Catalog-constrained models of tremor and slow slip

大约16年前，科学界意识到了两种以前未被认识到的断层滑动类型。人们曾认为，深部的断层要么大部分时间处于锁定状态，只在短暂的地震中滑动，要么以板块构造速率(每年厘米)稳步向前爬行。从2000年左右开始，人们认识到，沿着俯冲带中断层的深部延伸，断层并不是简单地从浅层被锁定到以板块速度在更深处爬行。相反，在这种中间状态下，断层周期性地加速滑动，速度可能是板块构造速率的100倍，但随后再次减速，没有发生地震。这些“幕式慢滑事件”在能量释放方面与6.5级地震相当，但持续了几天到几周，而不是几秒钟。此外，这些缓慢的地震(从大地测量观测到的)伴随着在地震仪上看到的一种新信号，称为“构造震动”。地震由无数次“低频地震”组成，这些地震的能量释放相当于1？2.5级地震，但持续时间大约是地震持续时间的10倍(十分之几秒)。对幕式慢滑和震颤的研究与地震灾害有关，因为它们代表了能够产生9级地震的俯冲带断层的浅层、锁定部分的应力速率增加的次数。也有人提出，地震的上升幅度可能被用来评估未来大地震的下降幅度，这对西雅图的地面震动具有重要意义。但我们需要更好地理解慢滑和震颤，然后才能自信地将这些想法转化为关于地震风险的陈述。拟议的研究旨在提高我们对这些类型的断层滑动的理解。当幕式缓慢滑动第一次被发现时，问题是断层如何自发地周期性地加速，然后减速而不产生地震。现在，我们的目标是利用对慢滑的观察来区分关于这一行为的相互竞争的假说。在过去的几年里，研究人员一直在改进震颤检测/定位算法，目的是生成高分辨率的星表，以扩大对慢滑的观测限制(震颤在这方面很有用，因为它可以比潜在的慢滑更准确地定位)。这项提议的目标是开发使用这些观测约束作为指导的慢滑数值模型。主要观测结果包括：(1)虽然主震锋沿断层以5-10公里/天的速度横向传播，但次生震锋出现在主锋后面，传播速度快数十倍至数百倍。(2)这些次生迁移具有快速震荡逆转的形式，它们都是从主锋开始并以相反的方向传播的，或者是沿主锋传播的，与主锋方向无关。一些最大的快速震动逆转开始于沿主锋的迁移。没有一种迁移是从主锋开始并沿长期传播方向传播的。(3)最靠近主锋的震颤迁移规模很小，而且重复的时间尺度太短，不能被潮汐驱动。随着这些间隔从几分钟逐渐增加到几个小时，最终最大的快速震动逆转被12.4小时的潮汐所调制。这项工作试图使用与速率和状态相关的摩擦模型来再现这些观测结果，包括相对均匀的模型和那些明确包括旨在模拟震动源的凹凸体的模型。重点将放在分析上理解为什么模拟会这样做，这样他们的结果就可以被外推到他们特定假设的狭隘范围之外。

About 16 years ago the scientific community became aware of two previously unrecognized styles of fault slip. It had been thought that faults at depth either spent most of their time locked, slipping only in brief earthquakes, or else crept along steadily at the plate tectonic rate (centimeters per year). Starting around 2000 it was recognized that along the deep extension of faults in subduction zones, the fault did not simply progress from being locked at shallow depth to creeping at the plate rate at greater depth. Instead, in this intermediate regime the fault periodically accelerated to slip speeds perhaps 100 times the plate tectonic rate, but then slowed down again without producing earthquakes. These "episodic slow slip events" were comparable in energy release to magnitude 6.5 earthquakes, but lasted days to weeks rather than several seconds. In addition, these slow earthquakes (observed geodetically) were accompanied by a new signal seen on seismometers, termed "tectonic tremor". Tremor consists of myriad "low frequency earthquakes" that correspond in energy release to magnitude 1?2.5 earthquakes, but that last roughly 10 times as long (a few tenths of a second). Studies of episodic slow slip and tremor are relevant to earthquake hazards because they represent times of increased stressing rate on the shallow, locked portion of subduction zone faults capable of producing magnitude 9 earthquakes. It has also been proposed that the up-dip extent of tremor might be used to assess the down-dip extent of future great earthquakes, with important implications for ground shaking in Seattle. But we need to understand both slow slip and tremor better before we can confidently translate such ideas into statements about seismic risk. The proposed study seeks to improve our understanding of these styles of fault slip. When episodic slow slip was first discovered, the question was how faults could spontaneously and periodically accelerate, but then decelerate without producing an earthquake. Now, the goal is to use observations of slow slip to distinguish between competing hypotheses for this behavior. For the past several years the researchers have been improving tremor detection/location algorithms, with the aim of generating high-resolution catalogs that expand the observational constraints on slow slip (tremor is useful in this regard because it can be located more accurately than the underlying slow slip). The goal of this proposal is to develop numerical models of slow slip that use these observational constraints as guides. Key observations include: (1) Although the main tremor front propagates laterally along the fault at speeds of 5-10 km/day, secondary tremor fronts arise behind the main front that propagate tens to hundreds of times faster. (2) These secondary migrations take the form of rapid tremor reversals, all of which start at the main front and propagate in the opposite direction, or migrations that propagate along the main front independent of the main front orientation. Some of the largest rapid tremor reversals begin as migrations along the main front. None of the migrations start at the main front and propagate in the long-term propagation direction. (3) Closest to the main front, the tremor migrations are small and recur on timescales far too short to be tidally driven. As these intervals gradually increase from minutes to hours, eventually the largest rapid tremor reversals become modulated by the 12.4-hour tides. This work seeks to reproduce these observations using models of rate- and state-dependent friction, including both relatively homogeneous models and those that explicitly include asperities intended to mimic tremor sources. Emphasis will be placed on understanding analytically why the simulations do what they do, so that their results can be extrapolated beyond the narrow confines of their specific assumptions.