Discussion on precise location, focal mechanism and seismogenic mechanism of Jiaokou reservoir earthquakes
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摘要: 利用双差定位法对宁波皎口水库2009—2021年地震进行重新定位，通过HASH方法计算了较大地震的震源机制解，基于精确定位后的地震震源位置分布、深度分布以及总体震源机制解特征，结合区域地质构造背景、库区地质条件以及水库水位变化，对发震构造及孕震机理进行探讨。结果表明，皎口水库2009—2012年地震序列与2018—2021年地震序列孕震机理相似，区域性断裂长兴—奉化可能为发震断层；库水通过断裂破碎带下渗，孔隙水压力扩散及库水对岩石的软化弱化长时间作用，造成断层面及其深部某些薄弱部位变为不稳定区域，最后在库水变化导致的加卸荷应力场作用下触发地震。Abstract: Jiaokou reservoir earthquakes in Ningbo from 2009 to 2021 were relocated using the double difference positioning method. The focal mechanism solutions of large earthquakes were calculated by HASH method. Based on the location distribution, depth distribution and overall focal mechanism solution characteristics of the earthquakes after precise positioning, the seismogenic structure and seismogenic mechanism were discussed by combining with the regional geological tectonic background, geological conditions in the reservoir area and the change of reservoir water level. The results show that the seismogenic mechanism of the Jiaokou reservoir earthquake sequence in 2009—2012 is similar to that in 2018—2021, and the regional fault Changxing—Fenghua may be the seismogenic fault. The reservoir water infiltrates through the fracture fracture zone, the pore water pressure diffuses, the reservoir water softens and weakens the rock for a long time, causing the fault plane and some weak parts in the deep part to become unstable areas, and finally triggering the earthquake under the loading and unloading stress field caused by the change of reservoir water.
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图 2 皎口水库地震序列震中分布图
Figure 2. Epicenter distribution map of Jiaokou reservoir earthquake sequence
图 4 2009—2021年皎口水库地震序列原始（a） 及重新定位后（b） 震中分布图
Figure 4. Epicenter distribution map of the original (a) and relocated (b) of Jiaokou reservoir earthquake sequence from 2009 to 2021
图 5 2009—2021年皎口水库地震震源深度分布图（投影到剖面AA′）
Figure 5. Focal depth distribution of Jiaokou reservoir earthquake sequence from 2009 to 2021 (projected to section AA′)
图 6 皎口水库2018—2021年地震序列精确定位后的地震分布在水平面（a）、断层面（b）、垂直于断层面的横断面上（c）的投影、以及地震距断层面距离的分布（d）。圆圈表示精确定位后的地震，粗线表示确定的断层面边界，AA′为断层上边界端点
Figure 6. The earthquakes accurately located in the 2018—2021 Jiaokou reservoir earthquake sequence are distributed on the projection of the horizontal plane (a), the projection of the fault plane (b), the projection of the cross-section perpendicular to the fault plane (c), and the distribution of the distance between the earthquakes and the fault plane (d). The circles indicate the accurately located earthquakes. The thick line indicates the determined boundary of the fault plane. AA′ is the endpoint of the upper boundary of the fault
图 7 皎口水库地震序列较大地震震源机制解
Figure 7. Focal mechanism solutions of the larger earthquakes in Jiaokou reservoir earthquake sequence
图 8 2009—2021年皎口水库地震总体震源机制解
