以2022年马尔康震群为例研究震群发震机制

Probing into the mechanism of earthquake swarms:a case study of the 2022 Barkam earthquake swarm

地震是人类所面临的危害最严重的自然灾害之一,了解地震的孕震、发震机理对防震减灾、国家地震安全等具有重要的科学和社会价值.相较于前震-主震-余震型、主震-余震型地震,震群型地震更易造成较大的灾害,因此对震群孕育发生机理的研究刻不容缓.2022年6月10日四川省阿坝州马尔康市发生震群事件,其中包括MS5.8、MS6.0、MS5.2等三次较大震级的地震,此次地震序列的精定位结果、可靠的震源参数为开展震群孕震机制的研究提供了良好的数据基础.本文基于数值模拟的方法,利用断层的运动学参数建立黏弹模型,根据弹性位错、孔隙流体扩散等理论,从发震断层的能量积累、释放两个角度探讨马尔康震群可能的发震机理.对发震断层系统应力加载的模拟结果表明,震区的应力加载较均匀的分配到两条近平行发震断层上,使其在相同的时期内积累了相近的震间构造应力,进而发生相似震级的震群事件.考虑震群序列之间可能存在的应力触发作用,同震库仑应力计算的结果显示马尔康震群序列中各主要地震未受到同震静态应力触发作用.而对发震断层等效摩擦系数的判断及其附近流体扩散的研究认为马尔康震群发震断层震前受流体活动的影响,等效摩擦系数下降从而导致断层破裂,震后流体调整导致的孔隙压力变化进一步促进后续地震的发生.

Earthquakes are among the most devastating natural disasters that affect humanity.Understanding the mechanism behind earthquake initiation and propagation holds crucial scientific and societal significance for seismic hazard mitigations.In contrast to foreshock-mainshock-aftershock and mainshock-aftershock seismic patterns,earthquake swarms are prone to causing more extensive damage.Hence,an in-depth investigation into the mechanisms governing earthquake swarm genesis is imperative.On June 10,2022,a swarm sequence of earthquakes,including three moderate to large earthquakes with magnitudes of MS5.8,MS6.0,and MS5.2,occurred in Barkam,Aba Prefecture,Sichuan Province,whose accurate relocation results and reliable source parameters provide a good data base for such research.In this paper,a viscoelastic model is established based on the kinematic parameters of faults,and the possible seismogenic mechanisms of the Barkam swarm are discussed from the perspectives of energy accumulation and release on seismogenic faults according to the theories of elastic dislocation and pore fluid diffusion.The results of stress loading simulations on the seismogenic fault system indicate that it is possible for parallel faults to be stressed evenly and simultaneously during the same interseismic period,which will lead to a swarm of events of similar magnitude.Considering the possible stress-triggering effect between the events in a swarm,this paper also calculated the coseismic Coulomb stress changes in the Barkam earthquake swarm sequence,and found that the events were not triggered by coseismic static stress transfers.Moreover,our simulation suggests that the effective friction coefficient of the seismogenic fault of the Barkam swarm was decreased by the fluid activity before the earthquake,which contributed to the rupture of the fault,and the pore pressure change caused by the fluid adjustment after the earthquake further promoted the occurrence of subsequent earthquakes.