STUDY ON THE SEISMOGENIC STRUCTURE OF THE 2022 GUJIAO ML4.1 EARTHQUAKE IN SHANXI PROVINCE BASED ON FOCAL MECHANISM AND SEISMIC LOCATION
Understanding the mechanism of earthquake sequence in the mining area is important for the time-dependent hazard assessment.An earthquake of ML4.1 occurred in Gujiao,Taiyuan,Shanxi on February 20th,2022,which caused strong ground motion in Gujiao and surrounding counties.The epicenter of this earthquake is located in the area of Lvliang uplift,where historical earthquakes are relatively rare.In addition,the coal resources are well developed in the earthquake source area which has attracted much attention from society and local governments.To investigate the mechanism and the seismogenic fault of Gujiao ML 4.1 earthquake,we first apply the double-difference location method to retrieve highly accurate hypocenter locations.The results show that the earthquakes mainly occur at a depth range of 3～5km,and display a dominant distribution direction nearly EW-trending,which differs significantly from the NE-trending fault distribution pattern in this region.We further collect the broad-band seismic waveforms from the regional network of Shanxi province to perform focal mechanism inversion.The inversion results show that the Gujiao earthquake is a left-slip seismic event with a moment magnitude of MW3.96.The optimal double-couple solution is characterized by a strike of 90°,dip of 80°,and a rake angle of-21° for fault plane Ⅰ,while for the fault plane Ⅱ,the strike is 184°,dip is 69°,and rake angle is-169°.The best centroid depth is estimated to be at 3km.This earthquake shows an extremely shallow focal depth.Moreover,By using cluster analysis method,we obtained the central solution for the seismogenic fault plane of the GuJiao earthquake,with a fault strike of 91°and a dip angle of 70°.The focal solutions show that the earthquake exhibit a strike-slip type,and the orientations of earthquake sequence coincide well with the focal mechanisms.In addition,to discuss the effect of Gujiao ML4.1 earthquake on regional stress,we calculate the stress drop of this seismic sequence.The results show that the stress drop is significantly smaller than that of the regional earthquakes,exhibiting at least one order of magnitude lower than that of the background earthquakes in the same region.This phenomenon reflects that the stress level in the focal area of the GuJiao earthquake is not high,suggesting that the background stress enhancement in the focal area is not obvious.Based on regional geological structure,we found that the known faults in the region are all high-angle normal faults,and the strike of these faults are inconsistent with the focal mechanism solution of Gujiao earthquake sequence,which suggests that the existing faults are not the seismogenic fault.Taking the regional mining activities into account,we speculated that mining may cause strong disturbance to the stress field,and lead to stress redistribution within the rock mass.Such coal mining activity may generate a high stress disturbance on the hidden fault plane,and then the fault become the carrier of stress transfer.So we conclude that the seismogenic mechanism of the Gujiao-seismic sequence may be related to coal mining activities near the focal area,which leads to local stress changes,thus resulting in the activation of preexisting hidden faults and triggering the occurrence of the Gujiao earthquake.
Gujiao earthquakefocal mechanism solutionhidden faultgCAP methodseismic stress drop
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