Abstract:

Using the fault model issued by the USGS, and based on the dislocation theory and local crust-upper-mantle model layered by average wave velocity, the co-seismic and post-seismic deformation and gravity change caused by the 2021 Maduo M_S7.4 earthquake in an elastic-viscoelastic layered half space are simulated. The simulation results indicate that: the co-seismic deformation and gravity change show that the earthquake fault is characterized by left-lateral strike-slip with normal faulting. The changes are concentrated mainly in 50km around the projection area of the fault on the surface and rapidly attenuate to both sides of the fault, with the largest deformation over 1 000mm on horizontal displacement, 750mm on the vertical displacement, and 150μGal on gravity change. The horizontal displacement in the far field(beyond 150km from the fault)is generally less than 10mm, and attenuates outward slowly. The vertical displacement and gravity change patterns show a certain negative correlation with a butterfly-shaped positive and negative symmetrical four-quadrant distribution. Their attenuation rate is obviously larger than the horizontal displacement, and the value is generally less than 2mm and 1 micro-gal. The post-seismic effects emerge gradually and increase continuously with time, similar to the coseismic effects and showing an increasing trend of inheritance obviously. The post-seismic viscoelastic relaxation effects can influence a much larger area than the co-seismic effect, and the effects during the 400 years after the earthquake in the near-field area will be less than twice of the co-seismic effects, but in the far-field it is more than 3 times. The viscoelastic relaxation effects on the horizontal displacement, vertical displacement and gravity change can reach to 100mm, 130mm and 30 micro-gal, respectively. The co-seismic extremum is mainly concentrated on both sides of the fault, while the post-earthquake viscoelastic relaxation effects are 50km from the fault, the two effects do not coincide with each other. The post-seismic horizontal displacement keeps increasing or decreasing with time, while the vertical displacement and gravity changes are relatively complex, which show an inherited increase relative to the co-seismic effects in the near-field within 5 years after the earthquake, then followed by reverse-trend adjustment, while in the far-field, they are just the opposite, with reverse-trend adjustment first, and then the inherited increase. The horizontal displacement will almost be stable after 100 years, while the viscoelastic effects on the vertical displacement and gravity changes will continue to 300 years after the earthquake. Compared with the GNSS observation results, we can find that the observed and simulated results are basically consistent in vector direction and magnitude, and the consistency is better in the far-field, which may be related to the low resolution of the fault model. The simulation results in this paper can provide a theoretical basis for explaining the seismogenic process of this earthquake using GNSS and gravity data.

Key words: Maduo earthquake, gravity change, displacement, viscoelastic relaxation, dislocation

摘要：

文中基于矩形位错理论及USGS发布的断层模型, 结合研究区地壳-上地幔平均波速分层结构, 模拟计算了弹性-黏弹分层半空间中2021年玛多M_S7.4地震产生的同震及震后地表形变和重力变化。 经分析发现, 同震形变和重力变化显示发震断层具有左旋走滑兼正断错动的综合特征, 其变化主要发生于断层在地表投影周边50km的范围内, 向断层两侧快速衰减, 向E最大水平位移量>1 000mm, 向N最大位移量达570mm, 垂直位移近750mm, 重力变化达150μGal; 远震区(与断层的距离>150km)的水平位移量值一般<10mm, 向外衰减较慢; 而垂直位移和重力变化图像呈现一定的负相关, 呈蝴蝶状的正负四象限对称分布, 向外衰减的速率明显强于水平形变, 变化量值一般<2mm和<1μGal。 震后效应随时间的推移逐步显现并持续增强, 其图像变化形态与同震类似, 表现出明显的继承性增强趋势; 震后黏弹性松弛效应的影响范围远大于同震, 震后400a间其影响量值在近场区一般≤同震的2倍, 但远场区均>3倍; 震后400a间黏弹性松弛对水平位移、 垂直位移和重力变化的影响可达100mm、 130mm和30μGal; 同震效应的极值区域主要集中在断层两侧, 且离断层越近量值越大, 而震后黏弹性松弛效应的极值区分布于离断层两侧约50km处, 两者并不重合; 震后水平位移主要表现为持续单调增强, 而垂直位移和重力震后的变化则相对复杂: 近场区在震后5a内呈现相对同震的继承性增强, 随后反向调整, 而远场区则相反, 先反向调整, 后呈继承性增强; 水平位移在100a后基本稳定不变, 而黏弹性松弛效应对垂直位移和重力变化的影响会持续到震后300a。 与GNSS实测结果对比后发现, 两者在运动方向和量级大小上基本一致, 远场符合更好, 这可能与断层模型的分辨率有关。 文中研究可为利用实际形变和重力资料解释此次地震的孕震过程研究提供理论依据。

关键词: 玛多地震, 重力变化, 形变, 黏弹性松弛, 位错