CHARACTERISTICS AND SEISMIC STRUCTURE ANALYSIS OF THE MS5.9 AND MS4.7 EARTHQUAKE SEQUENCES IN ALUKEERQIN BANNER,INNER MONGOLIA
The Alukeerqin Banner region in Chifeng city,Inner Mongolia,experienced two notable earthquakes,with magnitudes of MS5.9 and MS4.7,in 2003 and 2021,respectively.These were the most significant seismic events in the area in recent years,however,neither resulted in surface rupture.The distribution of aftershocks also did not align with known fault lines,and both the characteristics and mechanisms of these seismic activities remain unclear.To address this,we utilized data from the Inner Mongolia Seismic Monitoring Network to reposition the MS5.9 and MS4.7 earthquake sequences in Alukeerqin Banner.The results indicate that the MS5.9 and MS4.7 earthquake sequences occurred on the western and eastern sides of the Shuiquanzigou Tianshankou fault,respectively,aligning in a northwest-southeast(NW-SE)direction.The main shocks are approximately 45km apart,with focal depths of 2-12km and 8-22km for aftershocks.The main earthquakes are situated in the southeastern portion of the aftershock sequences,which also trend NW-SE.Depth-profile analysis of the aftershock zones shows relatively simple structures,with clusters oriented in a NW trend and inclined southwest(SW).The 2003 MS5.9 earthquake sequence exhibits a fault plane with a dip angle of approximately 60°,while the 2021 MS4.7 earthquake sequence has a nearly vertical fault plane.Using the CAP method and P-wave first-motion polarity analysis,we derived focal mechanism solutions and source depths for earthquakes of ML≥ 1.5 in the region.The focal mechanism solution indicates that the MS4.7 main shock primarily involved left-lateral strike-slip motion at a source depth of 19.9km,which closely matches the initial rupture depth of 21km obtained from relocation.Other significant earthquakes in the series similarly exhibit left-lateral strike-slip characteristics,with most developing along a NW-SE strike plane,consistent with the seismogenic fault characteristics identified in the relocated series.Previous research also shows that the main shock of the MS5.9 earthquake sequence involved left-lateral strike-slip motion,with the B-node plane orientation(NW direction)aligning with the distribution of fine-located events and the long axis of the macroscopic intensity isoseismal line.The temporal-spatial distribution and focal mechanism analyses of the MS5.9 and MS4.7 earthquake sequences suggest that their primary faults are consistent in strike and mechanical properties with the Shuiquanzi-Tianshankou Fault,trending NW but located at different positions.This confirms that the seismogenic structure of the MS5.9 earthquake is likely a left-lateral strike-slip secondary fault on the western side of the Shuiquanzi-Tianshankou Fault,trending SW.The seismogenic structure of the MS4.7 earthquake may be a concealed fault nearby with similar characteristics to the Shuiquanzi-Tianshankou Fault.We also analyzed 71 earthquakes of magnitude 2 and above in the southeastern segment of the Daxing'anling uplift since 2012,based on observation data from the China Earthquake Networks Center.Using comprehensive focal mechanism inversion,we determined the regional P-axis distribution,finding that the primary compressive stress direction in the southeastern Daxing'anling uplift is predominantly NW and nearly east-west(EW).In the vicinity of Alukeerqin Banner,the P-axis orientation is mainly EW,reflecting a relatively simple stress field.The focal mechanisms of the 2003 MS5.9 and 2021 MS4.7 earthquakes are consistent with this regional stress field,suggesting that these earthquakes were likely caused by faulting influenced by the nearly EW-oriented regional principal compressive stress.
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