Earthquakes on faults in the brittle upper crust evoke sudden changes in pore fluid pressure as well as postseismic viscoelastic flow in the lower crust and lithospheric mantle but the relative importance of these processes during the postseismic phase has not been systematically studied. Here, we use two-dimensional finite-element models to investigate how pore fluid pressure changes and postseismic viscoelastic relaxation interact during the earthquake cycle of an intracontinental dip-slip fault. To isolate the effects from pore fluid flow and viscoelastic relaxation from each other, we performed experiments with and without pore fluid flow and viscoelastic relaxation, respectively. In different experiments, we further varied the permeability of the crust and the viscosity of lower crust or lithospheric mantle. Our model results show poroelastic effects dominate the velocity field in the first months after the earthquake. In models considering poroelastic effects, the surfaces of both hanging wall and footwall of the normal fault subside at different velocities, while they move upwards in the thrust fault model. Depending on the permeability and viscosity values, viscoelastic relaxation dominates the velocity field from about the second postseismic year onward although poroelastic effects may still occur if the permeability of the upper crust is sufficiently low. With respect to the spatial scales of poroelastic effects and viscoelastic relaxation, our results show that pore fluid pressure changes affect the velocity field mostly within 10–20 km around the fault, whereas the signal from viscoelastic relaxation is recognizable up to several tens of kilometres away from the fault. Our findings reveal that both poroelastic effects and viscoelastic relaxation may overlap earlier and over longer time periods than previously thought, which should be considered when interpreting aftershock distributions, postseismic Coulomb stress changes and surface displacements.

中文翻译：

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正向地震和逆冲地震后孔隙弹性效应和粘弹性松弛对震后速度场的相对重要性：二维有限元建模的见解

脆性上地壳断层上的地震会引起下地壳和岩石圈地幔中孔隙流体压力以及震后粘弹性流的突然变化，但这些过程在震后阶段的相对重要性尚未得到系统研究。在这里，我们使用二维有限元模型来研究在陆内倾滑断层的地震周期中孔隙流体压力变化和震后粘弹性弛豫如何相互作用。为了将孔隙流体流动和粘弹性松弛的影响相互隔离，我们分别进行了有和没有孔隙流体流动和粘弹性松弛的实验。在不同的实验中，我们进一步改变了地壳的渗透率和下地壳或岩石圈地幔的粘度。我们的模型结果显示，地震发生后最初几个月，孔隙弹性效应在速度场中占主导地位。在考虑孔隙弹性效应的模型中，正断层的上盘和下盘表面以不同的速度沉降，而在逆冲断层模型中则向上移动。根据渗透率和粘度值，从震后第二年开始，粘弹性弛豫在速度场中占主导地位，尽管如果上地壳的渗透率足够低，仍然可能发生孔隙弹性效应。对于孔隙弹性效应和粘弹性弛豫的空间尺度，我们的研究结果表明，孔隙流体压力变化对速度场的影响主要集中在断层周围10-20公里范围内，而粘弹性弛豫信号在数十公里外也能被识别。从过失。我们的研究结果表明，孔隙弹性效应和粘弹性松弛可能比以前认为的更早、更长的时间段重叠，在解释余震分布、震后库仑应力变化和地表位移时应考虑到这一点。