北天山中段微震活动检测及其活动性和孕震构造分析

Discussion on the Seismicity and Seismogenic Structure in Fold-and-thrust Belt of Northern Tianshan Mountains

北天山地震带地处中国大陆强震高发区,孕震构造复杂,近年来陆续发生了2016年呼图壁M_S6.2地震和2017年精河M_S6.6地震。由于测震台网相对比较稀疏,该区域微震监测能力较弱。本文主要采用波形模板扫描法对北天山中段(43.5°N^44.5°N;85°E^87.5°E)进行微震事件检测,并反演精细的一维速度结构,重新定位地震;深入分析该区域的地震活动性和孕震构造特征。经过微震检测,得到该地区2014年1月至2018年9月期间57902个地震事件,是原地震目录的10倍,完备震级从1.2降至0.5。结果显示,北天山中段地震十分活跃,主要分布在北天山山前霍尔果斯—玛纳斯—吐谷鲁背斜带南翼的浅部和南玛纳斯—齐古背斜带深部,呼图壁地震震后地震活动性有增强的趋势。研究期间沿背斜构造带走向地震分布不均,霍尔果斯—玛纳斯—吐谷鲁背斜带西段地震活动多于东段,南玛纳斯—齐古背斜带东段地震活动显著强于西段。经过重定位,发现研究区的地震事件主要发生在褶皱内部的'盲断层'上,这些隐伏断裂与区域活动断裂和背斜构造共同组成的断层系孕育了北天山山前活跃的地震活动,并可能成为未来强震的发震构造。

The North Tianshan Seismic Belt in China has complex seismogenic structures with high risk of strong earthquake.The 2016 Hutubi M_S6.2 earthquake and the 2017 Jinghe M_S6.6 earthquake occurred successively in recent years.However,because the seismic network of North Tianshan is relatively sparse,the ability to monitor micro-seismicity in this area is weak.In this paper,we detected microseismic events from continuous waveforms with the waveform matched filter technique in the middle section of the North Tianshan Mountains(about 43.5°N^44.5°N,85°E^87.5°E),relocated them with revised velocity structure,and then analyzed the characteristics of seismicity and seismogenic structure in this area.The scan detected 57 902 events in the study area from January 2014 to September 2018,about 10 times earthquake events in CENC(China Earthquake Networks Center)catalog,and the complete magnitude decreased from 1.2 to 0.5.The results show that there are obvious swarm activities in the study area,which mainly occurred after the Shawan and Hutubi earthquakes and distributed in the shallow southern flank of the Huoerguosi-Manasi-Tugulu anticlinal belt and deep South Manasi-Qigu anticlinal belt.Earthquakes along the strike of anticlinal tectonic belt are unevenly distributed.Earthquakes in the western Huoereguosi-Manasi-Tugulu anticlinal belt are more than that in the eastern part.But the earthquake magnitude in eastern South Manasi-Qigu anticlinal belt is significantly larger than that in the western part.After relocation,we found that the seismic events in the study area mainly occurred on the'blind faults'within the folds.The regional large active faults and concealed faults in anticlinal structures gave birth to active seismicity in the piedmont of the North Tianshan Mountains,and would become the sources of future strong earthquakes.