NEW RESULTS FROM A PROPOSED PBO CASCADE VOLCANO CLUSTER I: DEFORMATION IN THE MEDICINE LAKE REGION OF NORTHEAST CALIFORNIA FROM LEVELING, GPS, AND INSAR

Medicine Lake, the largest volcano by volume in the Cascades, is a basalt-rhyolite Quaternary-Holocene shield located in northeast California. Geodetic leveling indicates that the volcano is subsiding by ~8 mm/yr at the summit, a rate that is stable in both space and time and decreases symmetrically on the flanks. Horizontal displacements at GPS sites on the upper flanks are radially toward the summit of the volcano. Interferometric synthetic aperture radar results also indicate subsidence centered on the volcano at a rate that is consistent with leveling measurements. Possible deformation mechanisms include volume loss at depth, surface loading by the volcano and dense intrusions, and crustal thinning due to Basin and Range extension. Models that include a deflating point source at depths of 6-10 km fit the existing data but the cause of the volume loss is unclear. GPS results do not favor a mechanism that requires crustal extension although several normal faulting events have occurred in the region over the past 25 years. To test the surface loading hypothesis, an 80 km traverse extending eastward away from Medicine Lake was leveled in June 2003. Preliminary comparison with a previous survey from 1940-41 suggests that no peripheral uplift, which should form an annulus around the load, exists within 120 km of Medicine Lake. However, the leveling does reveal that at least 1 and perhaps as many as 3 dip-slip faults east of the volcano have experienced vertical displacements of 5-10 cm in the last 60 years. This raises two possibilities: 1) that tectonic activity contributes to subsidence of the volcano, or 2) that strain from an annulus of uplift is accommodated by fault slip. Also in June 2003, a regional east-west transect of GPS stations (established in 1999) immediately south of the Medicine Lake volcano was reoccupied to clarify the tectonic setting of the region. Combining these spatially and temporally extensive geodetic datasets with existing surface and subsurface geological and geophysical data will provide additional constraints on the subsidence mechanism at Medicine Lake volcano and better define the tectonic setting of the southern Cascade Range.