We used first motion data for earthquakes at four Alaskan volcanoes, namely, Pavlof,
Dutton, Akutan and Makushin and computed composite focal mechanism solutions for
groups of earthquakes recorded between August 1996 and September 1998.
The composite focal mechanisms at the four volcanoes show considerable variation. For
a group of three events at Pavlof volcano we found a composite solution that represents
thrusting, with nodal planes striking WNW. This may represent dominantly
compressional stresses prevailing in the area oriented about 40 degrees off the direction
of plate convergence. At Dutton volcano, solutions were computed for two different
clusters of earthquakes. The solution for a cluster located NW of the summit represents
thrust faulting with the pressure axis oriented parallel to the direction of plate
convergence. For Akutan volcano, the solution for a cluster located east of the volcano
represents normal faulting with the pressure axis oriented orthogonal to the direction of
plate convergence. The solution for another cluster located NW of the summit shows
strike-slip faulting and pressure axis oriented in the E-W direction. Based on both of
these observations we suggest that local stresses may dominate at Akutan volcano, since
the P axes appear to be unrelated to plate convergence. At Makushin volcano we obtained
constrained solutions for three clusters. For a cluster located NE of the summit we found
strike-slip faulting with a normal component and the pressure axis nearly parallel to the
direction of plate convergence. For the cluster located east of the summit, the solution is
strike-slip faulting with the pressure axis oriented SE-NW (subparallel to plate
convergence). For a cluster located NW of Makushin's summit the composite solution
shows thrust faulting with the pressure axis oriented SE-NW. Again this suggests local
stresses superimposed on those due to plate convergence.