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

构造震动的原因是什么？

基本信息

批准号：
NE/S00968X/1
负责人：
Jessica Hawthorne
金额：
$ 36.68万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2019
资助国家：
英国
起止时间：
2019 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FS00968X%2F1
关键词：
What causes tectonic tremor Investigating

项目摘要

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.

在过去的二十年里，不断改进的地震和大地测量数据显示，许多断层通过一系列传统摩擦定律无法预测的现象积累滑动：通过慢地震，或平均滑动速率在0.1微米/秒和1毫米/秒之间的断层滑动事件，比典型的地震滑动速率1米/秒慢1千到1千万倍。现在，在大多数俯冲带都能发现慢地震，在孕震带的下倾区域，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是什么断裂带过程导致了我们在慢地震中观察到的慢地震滑动。重要的是提高我们对慢地震的认识，因为它们发生在孕震区附近。它们能够引发大规模的破坏性地震。在这个项目中，我们专注于最小但最丰富的慢地震：震颤。震颤是由成百上千万个小的、间隔很近的、缓慢的地震组成的。这些地震可以被迅速观测到，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。但与其他慢地震一样，人们对震颤的了解仍然很少。该项目的目标是确定哪种物理过程产生震颤并将其滑动率限制在1 mm/s左右。已经提出了几种震颤低滑动率的解释。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。震动之所以缓慢，只是因为断层的摩擦强度或法向应力较低，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关闭，从而抑制快速滑动。或者，震颤可能是一个失败的地震成核的集合，这是由于在名义上稳定的断层上的应力扰动而引起的。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个提议的震颤生成模型。我们将使用来自世界上观测最好的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。为了测试我们的模型预测，我们将首先研究震颤如何与较短和较长的慢地震相关。如果震颤是由相同的新的断层带物理控制的，控制较大的慢地震，那么应该有一个连续的慢地震，具有广泛的大小和滑动率。连续体的存在或不存在对于约束控制大型和小型慢地震的过程将是重要的，因为只有少数提出的大型慢地震模型与连续体的广泛滑动速率一致。我们将搜索0.05至1秒长的事件，在这个连续使用最近开发的地震分析技术。我们将研究震颤的聚集性，以便（1）在连续介质中识别可能的更大的、长达数小时的慢地震，（2）限制震颤和更大规模的滑动之间的关系。最后，为了进一步测试模型，我们将进入单个震颤事件的细节，并探索单个震颤地震中滑动的演变。我们将仔细研究由震颤产生的地震信号，以确定震颤的地震的持续时间，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属性是由特定的流变学控制的，有多少是由于当地的断层带结构。通过追求一套可以测试我们的模型的功能，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，以便我们更好地了解缓慢地震滑动，并更有把握地利用震颤来跟踪深度上的大规模滑动。