Dilatant Stabilization as a Mechanism for Slow Slip Events

在俯冲带中发现了“慢滑事件”，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。这些滑动事件以类似于地震的方式移动了地球表面，然而，它们不是持续几秒到几分钟，而是持续几天到几周，甚至几年。因为它们发生得如此缓慢，慢滑事件不会辐射出破坏性的地震波。它们似乎位于产生8至9级地震的巨型逆冲断层之下。卡斯卡迪亚的慢滑事件每隔10到16个月就会相当规律地发生一次，并逐渐增加锁定断裂带的应力。慢滑事件似乎很可能发生在锁定断层和稳定蠕动断层之间的摩擦过渡带。因此，了解慢滑的发生可能有助于改进对破坏性俯冲带地震的预测。人们对慢滑的物理学仍知之甚少。研究人员认为，速率状态摩擦在排水(恒定孔压)条件下形成滑移，但随着滑移的加速并变得有效地不排水(无流动)，膨胀引起的孔压降低消除了不稳定性。他们利用简化的等温膜扩散模型和Segall-莱斯[1995]膨胀本构定律对此进行了研究。数值模拟显示，慢速或快速(动态)滑移取决于扩容和摩擦参数，以及有效法向应力。低有效应力(高孔压)有利于稳定滑动，这与地震推断的某些慢滑带的Vp/VS值一致。他们将通过开发流体传输的有限差分计算来扩展这项工作，这些计算将与摩擦/弹性模拟相耦合。慢滑模型最终必须考虑热力和摩擦力的减弱。研究人员已经证明，在地震辐射之前的准静态滑动速度下，热加压变得重要；精确的加压速度取决于渗透率。这表明滑移是快是慢取决于扩容是否将滑移率限制在热压极限以下。要研究这一点，需要耦合热扩散和孔隙流体扩散，并考虑摩擦、弹性和扩容。对于滑移变得比跨主动剪切层的特征扩散时间更快的情况，他们必须在计算中明确考虑剪切带的有限厚度。这项工作应该会对慢滑的物理学以及最终控制滑移是否加速到具有破坏性地震特征的惯性限制速度的因素有重要的帮助。

"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.