STRESS DIRECTIONS ALONG THE ALASKA WADATI-BENIOFF ZONE FROM INVERSIONOF FOCAL MECHANISM DATA

The focal mechanisms of 137 intermediate depth (60-180 km) earthquakes in the Alaska Wadati-Benioff zone (WBZ) were derived from P-wave firs t-motions. The study focused on three earthquake clusters referred to as Iliamna, Talkeetna, and Denali. In all three clusters, the orientat ions of the principal stresses (pressure (P), null (B), and tension (T ) axes) are similar. The P axes for most of the events show NNE-SSW tr ends with shallow plunge conforming to the geometry of the WBZ in the study area. The rest of the events yielded E-W oriented P axes. The T axis for most of the events plunge at a steeper angle than P axes, but parallel the dip of the WBZ. We interpret this feature as indicting t he predominance of gravitational sinking of the associated underthrust lithospheric plate. The regional stress direction was estimated from the focal mechanisms using the inversion method of Gephart and Forsyth (1984). The results show that the maximum regional stress in the 60-9 0 km depth range plunges shallowly in the Iliamna (14 degrees) and Tal keetna (5 degrees) areas, and approximately in the direction of conver gence (N163 degrees E) of the Pacific and North American plates. In th is depth range, similar results for the Denali area could not be obtai ned due to a lack of data. However, the inversion of the focal mechani sm data for all three clusters for depths greater than 100 km yielded large values for both misfit and 95 percent confidence area compared w ith those in the depth range of 60-90 km. This may indicate the stress directions for depths greater than 100 km along the Alaska WBZ are mo re heterogeneous than in the depth range of 60-90 km.