| Abstrakt: |
To examine controls on the local stress field at Augustine Volcano, Alaska, before its 2006 eruption, we calculated fault plane solutions for volcano‐tectonic earthquakes from 2002 to 2006. The P‐axis orientation was first aligned to the regional maximum compression (NW) and then rotated by about 90° (perpendicular to the dike alignment) after the onset of surface deformation in mid‐August 2005. Using 3D finite element models, we systematically evaluated the effects of tectonic stresses, volcanic edifice densities, and dike overpressures on the local stress field orientation. Combining data and models to generate "phase diagrams" of different stress controls by these competing effects, we argue that moderate tectonic stress of 2–3 MPa at 600 m above sea level slightly exceeded the edifice loading before the precursory deformation and was then overprinted by a local stress field from dike opening with an overpressure of ∼15 MPa. Plain Language Summary: Volcano‐tectonic (VT) earthquakes are commonly observed before volcanic eruptions and provide critical information on pre‐eruptive rock ruptures due to the propagation of magma‐ or volatile‐filled cracks (i.e., dikes). Due to dike inflation, the direction of maximum compression near a dike may rotate ∼90° from sub‐parallel to the dike and regional maximum compression to perpendicular to the dike, as revealed by fault plane solutions (FPS). To understand the state of pre‐eruptive stress at Augustine Volcano, Alaska, and the trigger of its 2006 eruption, we calculate the FPS of VT earthquakes from 2002 to 2006 and observe a 90‐degree rotation of the pressure axis. Conceptually, whether the rotation of FPS can be observed depends on the relative contributions of different stress sources, including gravitational loading, tectonic force, and dike inflation. We systematically evaluate the effects of those sources by a series of numerical models. A "phase diagram" is generated to illustrate how different sources control the preferred stress orientation. Combined with the data, the model estimates the magnitude of tectonic stress and dike overpressure before the 2006 eruption of Augustine. FPS rotation is sensitive to volcanoes' stress changes which may provide critical early warnings to volcanic eruptions. Key Points: A 90° rotation of fault plane solution occurred when surface started to deform during the 2005–2006 volcanic unrest of AugustineA series of numerical models analyzed the competing effects of edifice loading, tectonics, and dike opening on the local stress fieldFault plane solution rotations can be more sensitive signals to detect volcano's subsurface pressure changes than deformation [ABSTRACT FROM AUTHOR] |
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