Using small events to constrain the physical mechanism governing slow slip

在传统模型中，断层可以以两种方式之一滑动：以接近板块速度的稳定速度滑动，或者在地震中偶尔滑动。然而，在过去的二十年里，研究人员发现了另一种行为：慢滑事件。在慢滑动事件中，板块界面滑动的大部分周期性加速，但在达到地震滑动率之前很久就停止了。令人困惑的是，究竟是什么物理机制导致了缓慢而断断续续的滑动？了解间歇性的慢滑是很重要的，因为慢滑事件发生在锁定带的正下方，这是危险的大逆冲地震的发生地。对慢滑的约束可以帮助我们约束震区是如何被加载到破坏的。慢滑甚至可以帮助我们理解大地震是如何开始的，因为控制慢滑开始的物理原理很可能与控制大地震开始的物理原理相同。缓滑也很重要，因为它是丰富的。在大多数仪器完备的俯冲带上观测到M bbb6事件，在深度上容纳了大部分板块界面滑动。如果我们要了解地震周期和俯冲地震的应力状态，我们需要了解慢滑。已经提出了许多物理机制来解释慢滑移。其中一些模型通常考虑到断层流变学的复杂性或断层性质的空间变化，从而再现了缓慢而偶然的滑动。然而，这些模型似乎不能再现所观测到的慢滑事件的所有性质。具体来说，这些模型被设计成缓慢且相对稳定的，因此它们很难再现在慢滑事件中日益观察到的复杂性。因此，我们似乎需要一个新的或修改的模型。挑战在于，有许多看似合理的模型需要考虑。因此，在这个项目中，我建议评估几个模型，并确定几个模型属性，这些属性对于再现观察到的慢滑至关重要。为了更好地评估这些模型，我建议将它们与现有的大型慢滑事件观测结果以及对卡斯卡迪亚中部更多小型慢滑事件的新分析数据进行比较。卡斯卡迪亚每年发生几十次小的慢滑事件。它们伴随着地震震颤：低振幅、持续的地震振动，被认为是由缓慢滑动事件驱动的无数微小地震组成的。我们将跟踪地震的位置，这样我们就可以看到小的慢滑事件是如何在太空中发展的。此外，我们将利用卡斯卡迪亚独特的高精度PBO钻孔应变网估计小事件的地震滑动震级。我们将使用这些新的分析来评估几个慢滑数值模型，并约束模型的基本特征。

In conventional models, faults can slip in one of two ways: steadily at rates near plate rate or episodically in earthquakes. However, in the past two decades researchers have discovered another behaviour: slow slip events. In slow slip events, large portions of the plate interface slip accelerate periodically but then stall long before reaching seismic slip rates. The puzzle is why: what physical mechanism allows for slow yet episodic slip?Episodic slow slip is important to understand because slow slip events occur just below the locked zone, which hosts hazardous megathrust earthquakes. Constraints on slow slip can help us constrain how the earthquake-generating region is loaded to failure. Slow slip may even help us understand how large earthquakes start, as the physics governing the beginning of slow slip is likely the same physics that governs the beginning of large earthquakes.Slow slip is also important because it is abundant. M>6 events have been observed on most well-instrumented subduction zones, accommodating large fractions of the plate interface slip at depth. If we are to understand the seismic cycle and the stress state at subduction ones, we need to understand slow slip.A number of physical mechanisms have been proposed to explain slow slip. And a number of these models have reproduced the slow yet episodic slip, usually by allowing for complications in the fault rheology or spatial variations in the fault properties. However, these models have not appeared capable of reproducing all the properties of observed slow slip events. Specifically, the models are designed to be slow and relatively stable, so they struggle to reproduce the complexity that is increasingly observed within a slow slip event. It thus seems that we need a new or modified model. The challenge is that there are many plausible models to consider. So in this project, I propose to evaluate several models and to identify several model properties that are essential to reproducing observed slow slip. To better evaluate these models, I propose to compare them with existing observations of large slow slip events as well as new analysis of data from the more numerous small slow slip events in central Cascadia. Several tens of small slow slip events occur annually in Cascadia. They are accompanied by seismic tremor: low-amplitude, sustained seismic vibrations thought to be composed of numerous tiny earthquakes driven by the slow slip event. We will track the tremor locations, so that we can see how the small slow slip events grow in space. In addition, we will estimate the magnitude of aseismic slip in the small events, using the unique network of high-precision PBO borehole strainmeters in Cascadia. We will use these new analyses to evaluate several numerical models of slow slip and constrain essential model characteristics.