Earthquake Swarms on June 28th of 2010, near Lingyun and Fengshan County, South China: Shallow Seismic Activity of Fault Induced by Surface Fluid in Karst Geology Area
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摘要: 2010年6月28日, 广西凌云-凤山地区在特大强降雨活动之后发生密集的浅源低震级地震活动, 造成严重经济损失.该震群活动是否与强降雨有关, 强降雨是否可以触发震群活动, 仍存在争议.基于该地区地震构造背景, 对震群的活动特征、震源机制解及其与断裂构造的关系进行了分析, 并建立了断层内流体孔隙压力触发断层滑动的力学模型.该浅层震群活动的发生时间、震中位置和活动频次均与特大强降雨具有密切的相关性.认为岩溶地区长期干旱和地下水缺乏有利于地壳浅层的应力积累.地表流体很难通过下渗扩散触发断层的完全解锁和深部滑动, 但岩溶裂隙和管道有利于地表流体快速汇聚下渗, 引起断层浅层强度的弱化, 导致断层部分解锁滑动.并在断层附近形成局部应力场异常和离散的封闭性超压流体, 触发密集的浅层低震级震群活动.Abstract: An instantaneous intense rainfall occurred in the Fengshan County area, Guangxi Province of South China on June 27, 2010. One day later, an strong seismicity with shallow depth and magnitudes less than ML3.0 occurred. It caused severe economic losses. It is not clear whether or not the activity of this shallow earthquake swarm is correlated with the heavy rainfall. Based on the tectonic setting in the study area, the activity characteristics, focal mechanism solutions of the earthquake swarm and its relationship with faults are investigated, and then a stress model is proposed to analysis fault slip triggered by pore-fluid pressure. It is concluded that the activity of this shallow earthquake swarm is highly correlated with the heavy rainfall after a prolonged drought in this region, including the occurrence time, epicenter location and the frequency. Prolonged drought and lack of groundwater in karst areas have a positive effect on the accumulation of stress in the shallow crust. In karst regions, the underground caverns, river and vertical cracks can be propitious to the fast penetration of surface water. Pore pressure changes cause the fault strength weakening, thereby trigger the unlocking and slip on the shallow part of fault. Discrete close overpressure fluid and abnormal stress field formed due to the shallow slip of fault. The shallow seismic activity comes from the slip and collapse of shallow sub-fractures and caves in karst regions.
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Key words:
- fluid /
- fault /
- Shallow earthquake /
- karst geology areas
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图 2 地质构造和震群位置
F1.百色-合浦断裂带；F2.巴马-博白断裂带；F3.河池-宜州断裂带；F4.木伦-东兰-逻楼断裂带；F5.更新-凌云-那能断裂带；f1.江州-高家洞断裂；f2.逻楼-江州断裂；f3.寅村断裂；f4.坡楼断裂；f5.那林断裂；f6.岩板断裂；f7.逻楼-平吕断裂；f8.陇仓断裂；f9.沙里-弄所断裂
Fig. 2. Geology setting and earthquake swarms
图 3 2010年6月江洲流动台记录的S-P波速差随时间变化
