| The fault system is widely distributed in the earth's crust,providing a good channel for fluid diffusion.The diffusion of fluid will increase the pore pressure of the fault zone and decrease the effective stress,thereby reducing the strength of the fault and making it easier to activate.Human activities related to water injection usually induce/trigger seismic activity,and the seismic activity in areas with active natural fluid is sensitive to the stress field response.The above seismic activities are closely related to changes in stress state and motion state of the fault caused by fluid diffusion.Therefore,studying the effect of industral injection and natural fluids on the seismicity of fault zones has important theoretical and practical significance for understanding of the stress state of fault zones,the mechanism of earthquake occurrence,and the evaluation of seismic risk.We take the Sichuan Basin with active industrial water injection activities and the northwest Yunnan area with active natural fluid activities as the research area.And we use a variety of seismicity analysis methods to study the effect of fluid on the seismic activity characteristics of the fault zone,including microseismic detection and relocation,temporal and spatial distribution of seismic activity,statistical parameter analysis,stress field inversion and fault stability analysis,etc.Regarding the effect of industial water injection,we focus on the seismic activity induced by long-term water injection in the Rongchang gas field,and the seismic mechanism and geomechanical conditions of the Changning M6.0 earthquake sequence.Regarding the effect of natural fluids,we focus on the seismic activity characteristics of the Weixi-Qiaohou fault on the northern section of the Red River fault zone.And we analyse the triggering effect and mechanism of remotely strong earthquakes on seismic activity in western Yunnan.The main results and understandings obtained are as follows.1.The Rongchang area is a long-term wastewater treatment site,with intermittent water injection for nearly 30 years.The largest induced earthquake was M 5.2.However,after the surrounding wastewater treatment wells were stopped in 2013,two earthquakes of M 4 occurred at the end of 2016.We conducted a comprehensive study on the seismic activity before and after the cessation of water injection in the Rongchang gas field.The statistical analysis of the ETAS model shows that the seismicity is greatly affected by external factors(water injection),and the proportion of externally triggered seismic activities exceeds 70%.The moment tensor inversion shows that the horizontal azimuth of the maximum principal stress field is 134°.The fluid overpressure required to trigger the above M ? 3.5 earthquake ranges from 2.2 to 16.4 MPa.After closing the surrounding water injection wells in 2013,two M 4 earthquakes occurred at the end of 2016,requiring fluid overpressures of 2.3 and 2.4 MPa,respectively.From the r-t diagram of the triggering front,it shows good hydraulic pressure diffusion,which may be because the depleted gas storage has good sealing,so the pressure is maintained for a long time.By analyzing the migration pattern of the source,calculating the fluid overpressure required for large earthquakes,and various statistical characteristics,these results show that the seismic activity after the cessation of water injection is the same as the seismic activity during the water injection process.It is caused by the delayed rupture of pre-existing faults in and around the reservoir.2.After the M 5.7 earthquake occurred in Xingwen area on December 18,2018,the M 6.0 earthquake occurred in Changning area on June 17,2019.We conducted a comparative study on the potential connection between water injection and seismic activity in the Changning salt mine area and the adjacent shale gas area.At the end of August 2019,about 400 M?3 earthquakes have been observed,including 40 M?4 and 5 M?5 earthquakes.The moment tensor results of M?4 earthquake show that the depth of centroid is between 1.3 and 7.6km.After the Xingwen M 5.7 earthquake,the seismic activity in the salt mining area increased significantly.The Xingwen earthquake may cause a unidirectional rupture along the NNW direction,and the end point is close to the northwest strike fault of the Shuanghe earthquake.Therefore,the seismogenic fault in the Changning earthquake may have terminated the fault rupture of the Xingwen earthquake,and the Xingwen earthquake may have promoted the Changning M 6 earthquake.By analyzing the temporal and spatial distribution characteristics of seismic activity,various statistical characteristics,stress field inversion results and calculation of fluid overpressure required for fault activation,it is found that seismic activity in Changning area is a tectonic earthquake due to fault activation caused by fluid injection,and there is a correlation with industrial mining of salt mines or shale gas.3.Based on the seismic data from temporary stations and regional stations in northwestern Yunnan,this paper uses waveform correlation methods to perform highresolution detection and high-precision location on continuous waveforms recorded from February 25,2018 to July 31,2019,and the seismicity characteristics of the WeixiQiaohou fault in the northern section of the Red River fault zone are analyzed.Studies have shown that,except for some special fault locations(such as terraces,intersections,etc.),currently the Weixi-Qiaohou fault has weak seismic activity,but there may be a hidden steep-dip right-lateral strike-slip fault along the west side of the fault.Small earthquakes on the fault are active.Statistical parameters,such as seismic frequency,energy release rate and b-value,indicate that the seismic activity in the Weixi-Qiaohou fault and its surrounding areas is relatively stable,and the regional stress enhancement is not obvious.The b-value is relatively high in most areas,and low b-value areas are mainly distributed in some special fault locations(such as terraces,intersections,etc.),but the scale is generally small.The statistical results of the ETAS model show that more than 40% of seismic activity may be affected by external factors such as deep fluid disturbance and remote strong earthquake triggers.Therefore,we believe that the Weixi-Qiaohou fault is currently not active,but the hidden branch faults on the west side clustered with small earthquakes have a tendency to increase.We need to pay attention to the occurrence of moderate-strong earthquakes on these branch faults.4.For the impact of long-distance earthquakes on seismicity in Yunnan,we analyze the triggering effect of 13 long-distance earthquakes on seismicity in Yunnan since 2006,focusing on the impact and mechanism of the June 17,2019 Changning Ms 6.0 earthquake.Studies have shown that the 12 strong earthquakes around 2006 to February 25,2018 have a significant impact on the Yunnan area.Among them,underground fluids are sensitive to co-seismic responses,which are manifested by water temperature rise,water level rise,flow acceleration,and significant changes in water radon content.Using ETAS model statistics,it was found that 18% of seismic activity was generated by external factors.Using the observation data of 30 temporary seismic stations,the waveform matching method was used to detect and locate the micro-earthquakes from February 25,2018 to July 31,2019,which showed that the earthquake clusters were distributed with very high permeability,such as at the end of the fault,fault bending,fault step,and fault intersection;ETAS model statistics revealed that 15 externally triggered seismic activity enhancement phases,all corresponding to the acceleration of underground fluid flow.After the occurrence of Ms6.0 Changning on June 17,2019,the co-seismic response of the underground fluid was significant,and the triggered seismic activity gradually increased and increased significantly on the fifth day after the earthquake.Using ? statistics,it was found that the triggered earthquake was still clustered and distributed repeatedly in the above faults.An area that was severely damaged by a rupture.The above results indicate that due to the widespread distribution of thermal fluids in Yunnan,long-range strong earthquakes are very likely to cause fluid flow acceleration and trigger seismic activity,and underground fluids themselves also have acceleration phenomena that lead to enhanced seismic activity.The permeability of some special structures of fault systems in Yunnan is very high,and the fluid is easy to move and form a high pore pressure,which may be the main mechanism for remote strong earthquakes to easily trigger seismic activity. |
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