Kuo-Liang Wen

Nonlinear soil amplification inferred from downhole strong seismic motion data

Nonlinear seismic response of soil is studied by comparison of the frequency-dependent soil amplification functions calculated on weak and strong motion. Amplifications are obtained by dividing Fourier amplitude spectra of acceleration at the ground surface by the spectra at the depths of 11 and 47 m in a borehole. Observed weak and strong motion spectral ratios are compared with those theoretically deduced from the one of the nonlinear soil models adopted in geotechnical engineering. Significant deamplification of the strong motion having PGA (peak ground acceleration) greater than 0.16 g, relative to the weak motion, is exhibited by the experimental ratios in the frequency range that is consistent with the model prediction. Existence of the different frequency bands, in which specific features of the nonlinear soil response are recognized in the theory, is also confirmed. These results give evidence of that nonlinear response can be observed at soft sedimentary sites from the real strong motion data.

Seismological evidence for nonlinear elastic ground behavior during large earthquakes

Abstract Amplification of earthquake-induced seismic waves by soft superficial deposits often causes significant damages in the urban areas. In predicting this effect for large future earthquakes, the linear elastic response of soils is customarily assumed. To check this assumption, we have analyzed surface and downhole acceleration data from the SMART1 and SMART2 strong motion arrays in Taiwan, covering peak accelerations of up to 0·3 g. First, frequency-dependent amplification induced by the alluvial deposits at the SMART1 array was estimated using spectral ratio technique, where the records at rock site were taken as a reference motion. Statistically validated reduction in soil amplification in the strong motion relative to the weak motion in the frequency range between approximately 1 and 9 Hz was detected. Secondly, relative site responses between the Pleistocene and recent sedimentary deposits at the SMART2 array were studied. Relative amplification was shown to be clearly dependent on the excitation level. Thirdly, we compared experimentally recorded uphole/downhole spectral ratios on weak and strong ground motion with the theoretical response yielded by the geotechnical code DESRA2 which assumes hysteretic constitutive relationship of soil. Major symptoms of nonlinear ground behavior predicted by the model were found in the observed data. Back-calculation of the shear wave velocities to the depth of 47 m shows nearly 50% decrease in the strongest quakes, also accounted for by the nonlinear soil behavior.

Geochemical variation of soil–gas composition for fault trace and earthquake precursory studies along the Hsincheng fault in NW Taiwan

The present study is proposed to investigate geochemical variations of soil-gas composition in the vicinity of the geologic fault zone of Hsincheng in the Hsinchu area of Taiwan. Soil-gas surveys have been conducted across the Hsincheng fault, to look for the degassing pattern of this fault system. During the surveys, soil-gas samples were collected along traverses crossing the observed structures. The collected soil-gas samples were analysed for He, Rn, CO(2), CH(4), Ar, O(2) and N(2). The data analysis clearly reveals anomalous values along the fault. Before selecting a monitoring site, the occurrence of deeper gas emanation was investigated by the soil-gas surveys and followed by continuous monitoring of some selected sites with respect to tectonic activity to check the sensitivity of the sites. A site was selected for long term monitoring on the basis of coexistence of high concentration of helium, radon and carrier gases and sensitivity towards the tectonic activity in the region. A continuous monitoring station was established at Hsinchu National Industrial Science Park (HNISP) in October 2005. Preliminary results of the monitoring station have shown possible precursory signals for some earthquake events.

Phase Velocity Variation at Periods of 0.5–3 Seconds in the Taipei Basin of Taiwan from Correlation of Ambient Seismic Noise

