2008年汶川地震近断层地震动的非平稳特征

Non-stationary characteristics of near-fault strong motions during the 2008 Wenchuan earthquake

利用中国强震动台网获得的汶川地震近断层强震动数据,研究汶川地震近断层地震动的强度和频率的非平稳特征,并对其非平稳特征的原因进行初步分析.研究表明近断层地震动强度和频率表现出很强的非平稳特性,这种非平稳特征主要受震源破裂机制、台站方位和局部场地条件的影响.破裂传播正向和反方向上地震动强度和频率的时间过程有显著的不同.在破裂正向上,地震动加速度记录表现为大幅值、短持时的强度过程,强度过程的持续时间为15 s左右,加速度时程的主要周期约为0.4 s,地震动速度时程有较明显的速度脉冲,脉冲周期可以达到8 s;在破裂的反方向上,地震动强度过程的时间跨度长达90 s,加速度记录的主要周期约为0.6 s,速度时程中没有明显的脉冲.小波功率谱的分析结果表明破裂正向上速度脉冲表现为在很小的时间尺度和频域内能量的集中,值得注意的是,观测到的速度脉冲的周期长达8 s,比Chi-Chi地震和Kobe地震中观测到的脉冲周期要长,这可能与汶川地震的震级相对较大有关.

Using near-fault strong motions from Wenchuan earthquake recorded by the National Strong Motion Observation Network System of China as database, non-stationary characteristics of amplitude and frequency content of strong motions from 2008 Wenchuan earthquake are investigated, and the reason for intense non-stationary characteristics of near-fault strong motions are analyzed. According to this study, non-stationary characteristics of near-fault strong motions are strongly affected by fault rupture mechanism, location of strong motion stations and site conditions. Obvious differences are shown in the time histories of strong motion in the forward and backward direction of the rupture fault. In the forward directivity region, the acceleration time history is characterized by large amplitudes and short durations, the main duration is as short as 15 s, and the predominant period of the acceleration time history is about 0.4 s. Large velocity pulses are shown in the velocity time histories, the period of velocity pulse can be as large as 8 s. In the backward region, the duration of acceleration time history is as long as 90 s, the predominant period is about 0.6 s, and no obvious pulse can be seen in the velocity time history. Wavelet analysis shows that the velocity pulse in the forward directivity region is characterized as intense concentration of energy in a short time and frequency range. A noticeable phenomenon is shown that the period of recorded velocity pulse in Wenchuan earthquake is much larger than the Chi-Chi and Kobe earthquake, as a result of a much larger magnitude for this event.