Catalog-constrained models of tremor and slow slip

颤动和慢滑移的目录约束模型

基本信息

批准号：
1645145
负责人：
Allan Rubin
金额：
$ 33.28万
依托单位：
Princeton University
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-03-15 至 2022-02-28
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1645145&HistoricalAwards=false
关键词：
Catalog constrained models tremor slow

项目摘要

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.

大约16年前，科学界意识到了两种以前未被认可的断层滑移风格。人们认为，深度的故障要么大部分时间都被锁定，只有短暂的地震滑倒，要么以板块构造速率（每年厘米）稳步爬行。从2000年左右开始，人们认识到，沿俯冲带的故障深度延伸，断层并不简单地从锁定在浅深度到板速度以更大的深度爬行。取而代之的是，在这种中间状态下，断层周期性加速到板块构造速率的100倍，但随后又放慢了速度，而不会产生地震。这些“情节慢速事件”在能量释放中与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（在大地上观察）伴随着在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，在能量释放中对应于1？2.5地震，但持续的时间约为10倍（十分之一一秒钟）。发作性慢速滑动和震颤的研究与地震危害有关，因为它们代表了俯冲区断层的浅层，锁定部分的压力率增加的时间，能够产生9级地震。还提出，震颤的上倾角可能用于评估未来大地震的下垂程度，这对西雅图的地面摇晃具有重要意义。但是，我们需要更好地理解慢速滑移和震颤，然后才能将这些想法转化为有关地震风险的陈述。拟议的研究旨在提高我们对这些断层滑移风格的理解。当首次发现情节慢滑滑时，问题是错误会在自发和定期加速的情况下如何发生故障，但随后在不产生地震的情况下减速。现在，目的是使用慢滑的观察结果来区分这种行为的竞争假设。在过去的几年中，研究人员一直在改善震颤检测/位置算法，目的是生成高分辨率的目录，以扩大慢速滑移的观察性约束（在这方面震颤是有用的（因为它可以比潜在的慢滑动更准确）。该提案的目的是开发使用这些观察性约束作为指南的慢滑的数值模型。主要观察结果包括：（1）尽管主要震颤前部以5-10 km/天的速度沿侧向传播，但次级震颤前线出现在主阵线后面，将几十次传播到数百次。（2）这些次级迁移采用快速震颤逆转的形式，所有这些逆转都始于主要前部，并向相反的方向传播，或者沿着主要前部传播的迁移独立于主要前端方向。一些最大的快速震颤逆转始于沿着主要战线的迁移。没有一个迁移始于主要前沿，并沿长期传播方向传播。（3）最接近主阵线的，震颤迁移很小，在时间尺度上反复出现，太短了，无法潮湿驱动。随着这些间隔逐渐增加到几分钟，最终最大的快速震颤逆转变得由12.4小时的潮汐调节。这项工作旨在使用依赖速率和州依赖性摩擦的模型来复制这些观察结果，包括相对均匀的模型以及明确包含旨在模仿震颤来源的浅色的模型。将重点放在分析上理解为什么模拟做他们所做的事情的原因，以便可以将其结果推断到其特定假设的狭窄范围之外。