Stress and surface deformation due to earthquakes and volcanoes in Alaska

Abstract

A new method has been developed to investigate stress homogeneity along plate boundaries based on the cumulative misfit between the theoretical and observed slip directions on fault planes of individual earthquake focal mechanisms, calculated using assumed stress tensors. This method allows identification of volumes with uniform stress directions, suitable for inversion for stress orientations, with a minimum of computing time. The method also affords an alternative estimate of the significance of differences in stress directions. Applying this method in the Aleutians, we found that the Aleutian plate boundary is segmented based on the observations that the misfits are relatively constant within segments of uniform stress orientation but that they change abruptly at segment boundaries. The segmentation boundaries correspond to fracture zones, boundaries of asperities and ends of aftershock zones of great earthquakes. Applying this method in the Alaska Wadati-Benioff Zone (WBZ), we depict the stress field at two different scales. The stress directions measured by large earthquakes (Ms ~ 5) are homogeneous, with extension down dip and greatest compression along strike. The unusual orientation of the greatest principal stress is attributed to the bend of the slab under central Alaska, which generates compressive stresses along strike. The stress directions measured by small earthquakes $\rm(M\sb{L}\sim3),$ reveal a great deal of heterogeneity as a function of depth and along strike, although they show a trend that confirms the overall stress field derived from the large events. We propose that the ratio of the dimensions of the stress field (sensed by earthquakes) to the rupture dimensions is about 20 to 50. Using ERS-1 Synthetic Aperture Radar (SAR) interferometry, we have constructed high resolution topographic maps, and detected several centimeters of uplift that accumulated during two years (1993-1995) at two neighboring volcanoes in Alaska's Katmai National Park: New Trident vent and Novarupta Dome. From the uplift gradient we estimate the depth of the pressure source, presumably a magma body or hydrothermal system, under New Trident volcano to be 2 km. These are the first observations of volcanic deformation in this area and will be helpful to characterize the volcano's structure and behavior.