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

地震成核是断层从闭锁到动态破裂的过程，由于观测有限，对它的约束仍然很差。了解这一过程可能有助于确定潜在的地震过程，如地震断层滑动，如何导致地震破裂。许多观测结果支持慢地震可能先于危险的规则地震的假设，但以前没有观察到从慢到快破裂的过渡过程。该项目表明，在阿拉斯加中部的下地壳中可以观察到地震成核过程，并且甚低频地震（VLFE），一类慢地震，可以直接转变为常规地震。这项研究将对阿拉斯加中部Minto Flats断层带的慢地震和地震成核信号进行彻底搜索，当地密集的地震网络正在运作，最终目标是编目这些过程与常规地震的关系。在单一构造环境中对这些过程进行分类将使我们能够模拟地震成核过程，而无需从不同的构造环境中推断结果。该项目使用新的，高质量的地震过程观测结果，这些观测结果来自阿拉斯加中部Minto Flats断层带密集部署的地震仪，目的是描述慢地震发生的特征以及慢地震与观测到的地震成核信号之间的关系。在俯冲带或板块边界环境之外，很少有VLFE的观测，因此在这个板内断层系统上发现VLFE将扩大我们对慢地震可能发生的地方的了解。VLFE的和成核信号发生在或低于最深程度的定期地震活动沿着断裂带，这表明脆-韧性转变起着重要的作用，在这里成核地震。为了表征慢地震过程，研究人员采用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是偶然发现的，但该项目将通过使用基于相关性的地震处理技术，对成核信号的其他实例进行有力的编目。这项工作将量化这些信号在明托平原断层带，并调查他们的关系，定期地震。此外，该项目将启动观测地震学和地震建模社区之间的合作关系，因为参与观测成核信号的研究生也将与建模社区的领导者合作，对地震成核过程进行基于物理的数值建模。本项目所研究的地震虽然较小（M~4），但提供了一个观测更大地震的桥梁，假设这些地震之前有缓慢的地震过程。

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.