Tectonic stress in the Pacific Northwest Washington is dominated by a N-S m
ajor compressive axis, sigma(1) and sigma(3) minor compressive axis, sigma(
3) 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 earthqua
kes in the central Washington Cascade Mountains to study, in detail, the un
iformity of the stress tensor in this volcanic are. Earthquakes from the Pa
cific Northwest Seismograph Network (PNSN) catalog were divided into severa
l subsets based on epicentral and depth groupings, and stress-tensor invers
ions using the Gephart and Forsyth technique were computed for each group.
As in previous similar studies, the maximum compressive stress axis, sigma(
1), 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 sigma(1
). For other subsets, the minimum compressive stress axis, sigma(3) deviate
s 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 z
one (WRSZ) have near-vertical sigma(3), direction, whereas ether depth rang
es in this area show a near-horizontal, E-W sigma(3) orientation. We hypoth
esize that the change in orientation of sigma(3) in the 10 to 14 km depth r
ange in the WRSZ may be due to the influence of the nearby Mount Rainier ma
gmatic system. Independent evidence for magma at this depth comes in the fo
rm of a few deep long-period (LP) events.