Figure 8. Overall focal mechanism solution of Jiaokou reservoir earthquake sequence from 2009 to 2021
图 9 2009—2021年皎口水库逐日水位（a） 、水位日变化量（b） 与地震活动对比图
Figure 9. Comparison diagram of daily water level (a)、daily variation of water level (b) and seismic activities of Jiaokou reservoir from 2009 to 2021
表 1 皎口水库大坝基坑开挖发现的断层（贯穿坝基上下游）
Table 1. Faults founded in the foundation pit excavation of Jiaokou reservoir dam (running through the upstream and downstream of the dam foundation)
编号 产状 特征 力学性质 走向 倾向 倾角 １ N60°—80°W SW 65°—80° 破碎带发育，夹少量断层角砾及褐色断层泥 张性 ２ N30°—35°W NE 67°—80° 破碎带局部有少量断层泥及铁镁稀浆（其中见有巨大的断层角砾） 张性 ３ N75°—85°W NE 55°—70° 破碎带夹少量的断层泥及断层角砾岩 扭性 
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表 3 皎口水库地震序列较大地震震源机制解
Table 3. Focal mechanism solutions of the larger earthquakes in Jiaokou reservoir earthquake sequence
序
号发震时间
年-月-日 时:分震
级节面Ⅰ 节面Ⅱ P 轴 T 轴 B 轴 类
型备注 走向
/°倾角
/°滑动角
/°走向
/°倾角
/°滑动角
/°方位
/°倾角
/°方位
/°倾角
/°方位
/°倾角
/°1 1993-02-26 12:12 3.9 291 84 33 197 57 173 60 18 159 27 300 57 SS 2 1993-03-04 06:59 3.4 81 76 21 346 70 165 212 4 304 25 113 65 SS 3 1994-09-07 14:32 4.7 195 53 −164 95 77 −38 48 36 150 16 260 50 SS 4 2009-09-10 20:21 3.1 110 75 162 205 73 16 158 2 67 23 251 67 SS 5 2009-09-12 06:40 2.9 101 73 169 194 79 17 327 4 58 20 225 70 SS 6 2009-09-12 08:21 3.3 290 89 146 21 56 1 341 23 240 24 109 56 SS 7 2009-09-12 20:03 2.7 120 65 −161 22 73 −26 339 30 72 5 171 59 SS 8 2009-09-24 11:08 3.0 120 57 −135 1 54 −43 333 54 240 2 149 36 NS 9 2009-10-09 05:34 3.3 120 86 151 212 61 5 170 17 72 23 293 61 SS 10 2010-01-07 14:47 2.6 288 83 −153 194 63 −8 154 24 58 13 301 62 SS 11 2019-05-16 17:13 2.4 91 52 164 191 77 39 316 16 58 36 206 49 SS 12 2019-05-22 04:01 2.4 105 58 172 199 83 32 328 17 67 27 210 57 SS 13 2019-05-25 14:48 2.2 111 41 −133 342 61 −59 300 61 50 11 146 27 N 14 2019-05-28 10:44 3.6 110 68 −144 5 27 −27 331 41 235 7 137 49 NS 15 2019-05-28 11:47 2.2 275 84 157 8 67 7 323 12 229 20 81 66 SS 16 2019-05-28 16:52 3.3 119 82 −144 23 51 −10 348 33 245 20 129 50 SS 17 2019-05-29 13:17 1.9 282 83 −105 168 17 −25 176 50 25 36 284 15 NS 初动、振幅 18 2019-05-29 19:03 3.4 99 40 −173 4 86 −50 308 37 62 29 180 40 SS 初动、振幅 19 2020-11-05 20:03 3.0 128 82 −137 31 48 −11 359 35 252 22 136 46 SS 20 2020-11-11 01:04 1.3 92 79 −107 330 20 −34 342 53 196 32 95 17 NS 21 2020-12-18 12:21 3.1 302 54 −167 204 80 −37 157 33 258 17 11 52 SS 22 2020-12-18 13:03 1.8 189 79 −101 55 16 −46 85 55 288 33 191 11 N 初动、振幅 23 2020-12-20 05:29 1.6 324 19 −89 143 71 −90 52 64 233 26 143 0 N 24 2020-12-20 00:19 2.5 123 60 −164 25 76 −31 340 31 77 11 183 56 SS 初动、振幅 25 2020-12-20 10:29 1.4 105 66 −85 273 24 −101 25 69 191 21 283 5 N 26 2021-02-21 02:12 1.0 113 82 −106 357 18 −27 5 50 217 35 115 16 NS 27 2021-01-19 06:25 1.8 214 88 141 306 51 3 267 25 162 28 32 51 SS 28 2021-08-26 11:32 2.6 179 20 −56 323 74 −102 217 60 62 28 327 11 N
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