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Roman, D.C., 2004

Changes in local stress field orientation in response to magmatic activity

Bibliographic Reference

Roman, D.C., 2004, Changes in local stress field orientation in response to magmatic activity: University of Oregon, Eugene, Ph.D. dissertation, 188 p., illust.


A complete understanding of the initiation, evolution, and termination of volcanic eruptions requires reliable monitoring techniques to detect changes in the conduit system during periods of activity, as well as corresponding knowledge of conduit structure and of magma physical properties. Case studies of stress field orientation at active volcanoes can be used to relate changes in local stress to magma movement through conduits; these relationships may be tested through numerical modeling of induced stresses. Stress field analyses of earthquakes recorded during magma eruption and intrusion at Iliamna Volcano and Crater Peak vent, Alaska indicate that during magmatic activity, the orientation of the local axis of maximum compressive stress was perturbed relative to the orientation of the regional or ambient stress field. The exact nature of the perturbation is not resolved at Iliamna Volcano, but data from Crater Peak clearly show that the local stress field rotated horizontally by approximately 90° during periods of magmatic activity. Similar horizontal rotations have been observed at Mt. Ruapehu, New Zealand, Usu Volcano, Japan, and Unzen Volcano, Japan in conjunction with eruptive activity. Stress field rotations are also observed during episodes of magma intrusion. In contrast, horizontal rotations are not observed at volcanoes erupting low-viscosity basaltic magma. Together these observations suggest that the observed horizontal rotation may reflect pressurization and inflation of a dike-like conduit system by an influx of magma, and may require magma to have appropriate rheological properties. Numerical modeling of Coulomb stress changes induced by inflation of dike-like conduits supports the hypothesis that conduit dilation results in a local reorientation of the axis of maximum compressive stress. Modeling results indicate that faults surrounding the conduit experience an increase in Coulomb stress of ten bars or more in response to ?1 m of conduit dilation for a 'rotated' sense of slip (with respect to the regional stress field), corresponding to the stress field rotation observed in case studies.

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