Slow Earthquakes and Earthquake Nucleation in the Lower Crust of Central Alaska

地震成核，即断层从锁定到动态破裂的转变过程，由于观测有限，仍然没有得到很好的约束。了解这一过程可能有助于确定潜在的前兆地震过程，如地震断层滑动，如何导致地震破裂。许多观察结果支持这样的假设，即缓慢地震可能先于危险的常规地震，但从缓慢破裂到快速破裂的过渡过程以前没有观察到。该项目表明，在阿拉斯加中部下地壳可以观测到地震成核过程，极低频地震（VLFE’s）是一类慢震，可以直接过渡到常规地震。这项研究将对阿拉斯加中部Minto Flats断裂带的慢震和地震成核信号进行彻底的搜索，在那里有一个当地密集的地震网络正在运行，最终目标是编目这些过程与常规地震的关系。在一个单一的构造环境中对这些过程进行编目将使我们能够模拟地震成核过程，而不需要从不同的构造环境中推断结果。该项目利用在阿拉斯加中部Minto Flats断裂带密集部署的地震检波器对地震过程进行新的高质量观测，目的是描述慢震发生的特征以及慢震与观测到的地震成核信号之间的关系。对于俯冲带或板块边界环境以外的极低频地震的观测很少，因此在这个板内断裂系统上发现极低频地震将扩大我们对可能发生慢震的地方的认识。迄今为止发现的VLFE和成核信号发生在断层带规则地震活动的最深处或以下，表明脆性-韧性转变在这里的成核地震中起着重要作用。为了描述缓慢地震过程，研究人员采用了基于相关性的技术来识别地震群、低频地震、VLFE和前震序列。迄今为止，阿拉斯加中部的地震成核信号都是偶然发现的，但该项目将通过使用基于相关性和地震学处理技术，对成核信号的其他实例进行强有力的分类。这项工作将量化Minto Flats断裂带的这些信号，并研究它们与常规地震的关系。此外，该项目将启动地震观测学和地震建模界之间的合作关系，因为参与观测成核信号的研究生也将与建模界的领导者合作，对地震成核过程进行基于物理的数值模拟。本项目研究的地震虽然很小（M~4级），但为较大的地震提供了一个观测桥梁，这些地震被假设为在缓慢的地震过程之前发生。

Earthquake nucleation, the process by which a fault transitions from locked to dynamically rupturing, remains poorly constrained due to limited observations. Understanding this process may help in determining how potential precursory earthquake processes, such as aseismic fault slip, can lead up to earthquake rupture. A number of observations support the hypothesis that slow earthquakes can precede hazardous regular earthquakes, but the transition process from slow to fast rupture has not been observed before. This project shows that the earthquake nucleation process can be observed in the lower crust of central Alaska, and that very low frequency earthquakes (VLFE's), a class of slow earthquakes, can transition directly into regular earthquakes. This research will undertake a thorough search for slow earthquake and earthquake nucleation signals throughout the Minto Flats Fault Zone in central Alaska, where a local dense seismic network is operating, with the ultimate goal of cataloging how these processes are related to regular earthquakes. Cataloging these processes in a single tectonic environment will allow us to model the earthquake nucleation process without needing to extrapolate results from disparate tectonic environments. This project uses new, high quality observations of earthquake processes resulting from a dense deployment of seismometers in the Minto Flats Fault Zone of central Alaska, with the goal of characterizing slow earthquake occurrence and the relation between slow earthquakes and observed earthquake nucleation signals. Very few observations of VLFE's outside of subduction zone or plate boundary environments have been made, so the discovery of VLFE's on this intraplate fault system will expand our knowledge of where slow earthquakes are possible. VLFE's and nucleation signals identified to date occur at or below the deepest extent of regular seismicity along the fault zone, suggesting that the brittle-ductile transition plays an important role in nucleating earthquakes here. To characterize slow earthquake processes, the researchers employ correlation-based techniques to identify earthquake swarms, low frequency earthquakes, VLFE's, and foreshock sequences. Earthquake nucleation signals in central Alaska to date have been identified by chance, but this project will robustly catalog additional instances of nucleation signals by using correlation-based and seismological processing techniques. This work will quantify these signals in the Minto Flats Fault Zone and investigate their relation to regular earthquakes. In addition, this project will initiate a collaborative relationship between the observational seismology and the earthquake modeling communities, as a graduate student involved with observing nucleation signals will also work with leaders in the modeling community to conduct physics-based numerical modeling of the earthquake nucleation process. The earthquakes studied in this project, though small (at M~4), provide an observational bridge to larger earthquakes, which are hypothesized to have been preceded by slow earthquake processes.