Development of a new method for determining the proximal cause of cryptovolcanic earthquake swarms

集中在中地壳深处的强烈微震群几乎是火山爆发的普遍前兆。然而，这样的地震群并不总是以火山爆发为高潮，这些非喷发性地震群与岩浆上升的关系往往是高度模糊的，这对负责评估群事件所指示的喷发活动可能性的科学家构成了重大挑战。虽然这种火山群如果发生在已知的火山口之下，通常被认为是岩浆侵入中地壳的停滞，但同样有可能的是，火山群代表了中地壳流体循环模式受到扰动而加速的构造过程。本研究旨在验证这样的假设：分析承载“隐火山”地震群的岩石体积中主应力方向的变化可以提供地震群近因（岩浆或构造）的强有力指示。地震群是地壳中部岩浆侵位的直接结果，应伴随着侵位岩浆加压造成的地壳应力场方向的系统性~90º变化。相反，构造过程所导致的震群应该仅仅反映环境应力场的方向。我们计划通过两个独立的应力场分析技术（断层面的解决方案分析和剪切波分裂分析）的地震数据，包括三个最近的隐火山地震群沿着阿拉斯加的阿留申弧的组合应用程序来测试这一假设。三个案例研究代表了一系列的岩浆构造亲和力的基础上，他们的位置相对于已知的火山口和其他指标的中地壳岩浆上升记录在每个群。此外，我们的目标是表明，独立的应力场分析技术的组合是必要的，以克服在每种技术固有的模糊性时，评估岩浆上升产生的应力变化。我们的研究结果旨在建立一个框架，用于分析和解释未来的隐火山地震群在火山监测的背景下，并提供洞察的性质和频率的非喷发性浅层岩浆侵入。

Intense swarms of microearthquakes centered at mid-crustal depths are a near-universal precursor to volcanic eruptions. However, such earthquake swarms do not always culminate in an eruption, and the relationship of these noneruptive earthquake swarms to magma ascent is often highly ambiguous, posing a significant challenge to scientists in charge of assessing the likelihood of eruptive activity indicated by swarm episodes. While such swarms, when they occur beneath a known volcanic vent, are frequently attributed to stalled intrusions of magma in the mid-crust, it is often equally possible that the swarm represents tectonic processes accelerated by perturbation to fluid circulation patterns in the mid-crust. This study aims to test the hypothesis that analysis of changes in the orientation of principal stresses in the volume of rock hosting a 'cryptovolcanic' earthquake swarm can provide a robust indication of the proximal cause (magmatic or tectonic) of the swarm. Earthquake swarms that are the direct result of midcrustal magma emplacement should be accompanied by a systematic ~90º change in the orientation of the crustal stress field resulting from pressurization by the emplaced magma. In contrast, earthquake swarms that are the result of tectonic processes should simply reflect the orientation of the ambient stress field. We plan to test this hypothesis through combined application of two independent stress field analytical techniques (fault-plane solution analysis and shear-wave splitting analysis) to seismic data comprising three recent cryptovolcanic earthquake swarms along Alaska's Aleutian arc. The three case studies represent a range of magmatic-tectonic affinities based on their location with respect to known volcanic vents and other indicators of mid-crustal magma ascent recorded during each swarm. Furthermore, we aim to show that a combination of independent stress field analytical techniques is necessary to overcome ambiguities inherent in each technique when assessing changes in stress produced by magma ascent. The results of our study are intended to establish a framework for the analysis and interpretation of future cryptovolcanic earthquake swarms in a volcano monitoring context, and to provide insight into the nature and frequency of non-eruptive shallow magma intrusions.