Improving seismic hazard mitigation of the densely populated metropol- itan area of and around the capital of Taiwan requires detailed knowledge of the 3D crustal structure of Taipei basin. The high levels of ambient noise and the low levels of regional seismicity of this region complicate investigations of crustal structure with traditional seismic exploration or earthquake tomography methods. We investigate the shallow crust in the metropolitan region using surface wave array tomography with time domain empirical Greens function (TDEGF) inferred from correlation of ambient seismic noise. Analysis of the TDEGF amplitudes suggests that the dominant sources of ambient seismic noise are the coastlines and shallow continental shelf of the Taiwan Strait, northwest of the study region. Our study demonstrates that ambient seismic noise tomography is feasible at periods of 0.5-3 s, which is much shorter than the 10-30 s used in most other studies, and which opens new opportunities for high resolution studies of near-surface heterogeneity. The lateral variation in Rayleigh wave phase velocity correlates well with surface geology and suggests that faults play an important role in the regional tectonic setting. High phase velocities mark the Tatun volcanic area, the Kuanyin Mountain dominated by Quaternary igneous rock, and the Miocene Western Foothills south of the Taipei fault. Low phase velocities characterize regions are along western and southeastern edges of the Taipei basin and the Pleis- tocene Linkou tableland. Main faults in the region are either marked by low phase velocities or define transitions between regions of high- and low-velocity anomalies.

Empirical Study of Sediment‐Filled Basin Response: The Case of Taipei City

We analyze the site response of the Taipei basin using the records obtained by the Taiwan Strong Ground Motion Instrumentation Program (TSMIP) network. Records of 66 earthquakes of M=2.6-6.5 with a hypocentral depth varying from 1 km to 118 km and hypocentral distances of up to 150 km are studied for 35 stations located within this triangle-shaped alluvium structure. The site response is obtained in terms of spectral ratios calculated by dividing of the site spectrum by the reference spectrum estimated for a hypothetical “very hard rock” site. The recently developed empirical source scaling and attenuation models for the Taiwan region are used for the reference spectra calculation. This approach allows us to evaluate the variability of spectral ratios due to uncertainties introduced by source and propagation path effects and variability in the site response itself. The characteristics of site response in the Taipei basin depend on the properties of soil deposits and, in general, may be described by 1-D models. However, there are some peculiarities of spectral ratios that show the influence of subsurface topography.

Why 1G Was Recorded at TCU129 Site During the 1999 1Chi-Chi, Taiwan, Earthquake

The strong-motion station TCU129 recorded a peak horizontal acceleration higher than 1 g during the 1999 Chi-Chi, Taiwan, earthquake. Yet no structural damages occurred in its vicinity. Even some old buildings not far away from the station were not damaged. There did not seem to be very strong ground motion in this area during the Chi-Chi earthquake. To resolve these conflicting phenomena, we performed microtremor surveys in this area and installed additional two strong-motion accelerographs inside station TCU129 to compare the ground-motion records between the original and new accelerographs. We also compared the ground accelerations between station TCU129 and a nearby station TCU076. The results indicate that the high peak acceleration recorded at TCU129 during the 1999 Chi-Chi, Taiwan, earthquake was due to the effects of the concrete recording pier at station TCU129, and not due to the source, path, or site effects of the earthquake. Therefore, the peak acceleration values recorded at station TCU129 should not be used in studies of peak acceleration attenuation. However, the records are still useful, especially the integrated velocity and displacement time histories, for other studies.

Comparison of the Taiwan Chi-Chi Earthquake Strong-Motion Data and Ground-Motion Assessment Based on Spectral Model from Smaller Earthquakes in Taiwan

The design of buildings and structures in earthquake-prone regions must be based on information relating to expected seismic effect. Estimations of time domain and spectral parameters of ground motion are obtained by empirical relations that connect these to earthquake magnitude, distance, and local soil conditions. In the Taiwan region, the models for estimating ground motion parameters were obtained recently on the basis of recordings of small to moderate (5.0 ≤ M L ≤ 6.5) earthquakes. A large collection of acceleration records from the recent M L 7.3 ( M w 7.6) Chi-Chi earthquake and aftershocks makes it possible to test the applicability of the established models in the case of larger events. We compared modeled Fourier amplitude spectra and peak accelerations and response spectra, which were calculated using the stochastic approach from the Fourier spectra, and the data obtained during the mainshock and the largest aftershocks ( M L 6.6-6.8). It has been shown that the previously established regional spectral model (Fourier spectra of ground acceleration) may be applied for evaluation of ground motion parameters for earthquakes (reverse faulting) of magnitudes up to M L 6.8-7.0 and hypocentral depth more than 10 km. To satisfy to the peculiarities of ground-motion propagation during shallow (depth less than 10 km) events, the model should be revised. The analysis of accelerograms from the mainshock ( M L 7.3, depth 8 km, 314 records) and large shallow aftershock ( M L 6.8, depth 10 km, 350 records) allows the authors to obtain the revised spectral model for average-soil conditions. Manuscript received 31 May 2001.

