peak frequencies of spectral ratios to the rock sites were stable for different events and coincided with those of horizontal to vertical spectral ratios for the S -wave portion and those of relative site factors estimated separately by the generalized inversion method using local small-events data in the frequency range higher than 2 Hz. Although spectral ratios for frequencies lower than about 1 Hz should be affected by 3D basin structure, 1D S -wave responses represent the amplification of ground motion in the sediment sites for frequencies higher than 2 Hz.

Forced vibration test of the Hualien large scale soil structure interaction model. (part 2)

A Large-Scale Seismic Test (LSST) programme conducted at Hualien (stiff soil site), Taiwan, has started (Tang et al., 1991) to provide earthquake-induced soil-structure interaction (SSI) data as an extension of the same kind of program conducted at Lotung (soft soil site) (EPRI, 1989). Two tests have been performed: a Forced Vibration Test (FVT) without embedment (Morishita et al., 1993) and the same test with embedment. Horizontal excitations were applied in two different directions both on the roof slab and on the basemat of a 14-scale containment model. Vertical excitations were applied on the basemat only. The concept of ‘Principal Axes (D1 D2)’ was introduced for the SSI system, and the basic dynamic characteristics of the soil-structure system were obtained for D1, and D2. As an example, resonance frequencies for horizontal excitations were 4.1 and 4.6 Hz without embedment, and 6.1 and 6.3 Hz with embedment for D1and D2 based on results transformed to principal axes. Finally, a soil impedance function was obtained by a newly defined error minimizing method. This function also simulates the classic embedment effect.