In 2017, Mount Agung produced a small (VEI 2) eruption that was preceded by an energetic volcano-tectonic (VT) swarm (>800 earthquakes per day up to M4.9) and two months of declining activity. The period of decreased seismic activity complicated forecasting efforts for scientists monitoring the volcano. We examine the time history of earthquake families at Mount Agung in search of additional insight into the temporal changes in the shallow crust prior to eruption. Specifically, we analyze the period of declining seismic activity ~5 weeks prior to the eruption when forecasting uncertainty was greatest. We use REDPy (Hotovec and Jeffries, 2016) to build a catalog of 6,508 earthquakes from 18 October 2017–15 February 2018 and group them into families with a cross-correlation coefficient of 0.8. We show that the evolution of earthquake families illustrates that Mount Agung was progressing toward eruption even though overall earthquake rates and seismic-energy-release declined. We find that earthquakes families that dominated seismicity during the beginning of the crisis ceased near the onset of tremor on 12 November 2017. Then, earthquake families took on characteristics commonly observed during effusive phases of eruptions on 15 November — a full six days before the first phreatomagmatic eruption on 21 November 2017. We interpret the transitions in seismicity as the manifestation of a three-phase physical model including an Intrusion Phase, a Transition Phase, and a Magmatic Phase. We conclude that the time history of earthquake families provides insight into the evolution of the stress distribution in the volcanic edifice, the development of the volcanic conduit, and seismogenesis of magma effusion. Our work suggests eruption forecasts can be improved by incorporating automatic processing codes to assist seismologists during sustained periods of high earthquake rates, even at sparsely monitored volcanoes.

中文翻译：

---

2017年印度尼西亚巴厘岛阿贡山动荡和爆发的多个阶段重复发生地震

2017年，阿贡火山（Mount Agung）发生了一次小规模（VEI 2）喷发，随后爆发了一次充满活力的火山-构造（VT）群（每天发生超过800次地震，直到M4.9级），并且活动减少了两个月。地震活动减少的时期使监测火山的科学家的预测工作变得复杂。我们研究了阿贡山地震家庭的时间历程，以期对爆发前浅层地壳的时空变化有更多的了解。具体来说，我们分析了在火山爆发前5周（预测不确定性最大时）地震活动下降的时期。我们使用REDPy（Hotovec and Jeffries，2016）建立了一个目录，记录了2017年10月18日至2018年2月15日的6,508次地震，并将它们归类为互相关系数为0.8的家庭。我们表明，地震家庭的演变表明，即使总体地震发生率和地震能量释放下降，阿贡山也正在向喷发发展。我们发现，在危机开始时主导地震活动的地震家庭在2017年11月12日震颤开始附近就停止了。然后，地震家庭呈现了11月15日喷发阶段通常观察到的特征，即第一次地震爆发前整整六天地震爆发于2017年11月21日。我们将地震活动的过渡解释为三相物理模型的表现，包括侵入相，过渡相和岩浆相。我们得出的结论是，地震家庭的时间历史可以洞悉火山大厦中应力分布的演变，火山管道的发展，以及岩浆积水的地震发生。我们的工作表明，可以通过结合自动处理代码来改善喷发预报，即使在稀疏监测的火山中，自动处理代码也可以在地震频发的持续期间为地震学家提供帮助。