HR: 0830h
AN: V51F-0351    [PDF]
TI: Horizontal rotation of the local stress field in response to magmatic activity: Evidence from case studies and modeling
AU: * Roman, D C
EM: droman@newberry.uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AB: 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 prior to, during, and after magmatic activity can be used to relate changes in stress field orientation to the state of the magmatic conduit system. These relationships may be tested through modeling of induced stresses. Here I present evidence from case studies and modeling that horizontal rotation of the axis of maximum compressive stress at an active volcano indicates pressurization of a magmatic conduit, and that this rotation, when observed, may also be indicative of the physical properties of the ascending magma. Changes in the local stress field orientation during the 1992 eruption sequence at Crater Peak (Mt. Spurr), Alaska were analyzed by calculating and inverting subsets of over 150 fault-plane solutions. Local stress tensors for four time periods, corresponding approximately to changes in activity at the volcano, were calculated based on the misfit of individual fault-plane solutions to a regional stress tensor. Results indicate that for nine months prior to the eruption, local maximum compressive stress was oriented perpendicular to regional maximum compressive stress. A similar horizontal rotation was observed beginning in November of 1992, coincident with an episode of elevated earthquake and tremor activity indicating intrusion of magma into the conduit. During periods of quiescence the local stress field was similar to the regional stress field. Similar horizontal rotations have been observed at Mt. Ruapehu, New Zealand (Miller and Savage 2001, Gerst 2003), Usu Volcano, Japan (Fukuyama et al. 2001), Unzen Volcano, Japan (Umakoshi et al. 2001), and Mt. St. Helens Volcano, USA (Moran 1994) in conjunction with eruptive activity. This horizontal rotation may reflect pressurization and inflation of a conduit system by an influx of magma, and may be related to physical properties (rheology) of the ascending magma. In this regard, horizontal rotations are not observed at volcanoes erupting low-viscosity basaltic magma (e.g., Miyakejima, Japan, Ukawa and Tsukahara 1996). Numerical modeling of Coulomb stress changes induced by inflation of dike-like and cylindrical conduits supports the hypothesis that conduit dilation results in a local reorientation of the maximum compressive stress axis. Modeling results indicate that faults surrounding the conduit experience an increase in Coulomb stress of ten bars or more in response to $\leq$ 1 m of conduit dilation for a `rotated' sense of strike-slip or thrust motion (with respect to the regional stress field), corresponding to the stress field rotation observed in fault-plane solution studies. Furthermore, differences in the patterns of Coulomb stress changes induced by inflating dike-like and cylindrical conduits make it possible to distinguish between these two geometries based on the locations of earthquakes with rotated fault-plane solutions. Finally, although both case study and modeling results indicate that conduit inflation is likely to produce a local reversal of the positions of minimum and maximum compressive stress axes, it is possible that this phenomenon requires the presence of favorably-oriented faults in the volume of rock surrounding the conduit.
DE: 7280 Volcano seismology (8419)
DE: 8164 Stresses--crust and lithosphere
DE: 8419 Eruption monitoring (7280)
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting