小地震与人工爆破记录的时频分析

基于Matlab软件,编制了对近震记录进行时-频分析试验研究的程序,搜集2006—2008年间首都圈地区震级介于ML2.4~2.7的5次天然地震和5次人工爆破的27个台站的宽频带记录,根据震中距离的相似性挑选5组天然地震和人工爆破地震,分析其时频特征。对于所有的数据都给出其三分向的时频图、原始波形图和频谱图,以及水平向和三分向的叠加图。分析结果表明,天然地震和人工爆破的记录具有明显不同的时频特征,天然地震的时频呈现出"多峰"特征,而人工爆破的时频分布则相对"少峰";天然地震的时频峰值分布在较宽的频率范围内,人工爆破的峰值则分布在频率较低的相对狭窄的范围内。通过以上实践,发现时频分析在分析不同时间点的频谱信息以及整个数据的时频分布特点时比傅里叶变换具有优势。

Time-frequency Analysis of Records of Small Earthquakes and Explosions

In this study, we developed a program to analyze the time-frequency spectrum of seismology events using MATLAB software. Among all the time-frequency analysis tools, we selected short-time Fourier transform (STFT) owing to its computational simplicity, adequate time and frequency resolution, and absence of undesirable cross terms. One disadvantage of STFT is the tradeoff between window length and time and frequency resolution. In order to determine the optimal window length, we wrote the code according to the sample rate of data records. We collected 82 broadband records, including data of 27 stations, of 5 natural earthquakes and 5 explosion events that occurred in the capital area between 2006 and 2008 having a magnitude in the range of 2.4 to 2.7. For the sake of convenience, we selected five pairs of events, each pair with one natural earthquake and one explosion event at similar epicenter distance, and analyzed the time-frequency spectrum. We plotted the time-frequency spectrum for each record with its original waveform and Fourier spectrum including the three components and their combination. Different time-frequency patterns exist for natural earthquakes and explosion events. In the first place, more peaks can be seen in the natural earthquake time-frequency spectrum, while the explosion events spectrum has fewer peaks. In the second place, the peaks of natural earthquakes are located in a broad frequency band, whereas the peaks of explosion time-frequency spectrum are concentrated in a narrow area. We analyzed all the data, not restricted to the five pairs of events, and the time-frequency spectrum pattern was found to be consistent. However, this pattern may be applicable only in the capital area because the explosion techniques are fairly complicated. The operator can use completely different explosion plans, and different local seismological wave propagation models produce diverse wave fields. When compared with Fourier analysis, the time-frequency spectrum, which is the time domain expansion of Fourier Spectrum, provides more information.