川滇局部区域甲烷地震监测可行性探索——以2021年9月四川泸县M6.0地震为例

Feasibility exploration of methane seismic monitoring in local areas of Sichuan and Yunnan:Taking the M6.0 earthquake in Luxian,Sichuan in september 2021 as an example

研究表明地震会引起大气中甲烷气体异常,本研究选取川滇固定区域,以2021年9月四川泸县地震为例,基于美国对地观测卫星AQUA/EOS上搭载的高光谱传感器大气红外探测仪(AIRS)获取的甲烷气体产品,采用成熟的RST算法开展地震前后甲烷异常信息提取,并对2008年以来区域内6级以上地震开展甲烷异常指数时序分析。研究结果表明:甲烷异常与地震有一定对应关系,主要表现为甲烷打破区域历史时空特征分布规律,随孕震过程总体呈现出起始增强—异常加强—高峰—衰减—平静的变化特征。异常幅度与震级无明显的关系,但是异常持续时间可能与震级有关,即地震引起的甲烷异常并不是偶发的,具备一定异常持续时间。异常可能对应一定区域内的地震,后续需综合分析区域内构造地质情况、震级、不同研究区域半径,开展更深入的研究。川滇局部区域基于遥感手段开展甲烷气体地震异常监测具有一定的可行性,这与该区域本身富含大量烃类气体有关,地震的发生会促使地下海量烃类气体沿岩石裂隙、断裂带、不整合面等薄弱地带快速运移、扩散释放至大气中。本研究区域以外不具备油气藏条件、构造地质差异大等情况是否可开展甲烷地震监测,监测效能等尚需通过大量工作开展深入分析。

Studies showed that earthquakes can cause anomalies of methane gas in the atmosphere.The present study selected a certain area in Sichuan-Yunnan Province,and the Luxian earthquake in Sichuan Province in September 2021 was taken as an example.Based on methane gas products obtained by the Atmospheric Infrared Sounder,a hyperspectral sensor was carried by the US Earth observation satellite AQUA/Earth Observation System(AQUA/EOS).The mature Robust Satellite Technique(RST)algorithm was used to extract methane anomaly information before and after the earthquake.The background field characteristics of methane in the study area were analyzed.Moreover,the methane anomaly index time series analysis was performed for earthquakes with a magnitude of 6 or above in the region since 2008.Results showed that the methane background field in the study area is related to topography and geomorphology and has obvious spatial and temporal distribution characteristics.In particular,the difference in biodiversity and temperature is caused by seasonal changes in time,and the methane concentration regularly changes.The spatial variation is mainly related to topography and geomorphology.The seismic-related characteristic is high methane concentration at the fault and plate tectonic junction.The methane anomaly has a certain correspondence with the earthquake.This finding mainly shows that the distribution law of the temporal and spatial characteristics of methane in the regional history is broken.Moreover,the overall changes in the earthquake preparation process show certain characteristics:initial intensification,abnormal intensification,peak attenuation,and calm.The magnitude of the anomaly has no obvious relationship with the earthquake’s magnitude.However,the duration of the anomaly may be related to the magnitude of the earthquake;that is,the methane anomaly caused by the earthquake is not accidental and has a certain duration of the anomaly.In particular,the amplitude of the anomaly is greater than 2,the duration of the anomaly is at least 1 month,the amplitude of the anomaly is greater than 1,and the duration is more than 3 months.The anomaly may correspond to earthquakes in a certain region.Further research should be conducted through the comprehensive analysis of the structural and geological conditions,earthquake magnitude,and comparison of different research area radii in the following years.Conducting seismic anomaly monitoring of methane gas is feasible based on remote sensing in the local Sichuan-Yunnan region.This finding is related to the fact that the region itself is rich in hydrocarbon gas.The occurrence of earthquakes prompts the rapid migration and diffusion of underground massive hydrocarbon gas into the atmosphere along the weak zone,such as rock fissures,fault zones,and unconformity surfaces.This study preliminarily shows that the study of seismic methane anomaly in this local area is suitable for earthquakes with a similar tectonic background within the scope.Given the limitations of the study area,the follow-up study will fully consider the differences in tectonic backgrounds,combined with the distribution of oil and gas reservoirs,to conduct in-depth research outside the local area and further explore the feasibility of seismic methane anomaly monitoring.