华北地区中东部岩石圈挠曲与均衡特性以及地震活动性分析

Flexure of the lithosphere, isostatic characteristic and seismicity in middle east area of North China

华北地区中东部涵盖北京、天津以及即将建设的雄安新区等大型城市,区内发育了张渤地震带等多条大型活动断裂,地震活动性较强,历史上发生过多次6级以上地震.本文利用Fan小波的布格重力异常一致性方法研究该区的岩石圈有效弹性厚度和均衡调整初始加载比分布,同时基于均衡调整方法计算该区垂向构造应力分布,并将以上结果与历史地震活动进行统计分析.岩石圈挠曲分析表明,华北地区中东部的岩石圈有效弹性厚度为10～65 km,分布特征为自东南向西北逐渐减小.均衡调整初始加载比为0.5～0.8,表明现今的岩石圈挠曲状态主要由莫霍面加载形成.该区地壳承载的垂向构造应力约为-20～20 MPa,中西部地区垂向构造应力向上,东北和西南地区向下.统计分析结果显示,华北地区中东部的地震活动性随着岩石圈有效弹性厚度和均衡调整初始加载比的增加而减弱,垂向构造应力零值区域地震活动性较弱.雄安新区的岩石圈有效弹性厚度大约为15 km,均衡调整初始加载比为0.5～0.6,垂向构造应力为15～20 MPa,岩石圈参数对应的地震活动性较强,相关结果对于新区建设具有一定参考价值.

Middle east area of North China covers many mega-metropolis,such as Beijing and Tianjin,as well as the soon-to-be-built Xiong′an New District.Several large-scale active fault zones such as the Zhangjiakou-Bohai seismic belt have been developed in this area.With a strong seismicity,there have been many earthquakes of magnitude 6 or above in history.The distributions of effective elastic thickness and the isostatic initial loading ratio in this area are studied by the bouguer gravity anomalies coherence method which bases on the Fan wavelet.Simultaneously,the distribution of the vertical tectonic stresses is calculated by the isostatic adjustment method.Furthermore,the results above and the historical seismicity are analyzed statistically.The analysis of lithosphere flexure shows that the effective elastic thickness in the middle east area of North China is about 10~65 km,and its distribution features decrease gradually from southeast to northwest.The isostatic initial loading ratio is about 0.5~0.8,which indicates that the present flexure state of lithosphere is mainly caused by the loading of Moho interface.The vertical tectonic stress of the crust bearing in this area is about-20~20 MPa,the stresses are upward in the central and western areas,and downward in the northeastern and southwestern areas.The statistical results show that the seismicity decreases with the increases of the magnitude of effective elastic thickness and the isostatic initial loading ratio in middle east area of North China.The seismicity is weak in the zero value area of the vertical tectonic stress.The effective elastic thickness of the lithosphere in the Xiong′an new district is about 15 km.The isostatic initial load ratio is 0.5~0.6,and the vertical tectonic stress is 15~20 MPa.The lithosphere parameters correspond to strong seismicity,and the relevant results have certain reference value for the construction of the new district.