| The transient movement of fluid induced by earthquake contains the information of dynamic changes in stress.The study and identification of the dynamic changes in stress is of great significance for seismic activity analysis and prediction.The direction and velocity of underground fluid flow can be deduced from the effects of fluid flow on amplitude and phase of periodic temperature time series at different shallow depths.However,previous studies mostly used daily periodic temperature fluctuations at depths within 1 m.The observation techniques of dynamic changes of bedrock temperature developed in recent years have confirmed that bedrock temperature,as a new geophysical observation property,contains information of the movement of shallow crustal fluids,and the observation depth has been extended to tens of meters in this technology.It provids a new opportunity to obtain the characteristics of fluid flow in the shallow crust from temperature data.In 2016,an observation network of bedrock temperature with 5 measurement stations in Kashgar,Xinjiang has established by Institute of Geology of China Earthquake Administration.It has accumulated more than 4 years of data so far,providing valuable temperature data for obtaining the information of fluid flow.Based on these observational data,we studied the coupling relationship between the dynamic change of bedrock temperature and the movement of underground fluid,and obtained the thermal diffusivity and the multi-year average velocity of underground fluid flow in the area.On this basis,we further obtained the characteristics of fluid movement over time,and explored the relationship between the fluid flow and seismic activity.The main research work and conclusions are as follows:(1)The existence of fluid results in the difference between the apparent thermal diffusivities obtained from the amplitude and phase methods,which is not equal to the true thermal diffusivity.The greater the fluid velocity,the greater the difference between the two apparent thermal diffusivities.When it is not obvious,the apparent thermal diffusivity obtained by the phase is closer to the true thermal diffusivity.In Kashgar area,the apparent thermal diffusivity?Acalculated from the amplitude of five measurement stations ranges from 0.92×10-6to 1.71×10-6m2·s-1,and the apparent thermal diffusivity??calculated from the phase varies from 1.53×10-6to m2·s-1.The true thermal diffusivity?ranges from 1.52×10-6to 8.91×10-6m2·s-1.There is a significant difference in values between?Aand??,which indicates that the heat transfer process at the measurement stations is greatly affected by the heat convection of the fluid.In addition,we confirmed that when the thermal effect generated by fluid migration is not obvious,the apparent thermal diffusivity obtained from the phase is closer to the true thermal diffusivity.(2)The estimated information of underground fluid flow at each measurement stations in Kashgar shows that the flow direction of fluid in the measurement stations area is all upward,the fluid velocity of the measurement stations near Tianshan area ranges from 0.10×10-7to 1.94×10-7m·s-1,and the fluid velocity of the measurement stations near Kunlun Mountain area various from 8.56×10-7to 9.71×10-7m·s-1.By discussing the relationship between the fluid migration process and seismic activity,it is found that after frequent seismic events,the fluid velocity at some measurement stations has a tendency to increase.The multi-year average velocities are 1.1×10-8?1.0×10-8?1.94×10-7?9.71×10-7?8.56×10-7m·s-1 at Baoeryikuotuo Township(PX),Kangsu Township(KS),Shangatushi Township(SA),Upal Township(WP)and Machang(MC)stations respectively,and the flow direction is all upward.There are obvious differences in fluid velocity at the measurement stations in different regions.The measurement stations near the Tianshan area are Baoeryikuotuo Township(PX),Kangsu Town(KS)and Atushi Township(SA),and the fluid velocity is smaller.And the measurement stations near Kunlun Mountain area are Wupal Township(WP)and Horse Farm(MC)with higher fluid flow rates.The difference among the estimated fluid velocities at different measurement stations in different regions may be related to the hydrogeological environment.On the basis of obtaining the movement process of the fluid in the study area,we discussed the relationship between the characteristics of fluid flow and the seismic activity.Analysis shows that after seismic events occur frequently,the velocity of the underground fluid flow at stations of Baoer Yikuotuo Township(PX)and Kang Su Township(KS)has an increasing trend.In sum,the research shows that the characteristics of fluid flow can be obtained from continuous bedrock temperature observations at multiple depths,which is expected to provide a potential analysis method of earthquake precursor in the further. |
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