Abstract

Tectonic stress in the Pacific Northwest Washington is dominated by a N-S major compressive axis, σ1, and a minor compressive axis, σ3, which varies from E-W to near vertical. Some variations in this pattern occur in different parts of the region. In this study, we used 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) catalog 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, σ1, is nearly horizontal and trending N-S to NNE-SSW in all but one subset. Shallower events directly under Mount Rainier have a near-vertical σ1. For other subsets, the minimum compressive stress axis, σ3, deviates from vertical to horizontal for different groups of events. In particular, events in the depth range of 10 to 14 km in the western Rainier seismic zone (WRSZ) have near-vertical σ3 direction, whereas other depth ranges in this area show a near-horizontal, E-W σ3 orientation. We hypothesize that the change in orientation of σ3 in the 10 to 14 km depth range in the WRSZ may be due to the influence of the nearby Mount Rainier magmatic system. Independent evidence for magma at this depth comes in the form of a few deep long-period (LP) events.

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