Dilatant Stabilization as a Mechanism for Slow Slip Events

在俯冲带，包括北美西部的卡斯卡迪亚俯冲带，利用精确的全球定位系统（GPS）网络发现了“缓慢滑动事件”。 这些滑动事件使地球移位？地球表面的地震与地震相似，但不是持续几秒到几分钟，而是持续几天到几周，甚至几年。 因为它们发生得很慢，慢滑动事件不会辐射破坏性的地震波。 它们似乎位于产生8至9级地震的巨型逆冲断层之下。 卡斯卡迪亚的缓慢滑动事件每10到16个月发生一次，并逐渐增加锁定断层带的压力。 看来，缓慢滑动事件发生在锁定和稳定蠕动断层之间的摩擦过渡带。 因此，了解缓慢滑动的发生可能会改善对破坏性俯冲带地震的预测。 对慢滑的物理学仍然知之甚少。 研究人员认为，速率状态下的摩擦成核排水（恒定孔隙压力）条件下的滑动，但作为滑动加速，并成为有效的不排水（无流量），粘性引起的孔隙压力降低淬火的不稳定性。 他们研究这一点，采用简化的等温，膜扩散模型和Segall-Rice [1995]的本构关系的粘性。数值模拟表现出缓慢或快速（动态）滑移取决于粘性和摩擦参数，以及有效的正应力。稳定的滑动是有利的低有效应力（高孔隙压力），符合地震推断的vp/vs比在一些慢滑带。 他们将通过开发流体输运的有限差分计算来扩展这项工作，该计算将与摩擦/弹性模拟相结合。慢滑移建模最终必须考虑热以及摩擦弱化。 研究人员已经表明，在地震辐射之前，在准静态滑动速度下，热加压变得很重要;精确的速率取决于渗透率。这表明，滑动速度是快还是慢取决于滑动率是否受双折射率的限制 低于热加压极限。 为了研究这一点，需要将热扩散和孔隙流体扩散与摩擦、弹性和粘性耦合起来。 对于与穿过活跃剪切层的特征扩散时间相比变得更快的滑移，他们必须在计算中明确考虑剪切区的有限厚度。 这项工作应该对缓慢滑动的物理学以及最终控制滑动是否加速到破坏性地震的惯性限制速度的因素有重要意义。

"Slow slip events" have been discovered in subduction zones, including the Cascadia subduction zone in western North America, with precise Global Positioning System (GPS) networks. These slip events displace the earth?s surface in a similar fashion to earthquakes, however instead of lasting seconds to minutes, they last days to weeks, and even years. Because they occur so slowly, slow slip events do not radiate damaging seismic waves. They appear to be located beneath the megathrust faults that generate magnitude 8 to 9 earthquakes. Slow slip events in Cascadia occur fairly regularly every 10 to 16 months, and incrementally increase the stress on the locked fault zones. It seems likely that slow slip events occur in a frictional transition zone between the locked and steadily creeping faults. Therefore, understanding the occurrence of slow slip may lead to improved forecasting of damaging subduction zone earthquakes. The physics of slow slip have remained poorly understood. Theh investigators suggest that rate-state friction nucleates slip under drained (constant pore-pressure) conditions, but as slip accelerates and becomes effectively undrained (no flow), dilatancy induced pore-pressure reductions quench the instability. Theystudy this, employing a simplified isothermal, membrane diffusion model and the Segall-Rice [1995] constitutive law for dilatancy. Numerical simulations exhibit either slow or fast (dynamic) slip depending on dilatancy and friction parameters, as well as effective normal stress. Stable slip is favored by low effective stress (high pore-pressure), consistent with seismically inferred vp/vs ratios in some slow-slip zones. They will extend this work by developing finite difference calculations of fluid transport that will be coupled to the friction/elasticity simulations. Slow slip modeling must ultimately consider thermal, as well as frictional, weakening. The researchers have shown that thermal pressurization becomes important at quasi-static slip speeds, before seismic radiation; the precise rates depend on permeability. This suggests that whether slip is fast or slow depends on whether or not dilatancy limits slip-rates below thermal pressurization limits. To investigate this requires coupling thermal and pore-fluid diffusion, with friction, elasticity, and dilatancy. For slip that becomes fast compared to the characteristic diffusion time across the actively shearing layer, they must explicitly consider the finite thickness of the shear zone in the computations. This work should shed significant light on the physics of slow slip, and the factors that ultimately control whether slip accelerates to inertially limited speeds characteristic of damaging earthquakes.