Seismicity and stress-tenser inversion in the central Washington Cascade Mountains

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.