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，如无地震断层滑动，如何导致地震破裂。许多观察结果支持这样的假设，即慢震可以先于危险的常规地震，但从慢震到快速破裂的过渡过程以前从未观察到过。该项目表明，在阿拉斯加中部的下地壳可以观察到地震成核过程，而且极低频率地震(VLFE‘s)是一种慢地震，可以直接过渡到规则地震。这项研究将在阿拉斯加中部的Minto Flats断裂带进行彻底的地震和地震成核信号的搜索，当地密集的地震台网正在运行，最终目标是将这些过程与常规地震联系起来。将这些过程归类到一个单一的构造环境中，将使我们能够对地震成核过程进行建模，而不需要从不同的构造环境中推断结果。该项目使用了在阿拉斯加中部明托平坦断裂带密集部署地震仪所产生的对地震过程的新的、高质量的观测，目的是描述缓慢地震的发生以及缓慢地震与观测到的地震成核信号之间的关系。对俯冲带外或板块边界环境外的VLFE的观测很少，因此在这个板内断裂系统上发现VLFE将扩大我们对哪里可能发生慢震的认识。迄今识别出的VLFE和成核信号发生在断裂带上规则地震活动的最深程度或以下，表明脆韧性转换在这里的成核地震中起着重要作用。为了表征缓慢的地震过程，研究人员使用基于相关性的技术来识别震群、低频地震、VLFE和前震序列。到目前为止，阿拉斯加中部的地震成核信号已经被偶然地识别出来，但这个项目将通过使用基于相关性的和地震处理技术来有力地编目更多的成核信号实例。这项工作将量化Minto Flats断裂带中的这些信号，并调查它们与常规地震的关系。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，因为一名参与观测成核信号的研究生还将与建模社区的领导人合作，对地震成核过程进行基于物理的数值模拟。该项目研究的地震虽然规模较小(M~4级)，但提供了一座通往较大地震的观测桥梁，据推测，较大地震之前存在缓慢的地震过程。

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.