What causes tectonic tremor? Investigating tremor's origins and implications with seismology

在过去的二十年里，不断改进的地震和大地测量数据揭示了许多断层通过一系列常规摩擦定律没有预测到的现象来积累滑动：通过慢地震，或者平均滑动速率在0.1微米/S到1毫米/S之间的断层滑动事件，比地震典型的1m/S滑动速率慢1000到1000万倍。现在在大多数俯冲带都发现了慢地震，在孕震带向下倾斜的地区，它们容纳了大约一半的板块界面滑动。但目前，我们还不知道哪些断裂带过程产生了我们在慢震中观察到的无震滑动。提高我们对慢震的理解是很重要的，因为它们发生在孕震带附近。它们能够引发具有破坏性的大地震。在这个项目中，我们关注的是最小但最丰富的慢地震：震颤。地震是由成百上千万个小的、密集的、缓慢的地震组成的。地震可以被快速观测到，可以用来跟踪更大范围的非地震滑动变化，并评估这种滑动是否会引发危险的地震滑动。但与其他慢震一样，人们对震颤的了解仍然很少。这个项目的目标是确定是什么物理过程造成了地震，并将其滑移率限制在1毫米/秒左右。对于地震的低滑移率，人们提出了几种解释。震颤可能与正常地震受相同的摩擦滑动过程支配。地震之所以缓慢，可能只是因为断层的摩擦强度或正应力较低，因此不能驱动快速滑动。或者，一种更新颖的物理过程可以限制震颤的滑动速度。孔隙流体压力的变化可能会关闭断层，抑制快速滑动。地震可能是由于名义上稳定的断层上的应力扰动而产生的失败地震形核的集合。在拟议的工作中，我们将使用有针对性的地震学分析来评估五种拟议的地震产生模型。我们将使用世界上观测得最好的一些地震的高质量地震数据来测试特定的模型预测：加利福尼亚州帕克菲尔德附近的地震。为了测试我们的模型预测，我们将首先检查地震与较短和较长的缓慢地震之间的关系。如果震颤与较大的慢震受相同的新断裂带物理学的支配，那么应该有一个具有广泛大小和滑动速率的慢地震的连续体。连续体的存在或不存在对于约束大小慢震的过程将是重要的，因为只有几个提出的大慢震模型与连续体的大范围滑移率一致。我们将使用最近发展起来的地震分析技术，在这个连续体中寻找0.05到1秒长的事件。我们将研究地震的聚集性，以(1)识别可能在连续体内发生的更大的、长达数小时的缓慢地震，以及(2)限制地震和更大范围滑动之间的关系。最后，为了进一步测试模型，我们将进入单个地震事件的细节，并探索单个地震中滑动的演化。我们将仔细研究地震产生的地震信号，以确定地震的持续时间、大小和复杂性在不同事件中的变化。这些数据将让我们确定地震的性质在多大程度上是由特定的流变学控制的，有多少是由于局部断裂带结构。通过追求一系列可以测试我们的模型的特征，我们将能够确定哪些物理过程产生了构成地震的众多小地震，以便我们可以更好地了解地震的缓慢滑动，并更有信心地利用地震来跟踪深度的大规模滑动。

Over the past two decades, improving seismic and geodetic data have revealed that many faults accumulate their slip via a suite of phenomena that are not predicted by conventional friction laws: via slow earthquakes, or fault slip events whose average slip rates are between 0.1 microns/s and 1 mm/s, a factor of 1 thousand to 10 million slower than the 1 m/s slip rates typical of earthquakes. Slow earthquakes are now found at most subduction zones, where they accommodate about half of the plate interface slip in the region down-dip of the seismogenic zone. But currently, we do not know which fault zone processes generate the aseismic slip we observe in slow earthquakes.It is important to improve our understanding of slow earthquakes because they occur next to the seismogenic zone. They are capable of triggering large and damaging earthquakes. In this project, we focus on the smallest but most abundant slow earthquakes: tremor. Tremor consists of hundreds to millions of small, closely spaced, slow earthquakes. The earthquakes can be rapidly observed and could be used to track larger-scale aseismic slip variations and to assess whether that slip could trigger hazardous seismic slip. But like other slow earthquakes, tremor remains poorly understood. The goal of this project is to determine which physical process creates tremor and limits its slip rates to around 1 mm/s.Several explanations of tremor's low slip rates have been proposed. It is possible that tremor is governed by the same frictional sliding process that governs normal earthquakes. Tremor may be slow only because the fault's frictional strength or normal stress is low, and thus is unable to drive rapid slip. Alternatively, a more novel physical process could limit tremor's slip speeds. Changes in pore fluid pressure might pull the fault shut, inhibiting rapid slip. Or tremor could be a collection of failed earthquake nucleations, which arise because of stress perturbations on a nominally stable fault.In the proposed work, we will use targeted seismological analysis to assess five proposed models of tremor generation. We will test specific model predictions using high-quality seismic data from some of the best-observed tremor in the world: that near Parkfield, CA. To test our model predictions, we will first examine how tremor is related to shorter and longer slow earthquakes. If tremor is governed by the same novel fault zone physics that governs larger slow earthquakes, there should be a continuum of slow earthquakes with a wide range of sizes and slip rates. The presence or absence of the continuum will be important for constraining the processes governing large and small slow earthquakes, as only a few of the proposed models of large slow earthquakes are consistent with the continuum's wide-ranging slip rates. We will search for 0.05 to 1-second-long events in this continuum using recently developed seismic analysis techniques. And we will examine the clustering of tremor, in order to (1) identify larger, hours-long slow earthquakes potentially within the continuum and (2) to constrain the relationship between tremor and larger-scale slip.Finally, to further test the models, we will move into the details of individual tremor events and probe the evolution of slip in individual tremor earthquakes. We will closely examine the seismic signals produced by tremor in order to determine how tremor's earthquakes' durations, sizes, and complexities vary from event to event. These data will let us determine how much of tremor's properties are controlled by particular rheologies and how much is due to local fault zone structure.By pursuing a suite of features that can test our models, we will be able to determine which physical processes generate the numerous small earthquakes that constitute tremor, so that we may better understand slow earthquake slip and more confidently use tremor to track large-scale slip at depth.