3D numerical model for viscoelastic postseismic deformation following the Maule MW8.8 earthquake in 2010
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摘要: 2010年智利马乌莱MW8.8地震发生在纳斯卡板块与南美板块的板块边界处，引起了显著的同震和震后效应。GPS台网数据显示记录到的同震海向位移最大约5 m，垂向沉降最大约50 cm。在经过对俯冲效应、季节变化等效应的校正后，震后6年的海向最大位移约68 cm，垂向抬升最大约20 cm。马乌莱地震显著的震后形变对该区域的地下三维黏弹性结构有良好的约束。本文建立了智利中南部俯冲带区域的三维有限元模型，黏弹性的地幔楔及海洋地幔均使用伯格斯体材料，并在断层面上设置2 km厚的软弱层以模拟震后余滑。在与GPS台站震后位移数据进行比较后，模拟结果表明，大洋地幔顶部存在约120 km厚，黏度为1×1019 Pa·s的软流圈。模拟震后余滑效应的软弱层黏度为5×1017 Pa·s，其等效震后余滑的最大值在震后前两年接近2 m，且随着时间的增长而快速衰减。Abstract: The 2010 MW8.8 Maule earthquake occurred near the plate boundary between the Nazca plate and the South American plate. The earthquake produced significant coseismic and postseismic deformation. The maximum coseismic motion is about 5 m in the horizontal direction and about 5 cm subsidence. After correcting the GPS data for secular, seasonal and annual trends, the postseismic cumulative motion within the first 6 years after the earthquake include up to about 68 cm in the horizontal direction and up to 20 cm uplift. The three-dimensional （3D） viscoelastic structure can be constrained by the postseismic deformation of the 2010 earthquake. We have constructed a 3D finite element model to study the effects of the rheological structure on the postseismic deformation of the 2010 earthquake. We assume the viscoelasticrelaxation of the upper mantle to be represented by the Burgers rheology. And in the paper, a 2 km thick weak shear zone attached to the megathrust is used to simulate the afterslip. Based on the comparison with the GPS observation data, the preferred model determined that a 120 km thick asthenosphere with a viscosity of 1×1019 Pa·s at the top of the oceanic upper mantle is required to fit the data. The afterslip simulated by shear zone with a viscosity of 5×107 Pa·s is up to 2 m in the first 2 years and decays rapidly with time.
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图 1 南美俯冲带2010年马乌莱MW8.8地震构造背景
Figure 1. The tectonic background of the 2010 MW8.8 Maule earthquake in South American subduction zone
图 2 拟合得到的震间速度场与震后位移场
（a） 震间速度场；（b） 震后六年累积位移场
Figure 2. Fitted interseismic velocity field and postseismic displacement field
（a） Interseismic velocity field；（b） Accumulated postseismic displacement field for 6 years
图 3 有限元模型
（a） 具有最优参数的有限元模型示意图；（b） 三维有限元网格图
Figure 3. Finite element model
（a） Schematic diagram for FEM with the optimal model parameters；（b） 3D FEM meshes
图 4 2010年马乌莱MW8.8地震的同震位移图
Figure 4. Coseismic displacement of the MW8.8 Maule earthquake in 2010
图 5 震后两年震后余滑与各部分黏弹性松弛效应的独立模拟及其综合效应
（a） 软弱层模拟的震后余滑效应；（b） 地幔楔应力松弛效应；（c） 大洋地幔软流圈应力松弛效应；（d） 深部大洋地幔应力松弛效应；（e） 图（a）−（d）的综合效应
Figure 5. Independent simulation of afterslip viscoelastic relaxation effect of each part and its combined effect in 2 years after earthquake
（a） Afterslip effects simulated by weak shear zone；（b） Stress relaxation effects of mantle wedge；（c） Stress relaxation effects of oceanic asthenosphere；（d） Stress relaxation effects of deep oceanic mantle；（e） Combined effects of figs.（a）−（d）
图 6 模型误差图 （每一个小方块代表一个测试模型）
（a） 仅考虑水平分量误差图；（b） 仅考虑垂直分量误差图；（c） 水平和垂向三分量误差图
Figure 6. Misfits of model results （Each cube represents one test model）
（a） Misfits only considered horizontal component；（b） Misfits only considered vertical component； （c） Misfits considered three-component data
图 7 最优模型 0—2年（a）、2—4年（b）、 4—6年（c）震后位移图
Figure 7. Postseismic displacement for the optimal model of 0−2 years （a），2−4 years （b） and 4−6 years （c）
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