Determination criteria of repeating earthquakes based on spectral element modeling
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摘要: 本文以宁夏区域地震台网为例,分析了波形互相关法在判定重复地震中可能存在的问题并讨论了相应的处理方法,通过构建三维非均匀体模型并利用谱元法数值模拟地震波的传播,统计了不同台站观测到的地震对波形互相关系数的分布,进一步研究了互相关系数与非均匀体性质及震源机制解之间的关系。结果表明:针对宁夏区域地震台网,利用波形互相关法判定重复地震比地震定位方法更有效;互相关系数在不同台站的取值与震源附近三维非均匀体强度和直达波与尾波的振幅比有关,对于相同的震源间距,较弱震源、较弱非均匀体或者较强振幅的直达波均会导致波形互相关系数变高,因此应选取更高的互相关系数阀值来判定重复地震。宁夏区域地震台网平均台间距为30—50 km,通过选取直达波较弱的台站或只截取尾波窗口计算互相关系数并设定较高的阀值,利用波形互相关法可有效地判定ML1.0—3.0重复地震,进而为重复地震的监测与研究提供依据。Abstract: Taking Ningxia regional seismic network for an example, we discussed the problems or difficulties in detecting repeating earthquakes via the waveform cross-correlation method and provided the corresponding solutions. We first created statistical models of 3D volumetric heterogeneities and then incorporated them into spectral element method to simulate 3D seismic wave propagation, so investigate the spatial distribution of the cross-correlation coefficients between earthquake pairs observed by different seismic stations, we found strong impacts of the volumetric heterogeneities and focal mechanism solution on the cross-correlation coefficients. The results show that the waveform cross-correlation method is more reliable and therefore practically useful in identify repeating earthquakes than the earthquake relocation method for the Ningxia regional seismic network. The cross-correlation coefficients are affected by the intensity of 3D heterogeneities near the hypocenter and the amplitude ratio of direct waves to coda waves. Given the same distance between two earthquakes, the stronger heterogeneities near earthquake source or stronger amplitude direct waves will result in higher waveform correlation coefficients. Thus, a higher threshold of cross-correlation coefficient is needed to reliably detecting repeating earthquakes. In summary, based on the data of Ningxia regional seismic network with an average station spacing of 30−50 km, choosing a proper threshold value of cross-correlation coefficient by only taking the time window of coda waves or data with weak direct P-waves, waveform cross-correlation method could reliably identify repeating earthquakes with a magnitude range ofML1.0−3.0, thus providing basis for the surveillance of repeating earthquakes.
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图 2 5—10 Hz带通滤波后的地震波南北向(a)和垂向(b)速度分量的波形相似序列
图中各波形右上角数字为地震事件与参考事件(2010-06-13)的波形互相关系数
Figure 2. High similarity waveforms with the north-south components (a) and the vertical components (b) after 5−10 Hz band-pass filtering
The numbers in top right corners give the cross-correlation coefficients between the associated seismogram and template seismogram (2010-06-13)
图 6 震源间距为97.5 m (a)和195 m (b)时的波形互相关系数分布
自相关尺度a=2 km,P波和S波的速度扰动均方根分别为2%和4%。图中震源机制的空间分布或相位信息均与图4和图5一致
Figure 6. Waveform cross-correlation coefficients distribution with the distance separation of earthquake pairs of 97.5 m (a) and 195 m (b)
The auto-correlation length a is 2 km,the P-wave and S-wave velocity perturbations are 2% and 4%,respectively. The focal mechanism of the earthquakes used here is exactly the same as Fig.4 and Fig.5
图 5 地震对震源间距为195 m (a)和290 m (b)时波形互相关系数分布
非均匀体模型参数及震源机制解与图4相同
Figure 5. Waveform cross-correlation coefficients distribution with the distance separation of earthquake pairs of 195 m (a) and 290 m (b)
The parameters of heterogeneities model and the focal mechanism of the earthquakes are exactly the same as Fig. 4
图 7 震源间距97.5 m时的波形互相关系数分布(a)和垂向速度分量合成地震图(b)
非均匀体模型与图4完全相同,但震源机制为倾角45°逆冲型,选取剖面台站为y=30 km,x在10—55 km范围内递增
Figure 7. Waveform cross-correlation coefficients distribution (a) and a distance profile of vertical velocity seismograms (b) for an earthquake pair with a distance separation of 97.5 m
The heterogeneity model used here is exactly the same as Fig.4,but the earthquake focal mechanism is replaced by a thrust type with a dip angle of 45°,this profile is composed of synthetics from stations at y=30 km and x ranging from 10 km to 55 km
图 4 走滑型地震对应的波形互相关系数分布(a)及部分台站波形合成图(b)
自相关尺度a=2 km,P波和S波的速度扰动均方根分别为4%和8%。图(a)中的地震对间距为97.5 m,震源机制为走向0°的走滑型,图(b)中垂直向速度分量为5—10 Hz带通滤波后的震中距剖面,选取剖面台站为y=30 km,x从10 km递增至55 km
Figure 4. Waveforms cross-correlation coefficients (a) and waveform synthetics of station (b) for a strike-slip earthquake
The auto-correlation length a is 2 km,the P-wave and S-wave velocity perturbations are 4% and 8% respectively. The earth-quake pair has a distance separation of 97.5 m,and the earthquake has a strike-slip focal mechanism with a strike of 0° in Fig.(a),Fig.(b) shows a distance profile of vertical velocity seismograms after 5−10 Hz band-pass filtering,and the profile is composed of synthetics from the stations at y=30 km and x ranging from 10 km to 55 km
表 1 本文所用的地震波速度、密度及衰减模型结构
Table 1 Velocity,density and attenuation model structure of seismic waves used in this study
厚度/km vP/(m·s−1) vS/(m·s−1) 密度/(g·cm−3) Qμ 地壳 30 6 800 3 900 2 900 80 地幔 10 8 110 4 490 3 380 80 -
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