Strong Ground Motion in the Taipei Basin from the 1999 Chi-Chi, Taiwan, Earthquake

The Taipei basin, located in northwest Taiwan about 160 km from the epicenter of the Chi-Chi earthquake, is a shallow, triangular-shaped basin filled with low-velocity fluvial deposits. There is a strong velocity contrast across the basement interface of about 600 m/sec at a depth of about 600–700 m in the deeper section of the basin, suggesting that ground motion should be amplified at sites in the basin. In this article, the ground-motion recordings are analyzed to determine the effect of the basin both in terms of amplifications expected from a 1D model of the sediments in the basin and in terms of the 3D structure of the basin. Residuals determined for peak acceleration from attenuation curves are more positive (amplified) in the basin (average of 5.3 cm/sec 2 compared to −24.2 cm/sec 2 for those stations outside the basin and between 75 and 110 km from the surface projection of the faulted area, a 40% increase in peak ground acceleration). Residuals for peak velocity are also significantly more positive at stations in the basin (31.8 cm/sec compared to 20.0 cm/sec out). The correlation of peak motion with depth to basement, while minor in peak acceleration, is stronger in the peak velocities. Record sections of ground motion from stations in and around the Taipei basin show that the largest long-period arrival, which is coherent across the region, is strongest on the vertical component and has a period of about 10–12 sec. This phase appears to be a Rayleigh wave, probably associated with rupture at the north end of the Chelungpu fault. Records of strong motion from stations in and near the basin have an additional, higher frequency signal: nearest the deepest point in the basin, the signal is characterized by frequencies of about 0.3 – 0.4 Hz. These frequencies are close to simple predictions using horizontal layers and the velocity structure of the basin. Polarizations of the S wave are mostly coherent across the array, although there are significant differences along the northwest edge that may indicate large strains across that edge of the basin. The length of each record after the main S wave are all longer at basin stations compared to those outside. This increase in duration of ground shaking is probably caused by amplification of ground motion at basin stations, although coda Q (0.67 – 1.30 Hz) is slightly larger inside the basin compared to those at local stations outside the basin. Durations correlate with depth to basement. These motions are in the range that can induce damage in buildings and may have contributed to the structural collapse of multistory buildings in the Taipei basin.

Frequency-Dependent Site Amplifications with f 0.01 Hz Evaluated from Velocity and Density Models in Central Taiwan

The frequency-dependent site amplifications at 87 free-field strong- motion station sites in central Taiwan are evaluated from the velocity and density structures constructed from borehole data measured at shallow depths and the veloc- ity models inferred from earthquake data at great depths. Results based on the quarter- wavelength approximation method proposed by Boore and Joyner (1997) show that the site amplifications increase with frequency and are larger in the Western Plain with thick Holocene alluvium than in the Western Foothill with Pleistocene and Miocene formations. Considering wave attenuation, site amplification first increases and then decreases with increasing frequency. The turning frequency, ft, associated with the maximum amplification varies site by site.

Site-dependent design input ground motion estimations for the Taipei area: a probabilistic approach

Abstract Design seismic forces depend on the peak ground acceleration (PGA) and on the shape of design spectrum curves dictated in building codes. At present there is no doubt that it is necessary to construct so-called “site and region-specific” design input ground motions reflecting influence from different magnitude events at different distances that may occur during a specified time period. A unified approach to ground motion parameters estimation is described. A collection of ground motion recordings of small to moderate (3.0–3.5≤ML≤6.5) earthquakes obtained during the execution of the Taiwan Strong Motion Instrumentation Program (TSMIP) since 1991 was used to study source scaling model, attenuation relations and site effects in Taiwan region. A stochastic simulation technique was applied to predict PGA and response spectra for the Taipei basin. “Site and region-dependent” uniform hazard response spectra were estimated for various geological conditions in the Taipei basin using a technique of probabilistic seismic hazard analysis.