Characteristics of seismic coda attenuation in Longtan reservoir of Guangxi region
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摘要:
按照高信噪比和计算要求，从 2006—2013 年广西龙滩水库地震台网的10个子台记录到的龙滩库区近场数字地震波形中挑选出 3 822 条三分向记录，经滤波和消除环境噪声后，采用Sato单次散射模型，计算了地震波传播路径上的尾波Q （ f ）值，对Q （ f ）与频率f之间的关系进行了拟合，并在此基础上分析讨论了库区尾波衰减的时空特征. 结果显示，龙滩库区总体呈低Q0值和依赖性指数η较高的特点，这两个参数值均低于浙江珊溪水库，与其地震活动程度高于后者的事实相一致。尾波Q值的空间分布显示，周围小震密集分布的台站和历史地震活跃地区的台站的Q0值低于相对于周围小震分布稀疏的台站的Q0值。库区尾波衰减参数与尾波采样体之间的关系显示，采样深度越深，尾波参数Q0值越低，因此认为该区域可能存在深部高衰减层。衰减参数随时间的变化过程表明，蓄水后地震尾波衰减参数仍主要反映了库区原有地壳内部介质的非均匀性和地震波吸收特性，库区地壳介质特性无显著变化，但局部敏感点有所调整。
Abstract:This paper firstly selected 3 822 three-component records from 10 stations of Long-tan reservoir seismic network from 2006 to 2013 according to the high signal-to-noise ratio and calculation requirements. Based on the Sato single scattering model, this paper calculated the coda Q（ f ） after filtering the data and eliminating ambient noises, and then obtained the relationship between coda Q（ f ） and frequency f. Finally we anayzed the spatio-temporal characteristics of seismic coda attenuation in the Longtan reservoir area. The results show that the Longtan reservoir area is characterized by low Q0 and high dependence index η, and its Q0 value and dependence index η are lower than that of Shanxi Reservoir, which is consistent with the fact that the its seismic activity is higher than the latter. The spatial distribution of the coda Q value shows that Q0 values of the stations with surrounding small earthquakes densely distributed and the stations in the areas with active historical earthquakes are lower than those of the stations with sparse distribution of surrounding small earthquakes. Further analyses on the relationship between attenuation parameters and sampling depth of the reservoir area suggest that there exist deep high attenuation layers beneath the studied area. By analyzing the change process of attenuation parameters with time, it is considered that after reservoir impoundment the coda attenuation parameters still mainly reflect the non-uniformity of the original crustal internal medium of the reservoir area, and the properties of the crustal medium have not been changed significantly, but some local sensitive points have been adjusted.
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Keywords:
- Longtan reservoir /
- reservoir earthquakes /
- coda /
- attenuation parameter
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图 1 龙滩水库地质构造以及蓄水后地震震中和台站分布图
F1：八腊断裂；F2：天峨—金谷断裂带；F3：望谟—逻西断裂带；F4：乐业—甘田断裂；F5：罗甸—望谟断裂；F6：里落—白塘—浪全断裂带
Figure 1. Geological structure of Longtan reservoir as well as location of epicenters after impoundment and stations distribution
F1：Bala fault；F2：Tiane-Jingu fault zone；F3：Wangmo-Luoxi fault zone；F4：Leye-Gantian fault zone；F5：Luodian-Wangmo fault zone；F6：Liluo-Baitang-Langquan fault zone
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图 2 龙滩库区尾波衰减参数计算示例
（a） EW向原始波形记录，t0表示发震时刻，t1表示尾波截断点时刻（t1−t0＝60 s）；（b） 频率f＝11×（1±1/3） Hz时的波形滤波示例；（c） 频率f为4—18 Hz时拟合波形与实际波形的相关系数；（d） Q（ f ）与频率f的函数关系
Figure 2. Demonstrations of calculation process of coda wave attenuation parameters in Longtan reservoir area
（a） E-W component original waveform record，where t0 is the origin time，t1 denotes cutoff point of coda waves used in the calculation （t0−t1＝60 s）；（b） Waveform filtered demonstration with f ＝11×（1±1/3） Hz；（c） Correlation coefficient between fitted waveforms and actual ones when f is 4−18 Hz；（d） Functional relationship between Q（ f ） and f
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图 3 全区域尾波衰减Q（ f ）与f的关系（a）及Q值残差分布（b）
Figure 3. The relationship between coda attenuation parameter Q（ f ） and frequency f in the whole region （a） and residual distribution of Q values （b）
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图 4 全区域尾波衰减参数Q0值（a）和指数η值（b）随地震序列的变化
Figure 4. Variation of coda attenuation parameter Q0 （a） and index η （b） with earthquake sequences in the whole region
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图 5 八广台（a）和小苗坡台（b）衰减参数Q的统计结果比较
Figure 5. Comparison of the statistic results of attenuation parameters Q between the stations Baguang （a） and Xiaomiaopo （b）
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图 6 八广台（a）和小苗坡台（b）衰减参数Q0的统计结果比较
Figure 6. Comparison of the statistic results of attenuation parameters Q0 between the stations Baguang （a） and Xiaomiaopo （b）
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图 7 八广台（a）与小苗坡台（b）衰减指数η值统计结果比较
Figure 7. Comparison of the statistic results of attenuation parameters η between the stations Baguang （a） and Xiaomiaopo （b）
表 1 龙滩水库地震台网各台站尾波衰减参数统计
Table 1 Statistics on coda attenuation parameters of the stations of Longtan reservoir seismic network
台站 台基岩性 Q0 衰减参数 记录数 δQ0 η δη 天峨 灰岩 50.38 9.96 0.85 0.07 270 仁顶 灰岩 49.00 7.55 0.88 0.06 198 坡皇洞 灰岩 53.46 11.05 0.83 0.08 492 甲龙 砂岩 52.83 10.82 0.82 0.07 430 八广 砂岩 55.07 14.95 0.85 0.10 628 里纳 砂岩 52.30 12.09 0.85 0.08 517 凉风坳 砂岩 48.92 10.09 0.86 0.07 354 小苗坡 砂岩 45.67 7.76 0.90 0.07 443 罗甸 灰岩 52.57 12.04 0.86 0.09 212 坪上寨 砂泥岩 48.63 9.73 0.83 0.06 278 
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1. 都燊，俞言祥，肖亮. 利用密集台阵资料研究宾川盆地浅层介质尾波Q值. 地震学报. 2023(06): 1011-1024 . 
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