A Geostatistical Study Of The Relationship Between Fluid Geochemistry And Tectonic Activity

In the field of earthquake monitoring and forecasting,fluid is a sensitive component to identify the distribution and activity of tectonic belt,numerous observations have shown that the anomalies of soil gas and dissolved gas of groundwater occur before earthquakes,but the relationship between the abnormal characteristics and tectonic and seismic activities is still unclear.In this paper,the relationship between soil gas as well as dissolved gas of groundwater in fault zone and regional tectonic distributions and activities is analyzed from time and space scale by using the geostatistical methods.The Tangshan area,which is suitable for the observation of soil gas in fault zone due to the thick sediments and less distribution of wells and springs,is chosen as the research area to analyze the effect of regional tectonic distribution on soil gas in fault zone.The eastern margin of Sichuan-Yunnan block and its adjacent area,which contain wells and springs and are suitable for dissolved gas of groundwater observation,are taken as the study area to analyze the influences on dissolved gas of groundwater variations caused by tectonic events.The relationship between fluid geochemistry and tectonic activity is revealed comprehensively.In Tangshan area,the variogram method was employed to calculate the spatial high-resolution data of gas geochemistry in Tangshan fault zones and to analyze the quantitative relationship between the activities of fault zones and the concentration of fault zone gas.The results indicate that:(1)Soil gas concentrations of Rn,Hg,He,H2 and CO2 in Tangshan ranged from 411 to 38470 Bq/m3,from 2 to 61 ng/m3,from 3.32 to 47.19 ppm,from 2.37 to 60.31 ppm,and from 498.98 to 19760.29 ppm,respectively;(2)the structural characteristic of the fault zones is the major factor influencing the spatial distribution of fault zone gas among gaseous resources,meteorologicalhydrological conditions and carrier gas etc.;(3)by means of the variogram,the correlation domain between gas concentration and tectonics is determined,thus the tectonic sensitive gas is determined in every segment of Tangshan faults,around fault zones,the key monitoring area of fault activities can be designated.In addition,the Kriging figures of fault zone gas illustrate that the Luanxian-Laoting Fault is the most active fault,the activity of the Tangshan Fault is second to the Luanxian-Laoting Fault,the Jiyunhe Fault is relatively stable.In the converged area,fault zone gas shows more sharply anomalous high concentrations.In the eastern margin of Sichuan-Yunnan block and its adjacent area,the fluctuation of dissolved radon of groundwater recorded at six different monitoring stations from 2009 to 2018 was analyzed.With the statistical approach(the residual signal processing technique and the Hurst exponent estimates),dissolved radon of groundwater variations caused by seismic events were unraveled,reflecting regional stress status changes.The results of this study indicated that(1)dissolved radon of groundwater exhibits distinct differences between the eastern margin of the SichuanYunnan Block and the South China Block,the variation range of dissolved radon concentration in groundwater at fluid monitoring stations GZ,XC,QJ,YS,YL and WX were 6.65~9.64 Bq/L,22.30~27.50 Bq/L,11.50~16.70 Bq/L,6.60~15.70 Bq/L,25.80~57.80 Bq/L,9.29~41.51 Bq/L,respectively;(2)the precursory anomalies of dissolved radon of groundwater are principally categorized into three types: positive,negative,and dual behaviors;(3)the relatively low level of the dissolved radon of groundwater results from the underlying rock masses resisting the compressional stress and the locking on the Xianshuihe fault,the Anninghe fault,and the Zemuhe fault.Additionally,the seismic response zone of each fluid monitoring station was delineated using historical seismic precursors.