Seismicity and stress tensor inversion in the Central Washington Cascade Mountains (USA)

Abstract

Tectonic stress in the Pacific Northwest Washington is dominated by a N-S major compressive axis s1 and a minor compressive axis s3 which varies from E-W to near vertical. Minor variations in this pattern occur in different parts of the region. In this study we used ca. 550 earthquakes in the central Washington Cascade Mountains to study in detail the uniformity of the stress tensor in this volcanic arc. Earthquakes from the Pacific Northwest Seismograph Network (PNSN) catalogue were divided into several subsets based on epicentral and depth groupings and stress-tensor inversions using the Gephart and Forsyth technique were computed for each group. As in previous similar studies the maximum compressive stress axis (s1) is nearly horizontal and trending ca. N-S and NNE-SSW in all but one subset. Shallower events directly under Mount Rainier have a near vertical s1. For other subsets the minimum compressive stress axis (s3) deviates from vertical to horizontal for different groups of events. In particular, events in the depth range of 10-14 km in the Western Rainier Seismic Zone (WRSZ) have near vertical s3 direction while other depth ranges in this area show a near horizontal, E-W s3 orientation. We hypothesize that the change in orientation of 3 for the 10-14 km depth range in the WRSZ is probably due to the influence of the nearby Mount Rainier magmatic system.