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表明，许多断层通过一系列常规摩擦定律无法预测的现象积累滑动：通过缓慢的地震，或断层滑动事件，其平均滑动速率在0.1微米/秒到1毫米/秒之间，比地震的典型滑动速率1米/秒慢1000万到1000万倍。现在在大多数俯冲带发现了缓慢地震，在那里它们容纳了孕震区下倾区域中大约一半的板块界面滑动。但目前，我们还不知道是哪个断裂带过程产生了我们在慢震中观察到的地震滑动。提高我们对慢震的认识是很重要的，因为它们发生在发震带附近。它们能够引发大而破坏性的地震。在这个项目中，我们专注于最小但最丰富的缓慢地震：震颤。震颤由数亿次小的、间隔很近的缓慢地震组成。这些地震可以被快速观测到，并可用于跟踪更大规模的地震滑动变化，并评估这种滑动是否会引发危险的地震滑动。但就像其他缓慢的地震一样，人们对震颤的了解仍然很少。该项目的目标是确定产生震颤的物理过程，并将其滑动速率限制在1毫米/秒左右。人们对地震的低滑动率提出了几种解释。有可能震颤是由控制正常地震的相同摩擦滑动过程控制的。地震可能是缓慢的，只是因为断层的摩擦强度或正常应力较低，因此无法驱动快速滑动。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使断层闭合，从而抑制快速滑动。或者，震颤可能是一系列失败的地震成核的集合，这是由于名义上稳定的断层上的应力扰动引起的。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震颤产生模型。我们将使用来自世界上一些观测最好的震颤的高质量地震数据来测试特定的模型预测：帕克菲尔德附近，CA。为了测试我们的模型预测，我们将首先检查震颤与短地震和长地震的关系。如果地震是由控制更大的缓慢地震的相同的新断层带物理控制的，那么应该有一个连续的缓慢地震，其大小和滑动速率范围很广。连续体的存在与否对于限制大小慢震的过程是重要的，因为只有少数提出的大慢震模型与连续体的大范围滑动率相一致。我们将使用最近开发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类，以便(1)识别连续区内可能发生的更大的、持续数小时的缓慢地震；(2)约束地震与更大规模滑动之间的关系。最后，为了进一步验证模型，我们将进入单个震颤事件的细节，并探讨单个震颤地震的滑动演化。我们将仔细研究由地震产生的地震信号，以确定地震的持续时间、大小和复杂性如何随事件而变化。这些数据将让我们确定有多少震颤的性质是由特定的流变学控制的，有多少是由于局部断裂带结构。通过追求一套可以测试我们模型的特征，我们将能够确定哪些物理过程产生了构成震颤的众多小地震，这样我们就可以更好地理解缓慢的地震滑动，并更自信地使用震颤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.