Inversion of Focal Mechanism Data for the Directions of Stress at Redoubt Volcano, Alaska.
Sánchez, J. J., M. Wyss, and S.R. McNutt.
Geophysical Institute, University of Alaska Fairbanks
Abstract.The directions of principal stress axes in the vicinity of Redoubt Volcano, Alaska, are investigated by inverting fault plane solutions (FPS) of volcano-tectonic earthquakes recorded by a temporary array deployed on the volcano in July 1991 and by the AVO permanent network between December 1989 and December 1991. We follow the method devised by Gephart and Forsyth (1984) to obtain the principal stress directions and a measure of their relative magnitudes from a group of FPS. For a group of 38 earthquakes located NE of the volcano in the depth range 3.2 - 5.5 km we found a well constrained solution with an average misfit of 4.6 degrees that fulfills the assumption underlying the method, namely, that stress is homogeneous within the volume sampled by the seismicity; with a near-vertical s1, subhorizontal s3, and subhorizontal s2 oriented SE-NW. We used this solution as a test stress tensor. Using the Cumulative Misfit Method we tested the hypothesis that the observed fault-plane solutions for the entire volume sampled by the seismicity in the Redoubt area are due either to a single uniform stress tensor orientation or, alternatively, they show changes in orientation related to variations in stress in space or time. By comparing the individual misfits of all the fault-plane solutions computed for earthquakes during 1989-1991 with our stress model we identified a possible variation with time of the stress field in the vicinity of the volcano during October 1990 as shown by a change of slope in the cumulative misfit curve. By using the same technique, we also identified possible changes in the stress field with depth near Redoubt volcano. From a rotation of the stress tensor during the intrusive period compared to that in the inter-eruption period, we estimate that the absolute stress levels of s1 and s2 are near 1 kbar.