EARTHQUAKES IN VIDA, OREGON, AND THE TRANSITION FROM COMPRESSIONAL TO EXTENSIONAL STRESS REGIMES IN THE PACIFIC NORTHWEST

	Analyses of small earthquakes in the western Cascades, and a compilation of regional focal mechanisms, provide clues to the location of the transition zone between compressional and extensional stress regimes in the Pacific Northwest. In May, 1997, a swarm of small earthquakes (M < 2.7) occurred 40 km east of Eugene, near Vida, Oregon, and were recorded by the Pacific Northwest Seismic Network. We re-located the 3 largest earthquakes with a 1-D velocity model, constructed focal mechanisms, and composed a net moment tensor. In the Vida sequence, the 3 largest earthquakes appear to have been on the same fault, with a slip plane that dips steeply and trends north-northwest. The net sense of motion is right lateral with a reverse component, similar to that of the1993, M 5.7 Scotts Mills earthquake, which was right-lateral with a much larger reverse component (Thomas et al., 1996). These earthquakes lie within the compressional stress regime. Southeast, within the extensional regime, the 1993, M 5.9 and 6.0 Klamath Falls earthquakes (Braunmiller et al., 1995) had normal fault motion. Thus the stress regime changes between Vida and Klamath Falls. We hypothesize that the regime boundary aligns with the edge of the Columbia Embayment, an accreted oceanic terrane underlying northwestern Oregon (Riddihough et al., 1986). In the transition zone, events may have a significant component of strike-slip motion, as observed in Vida. Regional focal mechanisms substantiate these possibilities. The map shows western Oregon, with mainshock moment tensors and the southern edge of the Columbia Embayment (dashed line).