HR: 08:45h
AN: S21C-04    [Abstracts]
TI: Evidence for Melt Injection in the Crust of Northern California?
AU: * Hayes, G P
EM: ghayes@geosc.psu.edu
AF: Penn State University Department of Geosciences, 542 Deike Building, University Park, PA 16802 United States
AU: Johnson, C B
EM: cjohnson@geosc.psu.edu
AF: Penn State University Department of Geosciences, 542 Deike Building, University Park, PA 16802 United States
AU: Furlong, K P
EM: kevin@geodyn.psu.edu
AF: Penn State University Department of Geosciences, 542 Deike Building, University Park, PA 16802 United States
AB: The volcanic history of northern California over the past 20Ma has been well documented. A series of volcanic centers follow in the wake of the northward migration of the Mendocino triple junction (Dickinson, 1997) to the most recent surface expression at Clear Lake, active between 2-0.1Ma. As the triple junction has moved approximately 40-80km further north since this time, surface volcanism is expected to also shift northwards. Detailed seismic reflection surveys in the region of Lake Pillsbury (Levander et al., 1998), approximately 35-50km north of Clear Lake, imaged highly reflective bodies in the mid-lower crust interpreted as lenses of melt associated with upwelling asthenosphere, indicating that this location is the most likely site for future volcanism some time in the next 400k.y. Here we present results from an analysis of a migrating swarm of earthquakes that occurred on a previously unmapped structure approximately 15km west of Lake Pillsbury in mid-2000. Activity increased from a pre-2000 background level of less than one event per month to over 90 events per month in May 2000. We have used the double-differencing approach to relocate these events with both catalog time-picks and cross-correlated waveforms, producing a detailed, highly accurate image of their relative structure and pattern of migration. Earthquakes systematically move southwards during the period of increased activity of the swarm (April-August 2000) before shifting back to the north post-2000 when activity once again decreased. The pattern of relocated seismicity shows a linear 'streak' at a depth of approximately 8km, and is consistent with the stress increase associated with a dyke injection below the swarm in a Coulomb failure model. This evidence suggests that these earthquakes may be associated with melt injection into a series of mid-lower crustal dykes, and thus provide further evidence of on-going volcanic processes in the crust of northern California.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7280 Volcano seismology (8419)
DE: 8123 Dynamics: seismotectonics
SC: Seismology [S]
MN: Fall Meeting 2005