Gravity field characteristics and seismogenic model in North China
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摘要: 本文采用基于贝叶斯原理的重力平差方法对华北地区2009—2017年的16期陆地流动重力观测资料进行了统一处理，结合研究区内2009年以来4次M≥4.5 地震活动，分析区域重力场动态演化特征。基于华北地区均衡重力异常、布格重力异常、沉积层重力异常和莫霍面重力异常，研究华北地区三维密度结构特征，分析区域重力场变化与深部密度结构之间的关系。结果表明：研究区内近年来M≥4.5地震均位于区域差分重力场和累积重力场“零值”等值线附近、异常变化剧烈的梯度带及梯度带转弯部位，以及深部结构中理论均衡厚度与实际地壳厚度存在差异的非均衡区和三维密度结构的高低密度过渡带内。Abstract: In this paper, we use a Bayesian principle-based gravity leveling method to uniformly process 16 periods of land-based mobile gravity observations from 2009-2017 in North China, and study the dynamic characteristics of the regional gravity field in conjunction with four earthquakes （M≥4.5） that have struck the studied area since 2009. Further, we study the characteristics of the isostatic gravity anomaly in North China, and analyze the relationship between regional gravity field changes and deep density structure based on the Bouguer gravity anomaly, sedimentary sequences gravity anomaly and Moho gravity anomaly in the studied area to characterize the three-dimensional density structure in North China. The results show that, the four earthquakes （M≥4.5） in the studied area in recent years have been located near the contours of the regional differential gravity field and the zero-value of the cumulative gravity field, the abnormally shifting gradient zones and the bends of the gradient zones, the unbalanced zones in the deep structure where the theoretical equilibrium thickness differs from theactual crustal thickness, and the high and low densities of the three-dimensional density structure inside the transition zone.
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图 1 华北地区陆地流动重力观测网络
Figure 1. Land-based mobile gravity observation network in North China
图 2 华北地区2009-2期（a）和2014-2期（b）平差点值精度
左侧为贝叶斯平差方法平差结果；右侧为经典方法平差结果
Figure 2. The leveling accuracy of the 2nd measurement in 2009 （a） and 2014 （b） in North China
The left is the results of Bayesian method and the right is classical method
图 3 华北地区4次M≥4.5地震前后陆地重力相邻两期差分动态变化（a，b，c）及相对首期累积动态变化（d，e，f）
Figure 3. Differential dynamical gravity changes （a，b，c） and cumulative gravity changes （d，e，f） from the first measurement before and after the four M≥4.5 earthquakes in North China
（a） 2010-1－2009-2；（b） 2010-2－2010-1；（c） 2011-1－2010-2；（d） 2010-1－2009-2；（e） 2010-2－2009-2；（f） 2011-1－2009-2
图 4 研究区莫霍面深度（a）、均衡深度（b）和均衡深度异常（c）的结果
Figure 4. Moho depth （a），equilibrium depth （b） and equilibrium depth anomaly （c） results of the studied area
图 5 研究区沉积层底深度（a）、沉积层重力异常（b）和莫霍面重力异常（c）结果
Figure 5. Results of sediment base depth （a），sediment gravity anomaly （b） and Moho surface gravity anomaly （c） in the studied area
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