Fig. 3. The time series of S-P velocity difference recorded by mobile Seismic station in Jiangzhou, June 2010
图 4 震源分布与断层关系剖面
f1.江州-高家洞断裂；f2.逻楼-高家洞断裂；震群精定位分析结果由广西地震局台网中心提供
Fig. 4. Cross section profile of earthquake swarms and faults
图 5 凌云凤山震群日降雨量与地震日频次统计
a.江州站降雨量；b.罗楼站降雨量；c.地震日频次；数据由广西区气象局提供
Fig. 5. The time series of the daily rain amount
图 6 断层内流体孔隙压力触发断层滑动的应力模型
a.岩溶管道和裂隙有利于地表流体快速下渗，从而弱化断层强度，引起断层浅层部位的局部解锁滑动；b.断层内流体孔隙压力触发断层滑动的影响深度，图中假定岩石平均密度为2.5×103 kg/m3，断层面摩擦系数为0.6；c.流体作用下断层滑动准的应力莫尔图解，φ为内摩擦角，σn为断层面上正应力，σn为断层面上剪应力
Fig. 6. Stress model showing fault slip triggered by pore-fluid pressure
表 1 凌云-凤山震群ML≥2.0地震震源机制(蒙荣国等，2012)
Table 1. Focal mechanism of the earthquake swarms with ML≥2.0
时间 纬度(N) 经度(E) ML 节面Ⅰ 节面Ⅱ P轴 T轴 滑动性质 走向 倾角 走向 倾角 走向 倾角 走向 倾角 6月28日12∶41 24.38° 106.90° 3.0 26° 22° 190° 69° 105° 23° 269° 66° 逆倾滑动 6月28日17∶36 24.38° 106.88° 2.0 142° 23° 321° 67° 50° 68° 231° 22° 正倾滑动 6月28日18∶02 24.35° 106.95° 2.4 112° 81° 6° 30° 173° 47° 45° 30° 正倾滑动 6月28日19∶55 24.40° 106.86° 2.6 281° 20° 67° 73° 142° 60° 346° 28° 正倾滑动 6月28日19∶58 24.38° 106.90° 2.6 292° 76° 180° 34° 347° 49° 225° 24° 正倾滑动 6月28日20∶08 24.38° 106.88° 2.3 266° 24° 59° 69° 132° 65° 337° 23° 正倾滑动 6月28日20∶34 24.40° 106.88° 2.4 210° 67° 320° 51° 348° 46° 88° 9° 正倾滑动 6月28日21∶06 24.38° 106.90° 2.3 271° 27° 5° 88° 299° 37° 70° 41° 走滑 6月28日21∶34 24.38° 106.88° 2.2 29° 26° 206° 64° 293° 71° 117° 19° 正倾滑动 6月28日22∶02 24.38° 106.88° 2.5 285° 19° 138° 74° 243° 60° 40° 28° 正倾滑动 6月28日23∶13 24.38° 106.90° 2.5 31° 23° 190° 69° 265° 65° 106° 23° 正倾滑动 6月29日02∶03 24.38° 106.88° 2.2 117° 89° 24° 33° 177° 38° 55° 35° 走滑 6月29日02∶04 24.41° 106.85° 2.2 36° 24° 220° 66° 314° 68° 129° 21° 正倾滑动 6月29日02∶01 24.38° 106.88° 3.1 120° 87° 26° 31° 182° 40° 56° 35° 走滑 6月29日03∶11 24.38° 106.88° 2.0 26° 36° 258° 66° 148° 17° 27° 59° 逆倾滑动 6月29日06∶09 24.40° 106.88° 2.5 18° 37° 262° 72° 148° 20° 30° 52° 逆倾滑动 6月29日06∶05 24.38° 106.86° 2.3 120° 87° 27° 43° 175° 34° 63° 29° 走滑 6月29日10∶38 24.38° 106.91° 2.5 24° 22° 195° 68° 279° 66° 107° 23° 正倾滑动 6月29日11∶22 24.38° 106.88° 2.3 30° 32° 152° 72° 208° 5° 82° 22° 正倾滑动 6月29日13∶06 24.38° 106.88° 2.3 275° 28° 185° 90° 300° 39° 70° 39° 走滑 6月30日10∶17 24.38° 106.91° 2.3 29° 76° 274° 31° 88° 51° 320° 26° 正倾滑动 6月30日13∶51 24.38° 106.90° 2.8 204° 48° 308° 74° 355° 41° 250° 16° 正倾滑动 7月01日10∶27 24.38° 106.86° 3.2 26° 23° 210° 67° 117° 22° 300° 68° 逆倾滑动 7月04日19∶45 24.45° 106.88° 2.6 197° 89° 290° 26° 311° 41° 83° 38° 走滑 7月05日03∶24 24.38° 106.88° 2.5 262° 27° 154° 81° 271° 48° 43° 31° 走滑 
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