张北地震序列的尾波Qc值研究

采用尾波单次散射模型,选取张北地震序列70条波形记录,计算了张北地区t=58.88s时的尾波QC值:QC(f)=132.6±20f0.96±0.14。研究中发现,随流逝时间的增加QC逐渐增大,当流逝时间>120s时,出现逆转,QC迅速减小。文中采用频率域方法计算QC值,尾波功率谱计算自t>2ts开始,选用垂直向波形记录,取时间窗长为5.12s,步长2.56s向前滑动,每个时间窗两端5%的数据与余弦函数相乘,然后做FFT变换,分别计算9个中心频点各时间窗的能量密度谱,尾波流逝时间取各时间窗的中心点。当信号的能量密度<4倍的背景噪声能量密度时,尾波能量密度取值结束。求得能量密度谱后,线性拟合可求得不同流逝时间各频点的QC值。

The Coda Wave Qc Value of Zhangbei Earthquake Sequence

We estimate the coda Q_0 value for Zhangbei area from 70 seismograms by using single scattering model and give the average of Q_0 value by Q_C(f)=1326±20f~(0.96±0.14). We find that Q_0 value gradually increases with lapse time, but Q_0 value rapidly diminishes when lapse time is greater than 120 seconds. We adopt frequency methods in estimatig Q_C value. We selecte vertical component waveform records and began calculating wave power spectrum from t>2t_s. The coda wave windows is subdivided into 5.12 seconds intervals, with 50% overlap between segments. For each segment, a 5% cosine-taper is applied to both ends of the time series and the spectrum obtained by fast Fourier transform. Lapse time is the center of various time windows. When the power spectrum of signal are 4 times less than that of noise, the records are excluded from further analyses. After obtaining coda power spectrum, we can calculate Q_C value of various frequency points by linear fitting.