河套地震带的震源机制类型时空分布特征

基于地质构造背景分析,收集2000年以来发生在河套地震带的ML≥2.8级地震作为研究对象,综合运用基于P波初动的振幅比方法(APAS)和基于波形拟合的CAP反演方法求出256次地震事件的震源机制解。以断层节面滑动角作为判定指标,分区域给出了河套地震带的震源机制类型空间和时间分布图像,从断层滑动角度呈现河套地震带应力场时空变化过程。结果显示:临河盆地断层节面滑动角主要在水平±20°方向存在优势分布,走滑型特征显著;具体来讲,狼山—色尔腾山山前断裂带、临河断裂、乌拉山山前断裂等主要以纯走滑型地震为主,巴彦乌拉山断裂与磴口—本井断裂之间的区域多分布正走滑型地震。呼包盆地断层节面解虽然也呈现出走滑型为主的特征,但滑动角分布较为离散,优势分布方向不明显;呼包盆地西侧的包头至西山咀凸起一带表现出以走滑为主的小范围震源应力场特征,呼包盆地内部及东侧由于显著的区域垂直差异运动,正断层和逆冲型地震所占比例较大,震源机制类型整体呈现出与构造相依的分布特点。分析认为,2000年以来,河套地震带的应力场存在一定的时空非均匀性变化,研究结果更多表现了河套地震带的震源应力场变化过程,而研究资料时间不够长和震级不够大是引起这种应力场非均匀性暂态特征的主要因素。

Space-Time Distribution Characteristics of the Focal Mechanism Type in Hetao Seismic Belt

Based on the analysis of the geological structure background,and the collected earthquake (ML ≥2.8)data in Hetao seismic zone since 2000,the focal mechanism of 256 is calculated by using P wave first motion amplitude ratio method(APAS)and CAP retrieval method.Taken the fault plane slide angle as the indicater,we made the space and time distribution chart of the focal mechanism types in different zones,which depicts the space and time variation of the stress field in Hetao seismic zone.The results show that the fault plane slide angle in Linhe basin appeared dominant in horizontal ± 20°,a significant feature of strike slip type.This type of earthquakes are distributed mainly in Langshan Seertengshan fault zone, Linhe fault and Wulashan fault zone. The normal-strike-slip type of earthquakes are distributed mainly between Bayanwulashan fault zone and Benjin fault.Hubao basin faults are mainly strike-slip type,but the slide rangles are dispersed with on clear direction and space correlation West side of Hubao basinand Baotou-Xishanzui rise show a small-scale slip focal stress field characteristics;thrust and normal earthquakes take a great proportion in the inside and east side of Hubao basin resulted from the vertical differential movement,and the focal mechanism is structure-dependent.According to the analysis in the study area and in the period ,the gravitational field in the seismic belt varies non-uniformly in space and time.The non-uniformty is mainly result by the limitation of the time span and the magnitude.