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

点击翻译按钮获取中文摘要

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.