Tectonic Geochemical Characteristics And Activity Of Major Active Fault Zones On The Northeastern Margin Of The Qinghai-Tibet Plateau

The northeast margin of the Qinghai-Tibet Plateau is mainly located in the intersection of the Qinghai-Tibet Plateau and the North China Craton,and is the forefront of the extrusion and expansion of the Qinghai-Tibet block from the near north-south direction to the northeast direction.For a long time,the region has been characterized by strong tectonic deformation activities and large earthquakes,and the need for earthquake prevention,disaster reduction and social stability development in the region has become increasingly prominent.As a new discipline and method,the application of tectonic geochemistry in earthquake monitoring and forecasting is steadily advancing.It is mainly used to segment the seismic hazard of the rupture zone by the spatial variability of fault gas concentration,to judge the degree of seismic hazard by the fluctuation of fault gas concentration in time,and to determine the magnitude of the earthquake by the characteristics of the rupture structure and historical earthquakes.However,the method is not unified in terms of the specification of technical methods,the establishment of analytical standards and the understanding of anomalous mechanisms,which seriously restricts the development of the discipline and its promotion and application.How to make these unified and standardized is the key to the current practice of tectonic geochemical methods in short-range earthquake forecasting.This manuscript takes the main active fractures in the northeast margin of the Qinghai-Tibet Plateau(the middle-east section of the Qilianshan fault zone and the north margin of the West Qinling fault zone)as the research objects,and systematically investigates the tectonic geochemical characteristics of fault gas(radon,hydrogen and carbon dioxide)in the main active fracture zones in the northeast margin of the Qinghai-Tibet Plateau based on the results of soil gas detection in the fracture zones for many years,and deeply explores the coupling relationship between the intensity of fault gas concentration and the spatial distribution characteristics of the b-value of seismic activity parameters by combining the spatial and temporal distribution characteristics of historical and present-day seismic activities,so as to build a bridge for using tectonic seismic chemistry for tectonic activity studies and earthquake prediction.Finally,the solid body seismogenic model and its numerical simulation results are used to analyze the earthquake cases on the northeastern margin of the Tibetan Plateau,and give the indexes for determining the anomalies of medium-to strong-earthquake fault gas precursors to lay the foundation for future precursor analysis and earthquake forecasting.The main conclusions of this paper are as follows.(1)Soil gas from major active fault zones on the northeastern margin of the Tibetan Plateau is a clear indicator of fault geometry and activity characteristics.The fault gas concentration curves in the middle-eastern section of Qilianshan fault zone and the northern edge of West Qinling Ridge fault zone have obvious characteristics,and most of the measurement lines have more typical curve patterns,indicating that both active faults are favorable for fault soil gas measurement.Among them,the anomalous characteristics of radon are obvious,and the synchronization of radon and carbon dioxide is relatively good.(2)The solid body seismogenic model can better explain the relationship between tectonic geochemical features and active fracture occlusion segments,and provide theoretical support for determining the time,space and intensity of earthquake occurrence.Analysis of existing seismic cases shows that the gas tectonic geochemical descent characteristics are well indicative of the occluded section of fault tectonic activity,suggesting a background of strong earthquake incubation in the region.The descending and turning characteristics of fault gas tectonic geochemistry are indicative of the urgency of moderate to strong earthquakes,and the more obvious the turning,the closer to the source area.The greater the magnitude of the fault gas anomaly,the stronger the earthquake magnitude.Based on this analysis and summary,the precursor anomaly determination index is extracted.(3)The spatial and temporal variation characteristics of soil gas geochemistry in the middle-eastern part of the Qilianshan fault zone and the northern edge of the West Qinling Ridge fault zone are obvious indications of the regional tectonic motion state and seismogenic background.The spatially segmented characteristics are consistent with the historical present-day seismicity.The characteristics of trend change in time are closely related to the urgency of earthquakes.(4)The soil gas of the main active rupture zone on the northeast edge of the Qinghai-Tibet Plateau is an obvious indication of the rupture zone activity and seismic hazard.The western section of the middle-eastern part of the Qilianshan fault zone and the Ganjia and Buzicun-Diaogoumen section of the northern edge of the West Qinling Ridge fault zone have a moderately strong earthquake inception background and are future seismic hazard sections.