Investigation of source spectral models and scaling for slow slip processes in subduction zones

该奖项是根据2009年美国复苏和再投资法(公法111-5)资助的。最近，在全球众多俯冲带发现了一种耦合变形和弱地震辐射的新模式。缓慢滑动和弱地震以准规则的间隔从巨型逆冲界面的锁定部分周期性地向下倾斜，被称为幕式震颤和滑动(ETS)。ETS可能与俯冲带锁定和危险的部分相互作用，这要求更好地理解这一有趣的现象。研究人员将开发适当的模型来关联这些慢滑过程的不同大小和持续时间。这些模型将有助于比较这些过程的不同实例，以及比较不同频率范围内的现有数据。这是了解慢滑过程的物理学与更好地理解的地震物理学的不同之处的途径。更广泛的影响包括进一步教育一名或多名研究生，可能还有本科生。这项工作将阐明一种新的断层滑动模式，并可能对地震学、大地测量学、摩擦学和岩石力学的研究产生革命性的影响。更好地了解俯冲带中较小和较大的慢滑事件之间的关系，可能有助于预测那里发生的大地震，从而可能与地震危险性直接相关。此外，间歇性地震和滑动事件提供了一个宝贵的机会，以提高公众对区域地震危险的认识。目前，我们对ETS源过程的本质和基本物理缺乏基本的了解。我们也不了解最近观察到的越来越多的慢滑过程之间的关系，其中包括微小的低频地震，稍大的甚低频率地震，慢滑事件，幕式震动和滑动，以及大的无声地震。这些慢滑过程的特点是开始突然，幅度小，持续时间长。与类似时刻的常规地震相比，它们的频谱耗尽了高频能量。地震学家理解规则地震的一种方法是通过震源频谱模型，如欧米伽平方模型。对于常规地震，震源谱模型代表了我们对地震的理解，即地震是一个尺度不变的过程，在非常大的地震矩范围内是自相似的。对于慢滑现象，缺乏这样一个统一的模型。我建议为慢滑过程发展适当的震源尺度模型，并与不同频率范围的现有数据进行比较，以限制慢滑事件源谱的可能模型。建立这样的模型包括确定频谱斜率、拐角频率、应力降和传播速度之间的关系和约束。从俯冲带，特别是卡斯卡迪亚和日本，对慢滑过程的大量观测使这一提议变得及时。一个合适的光谱模型将有助于比较来自不同俯冲带、频带和ETS事件震级的数据。在某些情况下，这些数据应该根据绝对力矩速率谱进行比较。在其他情况下，只能进行相对比较，但这仍可能对潜在的源光谱模型施加强烈的限制。虽然常规地震具有大致恒定的破裂速度，这支持了它们的自相似性质，但对于慢滑移事件，有证据表明破裂传播速度与尺寸有关。类似地，尽管在很大范围的常规地震中应力降是恒定的，但对于缓慢滑动过程，应力降可能与大小有关。约束有效的源谱模型将揭示应力降、破裂速度和动量等重要物理性质之间的关系。所提出的源谱模型的构建和与各种类型的数据的比较将解决许多关于慢滑移过程的关键问题，并进一步阐明ETS过程的物理。了解慢滑过程是否以及如何随规模扩大将代表着一项根本性的进步，因此具有真正变革的潜力。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)Recently, a new mode of coupled deformation and weak seismic radiation has been discovered in numerous subduction zones globally. Slow slip and weak tremor occur episodically down-dip from the locked portion of the megathrust interface at quasi-regular intervals and have been termed Episodic Tremor and Slip (ETS). The likely interaction of ETS with the locked and hazardous portions of subduction zones mandate better understanding of this intriguing phenomenon.The researcher will develop appropriate models to relate the different sizes and durations of these slow slip processes. The models will facilitate comparison of different instances of these processes, as well as comparisons of available data at various frequency ranges. This is an avenue to understanding how the physics of slow slip processes differs from the better-understood physics of earthquakes.Broader impacts include furthering the education of one or more graduate students, and possibly undergraduates. This work will elucidate a new mode of fault slip, and could have a transformative impact on studies in seismology, geodesy, friction and rock mechanics. A better understanding of the relationship between smaller and larger slow slip events in subduction zones may help forecast the occurrence of large earthquakes there, and thus could be directly relevant to seismic hazard. As well, episodic tremor and slip events provide a valuable opportunity to advance public awareness of the regional seismic hazard. At present we lack a fundamental understanding of the nature and basic physics of the ETS source process. Nor do we understand the relationship between the growing menagerie of recent observations of slow slip processes, which include tiny low frequency earthquakes, somewhat larger very low frequency earthquakes, slow slip events, episodic tremor and slip, and large silent earthquakes. These slow slip processes are characterized by emergent beginnings, weak amplitudes, and long durations. Their spectra are depleted in high-frequency energy compared with regular earthquakes of similar moment.One way in which seismologists understand regular earthquakes is through models of the source spectrum such as the omega-squared model. For regular earthquakes, a source spectral model represents our understanding of earthquakes as a scale-invariant process that is self-similar over a very large range of seismic moments. Such a unifying model is lacking for slow slip phenomena. I propose to develop appropriate source scaling models for slow slip processes and compare with available data at various frequency ranges to constrain possible models of the source spectrum of slow slip events. Constructing such models involves determining relations among and constraints on spectral slope, corner frequency, stress drop, and propagation velocity.Burgeoning numbers of observations of slow slip processes from subduction zones, particularly Cascadia and Japan render this a timely proposal. An appropriate spectral model will facilitate comparison of data from different subduction zones, frequency bands, and magnitudes of ETS events. In some cases these data should be compared in terms of absolute moment rate spectra. In other cases only a relative comparison can be made, but that may still place strong constraints on potential source spectral models. Whereas regular earthquakes have a roughly constant rupture velocity underpinning their self-similar nature, for slow slip events there is evidence that rupture propagation velocity is size-dependent. Similarly, stress drop, although constant over a very large range of sizes of regular earthquakes, may be size-dependent for slow slip processes. Constraining valid source spectral models will reveal relationships between important physical properties such as stress drop, rupture velocity, and moment.The proposed source spectral model construction and comparison with various types of data will address many key questions about slow slip processes and further elucidate the physics of the ETS process. Understanding whether and how slow slip processes scale with size would represent a fundamental advance, and as such has the potential to be truly transformative.