Temporal-spatial variations of stress at Redoubt volcano, Alaska, inferred from inversion of fault plane solutions

We inverted fault plane solutions (FPS) of 420 earthquakes, with a largest magnitude of 3.1, mostly occupying a 4×4×10-km3 volume beneath Redoubt volcano, for principal stress directions during 1989–1998. On average, FPSs were computed with 8 P-wave readings, inversions performed with 48 FPS, yielding misfits of 7°, and the size of the 95% confidence regions of the stress directions is 30°. During the 1989–1990 eruptions stresses were typically: sub-horizontal σ1 and σ2 striking ESE–WNW and NNE–SSW, respectively, and near-vertical σ3, whereas during July 1991–January 1998 we found: near-vertical σ1, and sub-horizontal σ2 and σ3 striking SSE–NNW and ENE–WSW, respectively. We found differences in plunge of σ1, up to 60°, between 1990 and 1991, and sub-volumes with different stress at the 99% confidence level, evaluated with the z-test. Based on the changes in stress directions from the eruptive to the inactive period, we infer that the magnitudes of principal stresses increased progressively during the eruptions and had subsequently decreased by the post-eruption period. The observations can be explained by expansion and contraction of a plexus-like magma body together with changes in physical properties of the magmas involved. We estimate that the absolute values of principal stresses are similar to each other and are in the range 182–187 Mpa.