Earthquakes Indicated Stress Field Change During the 2006 Unrest of Augustine Volcano, Alaska

Abstract

The crustal stress field around a volcano is affected by many competing and temporally varying factors, such as gravitational loading, tectonic processes, pre-existing structures, and pressurizing and ascending magmas or hydrothermal fluids. Volcano tectonic (VT) earthquakes, indicating shear ruptures, often provide early warnings of volcanic unrest and reveal stress changes due to magma or hydrothermal fluid (Roman & Cashman, 2006). Unfortunately, the absolute magnitude of stresses triggering the volcano tectonic (VT) seismicity cannot be measured directly. However, the relative influence of different sources, such as dike opening and tectonic processes, on the stress field can be inferred from the stress orientation or its changes (Bonafede & Danesi, 1997). For example, a ∼90° rotation of the fault plane solution (FPS) P-axis from regional maximum compression is sometimes observed when a dike opens (Fukuyama et al., 2001;Roman et al., 2021), indicating a possible reorientation in the local stress field. From numerical modeling, the influence of tectonically-and magmatically generated stress fields on the local stress field can be quantified (Roman & Heron, 2007;Vargas-Bracamontes & Neuberg, 2012). However, the effect of gravitational loading due to volcanic edifices is usually ignored, and a systematic study integrating numerical models with seismicity and geodetic data from an active volcanic system is needed.Geophysical monitoring of the 2006 eruption of Augustine Volcano in Cook Inlet, Alaska (Figure 1) provides a wealth of data with which to study the stress field changes during its unrest and subsequent eruption. Starting on 30 April 2005, the number of volcano tectonic (VT) earthquakes generally increased from several to >1,000 per

Mechanical modeling of pre-eruptive magma propagation scenarios at calderas

“…Stress models, however, are still critical and not yet fully understood. In a real-case application, our scenarios would be the end point of a stress calibration, whereby the stress state of a volcanic region is constrained through a statistical procedure aiming at matching dike simulations with observations, such as past vent locations (Rivalta et al, 2019), orientation of exposed dikes (Maerten et al, 2022) or focal mechanisms (Zhan et al, 2022). Our model is well-suited for such purpose.…”

Earthquake Focal Mechanisms as a Stress Meter of Active Volcanoes

“…Numerical methods such as those done by Zhan et al. (2022) might lead to a more precise stress modeling to gain more insights into the origin of stress changes observed as temporal changes of seismic anisotropy.…”

“…While many of these studies construct a model to fit deformation measurements, incorporating other geophysical observations such as the amount and composition of gas emission, seismicity, and focal mechanisms, as Zhan et al. (2022) investigated, will lead to reducing uncertainties of key model parameters. This development leads to more precise physical models, and better forecasts of the volcanic activity (e.g., Poland & Anderson, 2020).…”

“…Zhan et al. (2022) investigated the temporal changes of focal mechanisms associated with the 2006 unrest of Augustine volcano, Alaska. They then modeled the observation by an intrusion of a magma‐filled crack, or dike, which is a ubiquitous form as an intrusion of magma with low to intermediate viscosity (Rivalta et al., 2015; Rubin, 1993).…”

“…If the dike pressure is large enough, the direction perpendicular to the dike can turn from the minimum compressional stress axis, or σ 3 , to the most compressional stress axis, or σ 1 . Zhan et al (2022) also considered the stress given by the volcano edifice. Loading by a volcanic edifice makes the stress field more symmetric with respect to the summit (e.g., Araragi et al, 2015) because it is usually close to axis-symmetric.…”