Catalog-constrained models of tremor and slow slip

大约16年前，科学界开始意识到两种以前未被认识的断层滑动类型。 人们一直认为，深层断层要么大部分时间都是锁定的，只在短暂的地震中滑动，要么以板块构造的速度（厘米/年）沿着稳步前进。 从2000年左右开始，人们认识到，沿着俯冲带断层的深部延伸，断层并不是简单地从浅部锁定到更深处以板块速率蠕动。 相反，在这种中间状态下，断层周期性地加速滑动速度，可能是板块构造速度的100倍，但随后又减速，没有产生地震。 这些“间歇性缓慢滑动事件”的能量释放与6.5级地震相当，但持续数天至数周而不是数秒。 此外，这些慢地震（测地观测）伴随着地震仪上看到的新信号，称为“构造震颤”。 震颤由无数的“低频地震”组成，其能量释放相当于震级1？2.5地震，但持续时间约为10倍（零点几秒）。 对阶段性缓慢滑动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锁定部分上应力速率增加的时间。也有人提出，地震的上倾程度可能被用来评估未来大地震的下倾程度，这对西雅图的地面震动有重要意义。但是，我们需要更好地了解慢滑和震颤，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。 当幕式缓慢滑动首次被发现时，问题是断层如何能够自发地和周期性地加速，但随后减速而不产生地震。 现在，我们的目标是利用慢滑的观察来区分这种行为的竞争假设。 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目录，扩展对慢滑的观测约束（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）。 这项建议的目标是开发数字模型的慢滑，使用这些观测约束作为指导。 主要意见包括：（1）虽然主震阵面以5-10 km/天的速度沿断层沿着横向传播，但在主震阵面后面产生的次级地震阵面传播速度要快几十到几百倍。(2)这些二次迁移采取快速震颤反转的形式，所有这些都在主锋处开始并在相反方向上传播，或者独立于主锋方向沿着主锋传播的迁移。 一些最大的快速地震逆转开始作为迁移沿着主要战线。 没有一个迁移是从主锋开始的，并在长期传播方向上传播。(3)最靠近主锋的地震迁移很小，而且周期太短，不可能由潮汐驱动。随着这些间隔从几分钟逐渐增加到几小时，最终最大的快速震颤逆转会受到12.4小时潮汐的调制。 这项工作旨在重现这些观察使用模型的速率和状态相关的摩擦，包括相对均匀的模型和那些明确包括凹凸旨在模仿震颤源。 重点将放在理解分析为什么模拟做他们做什么，使他们的结果可以外推超出其特定假设的狭窄范围。

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.