In this article, the empirical mode decomposition method combined with the Hilbert spectrum method (EMD ‫ם‬ HHT) is used to analyze the free-field ground motion and to estimate the global structural property of building and bridge structure through the measurement of seismic response data. The EMD ‫ם‬ HHT method provides a powerful tool for signal processing to identify nonlinear and nonstationary data. Based on the decomposed ground-motion signal, the absolute input energy of each decomposed wave was studied (the fling step [pulselike wave] can be separated from the recorded near-fault ground motion). Through application of the EMD ‫ם‬ HHT method to building and bridge seismic response data, the time-varying system natural frequency and damping ratio can also be estimated. Damage identification from seismic response data of buildings and bridges, particularly from the Chi-Chi earthquake data, is also described.

SUMMARYThe purpose of this paper is to investigate the ground motion characteristics of the Chi-Chi earthquake (21 September 1999) as well as the interpretation of structural damage due to this earthquake. Over 300 strong motion records were collected from the strong motion network of Taiwan for this earthquake. A lot of near-"eld ground motion data were collected. They provide valuable information on the study of ground motion characteristics of pulse-like near-"eld ground motions as well as fault displacement. This study includes: attenuation of ground motion both in PGA and spectral amplitude, principal direction, elastic and inelastic response analysis of a SDOF system subjected to near-"eld ground motion collected from this event. The distribution of spectral acceleration and spectral velocity along the Chelungpu fault is discussed. Based on the mode decomposition method the intrinsic mode function of ground acceleration of this earthquake is examined. A long-period wave with large amplitude was observed in most of the near-source ground acceleration. The seismic demand from the recorded near-"eld ground motion is also investigated with an evaluation of seismic design criteria of Taiwan Building Code. Copyright 2000 John Wiley & Sons, Ltd.KEY WORDS: ground motion analysis; response spectrum; seismic demand analysis INTRODUCTIONThe Chi-Chi earthquake that occurred in Taiwan at 1:47 a.m. on 21 September 1999 directly struck the central part of Taiwan. The epicentre of the earthquake was at 23.853N and 120.783E with a focal depth of 7.5 km and magnitude 7.3 (M * "7.3, Central Weather Bureau Earthquake News). A surface rupture along the Chelungpu fault with a length of 105 km was observed. The largest measured vertical o!set reached more than 9 m. After the major shock a total of 10 252 aftershocks were identi"ed (till 10 October 1999). Among them four aftershocks with a magnitude greater than 6.5 were also identi"ed. As a direct result of this earthquake, 2333 lives were lost and 10 002 people were injured, and over 8000 buildings were damaged. On the basis of the number of dead, this was Taiwan's worst disaster since the Shin-Chu Taichung earthquake of magnitude 7.1 (21 April 1935) where 3325 lives were lost. This earthquake was the movement of Chelungpu fault (thrust fault) and the overhanging wall is on the east side of the fault. Many buildings, including reinforced concrete buildings, were damaged but other facilities and structures such as bridges and lifelines were also damaged by this earthquake. The damage done by this earthquake in the central part of Taiwan is the "rst example of severe damage to civil infra-structures. The purpose of this paper is to study the ground motion characteristics of the Chi-Chi earthquake using the strong motion data collected by the Central Weather Bureau under the Taiwan Strong Motion Instrumentation Program (TSMIP). These analyses include: ground motion attenuation analysis, spectral amplitude study, principal direction and inelastic response analysis of a SDOF system s...

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