Absolute stress levels in models of low-heat faults: Links to geophysical observables and differences for crack-like ruptures and self-healing pulses
Absolute stress levels in models of low-heat faults: Links to geophysical observables and differences for crack-like ruptures and self-healing pulses
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低热断层模型中的绝对应力水平：与地球物理观测值的联系以及类裂纹破裂和自愈脉冲的差异
DOI：
10.1016/j.epsl.2023.118277
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发表时间：
2023
影响因子：
5.3
通讯作者：
Lapusta, Nadia
中科院分区：
文献类型：
--
作者：
Lambert, Valère;Lapusta, Nadia
断层上的绝对应力水平对地震物理学和断层力学具有深远的意义。许多观察表明，发育良好的成熟断层，如圣安德烈亚斯断层，通常是“弱”的，即在较低的剪切应力水平下运行，而在发震深度，预期的剪切阻力较高（约100兆帕）。特别是，低热流测量表明，在高度局部化的断层上，剪切应力水平为~ 10 MPa或更小。基于地形和类似考虑的地球动力学约束也支持“弱”断层运行，并且对一些成熟断层的约束与基于热的约束相当，但对于地形丰富的地区可能更高。在这里，我们研究了平均断层剪应力的测量及其与地球物理可推断量的关系，利用数值模拟的地震序列，在低产热的速率和状态断层上，由于长期流体超压和/或孔隙流体热加压增强的动态减弱。我们回顾了地震能量平衡，重点关注基于能量的平均剪应力定义，以及平均断层预应力（一种与地球动力约束合理相关的断层强度测量）如何表示为基于耗散的平均破裂应力（原则上可以从剪切加热约束推断）和地震学上可推断的震源性质，如静态应力降和表观应力。我们的模型表明，断裂过程中剪切阻力的快速动态减弱和愈合，如自愈脉冲所示，允许断层在低动态阻力和实际静态应力下降的情况下保持较高的平均地震间应力水平，这为基于地形和基于热的断层剪切应力约束之间的潜在差异提供了物理解释。在我们的模型中，差异与应力欠射和表观应力有关，根据我们的模拟，它们可能高达静态应力降的1-3倍。然而，对于自然地震，很少推断出适当大的表观应力值（以及因此产生的辐射能量），这要么是因为辐射能量被低估了，要么表明大多数大地震没有像足够尖锐的自愈脉冲那样传播，并有足够大的低冲。我们的研究结果强调了在将震源观测与断层应力的绝对水平联系起来时，动态和静态应力变化之间的区别，并表明，结合有限断层数值模拟的输入，回顾大地震的辐射能量和静态应力下降的估计，可能会改善对绝对断层应力水平的约束。
Absolute levels of stress on faults have profound implications for earthquake physics and fault mechanics. A number of observations suggest that well-developed, mature faults such as the San Andreas Fault are generally “weak,” i.e. operate at much lower levels of shear stress compared to the higher expected shear resistance ∼100 MPa at seismogenic depths. In particular, low heat flow measurements suggest shear stress levels of ∼10 MPa or less on highly localized faults. Geodynamic constraints based on topography and similar considerations also support “weak” fault operation, and are comparable with heat-based constraints for some mature faults, but potentially higher for regions with substantial topography. Here, we investigate measures of average fault shear stress and their relationship to geophysically inferable quantities using numerical simulations of earthquake sequences on rate-and-state faults with low heat production, due to chronic fluid overpressure and/or enhanced dynamic weakening from the thermal pressurization of pore fluids. We review the earthquake energy balance, focusing on energy-based definitions of average shear stress and how the average fault prestress (a measure of fault strength plausibly relevant to geodynamic constraints) can be expressed as the sum of the dissipation-based average rupture stress (which can, in principle, be inferred from shear-heating constraints), and seismologically inferable source properties, such as the static stress drop and apparent stress. Our modeling demonstrates that rapid dynamic weakening and healing of shear resistance during ruptures, as exhibited in self-healing pulses, allows faults to maintain higher average interseismic stress levels despite low dynamic resistance and realistic static stress drops, providing a physical explanation for potential differences between topography-based and heat-based constraints on fault shear stress. In our models, the difference is related to stress undershoot and apparent stress, which can be as large as 1-3 times the static stress drop based on our simulations. Yet suitably large values of apparent stress (and hence radiated energy) are rarely inferred for natural earthquakes, either because radiated energy is underestimated, or suggesting that most large earthquakes do not propagate as sharp enough self-healing pulses with sufficiently large undershoot. Our results emphasize the distinction between dynamic versus static stress changes when relating earthquake source observations to absolute levels of fault stress and suggest that reviewing estimates of radiated energy and static stress drop from large earthquakes, with input from finite-fault numerical modeling, may improve constraints on absolute fault stress levels.
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DOI：
--
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DOI：
--
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