Investigating the seismic signatures of volcanic unrest and eruption: Spatiotemporal distribution and source origin of tiny long-period seismicity

世界各地的火山观测站利用地震活动来监测火山，并设定警戒级别以保护生命和财产，但并不完全了解地震信号是如何产生的。这项研究将更好地阐明火山动荡和喷发期间的震源过程，有可能提高火山灾害监测和减灾能力。长周期（LP）地震活动是火山地震活动的一种特殊类型，火山监测科学家经常使用它来预测和评估火山爆发并减轻灾害，但其来源仍然存在争议。LP事件的序列和更连续的火山震颤信号（统称为LP地震活动性）通常出现在喷发前的地震图上，提供即将爆发的警告，但不能保证。该项目将对一种有趣的、在很大程度上被忽视的火山地震活动的起源进行详细调查：伴随着常规LP事件的许多微小振幅LP子事件。LP子事件可能是监测火山的有用新信号。这项研究将导致改进多个火山的地震活动性的描述和目录，生成一个公共信息数据库，适合其他研究人员研究火山和地震活动。这些产品将引起地球物理学、地质学、地震学和火山学团体的兴趣。该项目启动了加州大学圣巴巴拉分校、加州大学圣地亚哥分校和美国地质勘探局之间的合作。它通过为研究生提供资金来支持加州大学圣巴巴拉分校的教育计划。火山-地震事件编目代码将向公众提供，将有助于火山地震学家进行研究和监测，该项目将利用新的计算密集型处理方法，这些方法是根据对南加州和夏威夷区域地震活动的研究而改编的。在世界各地的多座火山中，显然都记录到了微小的LP子事件，但它们的起源仍然是个谜。在2004 - 2008年圣海伦火山（MSH）爆发期间，密集的地震网络非常好地记录了数百万个微小的LP子事件，但没有进行编目或分析。这些LP子事件包含丰富的，未开发的信息，有可能更好地阐明产生火山地震活动的过程。这项研究将高精度地绘制数百万个微小LP子事件的时空分布和震源机制图，并确定它们与其他火山地震活动和喷发活动的关系。主要数据集来自MSH，但来自加利福尼亚州猛犸山、基拉韦厄火山、HI和其他火山的其他数据集将用于多个火山系统的比较分析和假设检验。该项目将使用匹配和定位模板匹配方法系统地检测和定位微小LP子事件，执行高精度重新定位和聚类分析，堆叠相似事件（多重）组，并执行全波形反演以揭示矩张量和单力等效点源。综合这些结果将有助于解决地震活动与岩浆、热液和固体挤出管道过程之间的关系。结果将解决以下问题：（1）重新定位的地震活动可能会聚集成紧凑的体积，或沿着断层，岩浆管道，或热液结构：这些结构揭示了什么关于潜在的火山地震过程？(2)震源机制和子事件的位置是否揭示了火山地下水系统中的树枝状裂缝丛，或管道边缘的固体挤出过程？(3)LP和子事件之间的关系是什么？(4)LP和子事件会互相触发吗？(5)MSH的这些现象是独特的，还是在其他火山上观察到的类似行为？(6)造成这种地震活动的潜在物理过程是什么？(7)子事件可以用来改善火山爆发预测吗？

Volcano observatories worldwide use seismicity to monitor volcanoes and set alert levels to protect life and property, but do not fully understand how the seismic signals are generated. This research will better elucidate seismic source processes during volcanic unrest and eruption, potentially improving capability in volcanic hazard monitoring and mitigation. Long-period (LP) seismicity, a particular type of volcanic seismicity, is used routinely by volcano monitoring scientists to forecast and assess eruptions and mitigate hazards, but its source origin remains controversial. Sequences of LP events and more continuous volcanic tremor signals (collectively called LP seismicity) commonly appear on seismograms before eruptions, providing a warning, but not a guarantee, of impending eruption. This project will perform detailed investigations into the origin of an intriguing and largely overlooked additional type of volcanic seismicity: numerous tiny-amplitude LP subevents that accompany the regular LP events. LP subevents could be a useful new signal for monitoring volcanoes. This research will lead to improved descriptions and catalogs of seismicity for multiple volcanoes, generating a public database of information suitable for other researchers in their studies of volcanoes and seismicity. These products will be of interest to the geophysics, geology, seismology, and volcanology communities. This project initiates collaboration between UC Santa Barbara, UC San Diego, and the USGS. It supports the educational program at UC Santa Barbara by providing funds for graduate student support. The volcano-seismic event cataloging code will be made publicly available and will be useful to volcano seismologists for research and monitoring.This project will utilize novel computationally intensive processing methods adapted from studying regional seismicity in Southern California and Hawaii. Tiny LP subevents have apparently been recorded at multiple volcanoes worldwide, but their origin remains mysterious. Millions of tiny LP subevents were exceptionally well recorded by a dense seismic network during the 2004-2008 eruption of Mount St. Helens (MSH), but were not cataloged or analyzed. These LP subevents contain rich, unexploited information that has the potential to better elucidate the processes generating volcanic seismicity. This research will map the spatiotemporal distribution and source mechanisms of millions of tiny LP subevents to high precision and determine their relation to other volcanic seismicity and eruptive activity. The primary dataset is from MSH, but additional datasets from Mammoth Mountain, CA, Kilauea Volcano, HI, and other volcanoes will be exploited for comparative analyses and hypothesis testing across multiple volcanic systems. This project will systematically detect and locate tiny LP subevents using a match-and-locate template matching approach, perform high-precision relocation and cluster analysis, stack groups of similar events (multiplets), and perform full-waveform inversion to reveal the moment-tensor and single-force equivalent point-sources. Integrating these results will help to resolve the relationships between seismicity and magmatic, hydrothermal, and solid extrusion conduit processes. The results will address the following questions: (1) Relocated seismicity will likely cluster into compact volumes, or align along fault, magma conduit, or hydrothermal structures: what do these structures reveal about the underlying volcano-seismic processes? (2) Do source mechanisms and locations of subevents reveal a dendritic plexus of cracks in the volcano groundwater system, or solid extrusion processes at the conduit margin? (3) What is the relationship between LPs and subevents? (4) Do LPs and subevents trigger each other? (5) Are these phenomena at MSH unique or is similar behavior observed at other volcanoes? (6) What is the underlying physical process causing this seismicity? (7) Can subevents be used to improve eruption forecasting?