Crustal 2-D QS tomography in Longtan reservoir，Guangxi region
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摘要: 本文收集了广西龙滩水库地震监测台网2006年9月至2016年12月精定位后的3 382次ML≥0地震的数字波形资料，采用二维衰减成像技术获得了龙滩水库库区的QS二维分布图像。结果显示：龙滩水库库区QS的横向不均匀变化明显，QS低值区围绕着库区近似呈环形分布；在QS低值分布区附近，大多为河流与断裂带的交汇处；QS低值分布主要对应于透水性较强的岩性地区。以上现象表明QS低值分布受水、断裂、岩性等3种因素的影响，由此初步推断库水可能沿着断裂上的岩石破碎带及节理、裂隙发育地区和具有较强透水性岩层区域向下渗透，使得岩石孔隙中充满流体，内摩擦增大，地震波剧烈衰减，从而使QS值大幅下降。结果还表明：大多数地震发生在QS高、低值过渡区域，这种“软”、“硬”介质的交界处，容易积累应变能，孕育地震。这是由于当QS高、低值过渡区域受到水的加载作用及其对裂隙边界的润滑作用，发震断层的抗剪强度降低，使滑动容易产生，从而诱发地震。Abstract: In this study, we collected digital waveform records of 3 382 seismic events with ML≥0 recorded by Longtan reservoir induced-earthquake monitoring network in Guangxi region from September of 2006 to December of 2016 and these data have been precisely located. By using 2-D attenuation tomography technique, we extracted the 2-D QS tomography of Longtan reservoir area. The result show that there is a significant lateral heterogeneity in QS distribution in Longtan reservoir area. Approximately, there are low QS value annular regions surrounding the reservoir, and most of the low QS value regions are located at the intersections of rivers and fault zones. From the point of view of lithology, the main distribution of the low QS value corresponds to the lithologic area with strong permeability. All the above phenomena may indicate that the low QS value regions are affected by reservoir water, fault zones, and the lithology. The preliminary conclusion is: the reservoir water infiltrates through rock fracture zones, joints and crack regions of faults, as well as the regions of rock stratum with strong permeability, filling with fluid in the rock pores, increasing the internal friction, and causing significant attenuation of seismic wave, resulting in the decline of QS value. The result also shows that most of the earthquakes occurred at the transitional edge regions of high and low QS. This is perhaps a result of accumulation of strain energy in the transitional regions of hard and weak medium zones. Due to the loading effect of water mass and the lubrication effect on the crack boundary of the water in the high and low QS transition region, the shear strength of the seismogenic faults is decreased, leading to the relative sliding easily and inducing earthquakes.
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Keywords:
- Longtan reservoir /
- Q value /
- source spectra /
- 2-D QS tomography
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图 1 龙滩水库地质构造
F1：罗甸—望谟断裂；F2：高圩—八茂断裂；F3：凤亭—下老断裂；F4：马耳—拉浪断裂；F5：达恒—达良断裂；F6：党明—桂花断裂；F7：望谟—逻西断裂；F8：长里—八南断裂；F9：龙凤—八腊断裂，下同
Figure 1. Structural outlines of Longtan reservoir
F1：Luodian-Wangmo fault；F2：Gaoyu-Bamao fault；F3：Fengting-Xialao fault；F4：Maer-Lalang fault； F5：Daheng-Daliang fault；F6：Dangming-Guihua fault；F7：Wangmo-Luoxi fault；F8：Changli-Banan fault；F9：Longfeng-Bala fault. The same below
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图 2 重定位后的地震分布及本文所使用的台站分布
Figure 2. Distributions of relocated earthquakes and the stations used in this study
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图 3 龙滩水库库区地震活动M-t图及水位变化
Figure 3. M-t map of seismicity and the change of water level in Longtan reservoir
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图 4 仁顶台记录到的2007年12月10日5时44分贵州罗甸ML1.0地震的两个水平分量波形（a）及其合成位移谱和噪声谱 （b）. 图（b）中观测谱和拟合谱均未作几何扩散校正
Figure 4. Two horizontal seismic waveforms of a ML1.0 earthquake recorded by Rending station （a），and the synthesize displacement spectra and noise spectra （b）
The earthquake occurred in Luodian city，Guizhou Province，at 5:44 on December 10，2007. In Fig. （b） the geometrical spreading corrections were conducted neither for the observed spectrum nor the fitted spectrum of S wave
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图 5 S波走时t、衰减算子t*与震源距Δ的关系
（a） S波走时t与震源距Δ的关系；（b） 衰减算子t*与震源距Δ的关系；（c） 根据一倍均方根准则筛选出的衰减算子t*与震源距Δ的关系
Figure 5. The travel times t and attenuation operator t* of S wave versus hypocentral distances Δ
（a） The relationship between the travel times t of S wave and hypocentral distances Δ；（b） The relationship between the attenuation operator t* and hypocentral distances Δ；（c） The selected t* based on one-root-mean-square criterion varied with hypocentral distances Δ
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图 6 反演前（a）、后（b）t*残差δt*随震中距Δ的变化
Figure 6. The residuals of attenuation δt* varied with hypocentral distances Δ before （a） and after （b） inversion
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