HR: 0800h
AN: G51A-0068    [Abstracts]
TI: Newly discovered subsidence at Lassen Peak, southern Cascade Range, California, from InSAR and GPS
AU: * Poland, M
EM: mpoland@usgs.gov
AF: USGS - Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683-9589 United States
AU: Bawden, G
EM: gbawden@usgs.gov
AF: USGS - Water Resources Division, 3020 State University Drive East - Modoc Hall Suite 4004, Sacramento, CA 95819 United States
AU: Lisowski, M
EM: mlisowsi@usgs.gov
AF: USGS - Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683-9589 United States
AU: Dzurisin, D
EM: dzurisin@usgs.gov
AF: USGS - Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683-9589 United States
AB: ERS interferograms spanning the 1996-2000 time interval indicate up to 1 cm/yr of subsidence over a ~40 km-diameter area centered about 5 km southeast of Lassen Peak, California. Dry tilt and EDM measurements of the volcanic region completed in 1981, 1982, and 1984 showed no significant deformation, but this interval is too short to accumulate detectible deformation at the 1996 to 2000 rates. Since it is unclear based on InSAR results alone whether the subsidence is a recent or more long-lived process, we resurveyed one dry tilt array and measured five EDM line lengths with GPS in July 2004. Results from the three-station dry tilt array were inconclusive, likely due to an unstable benchmark in the array. Line lengths, however, shortened by up to 14 cm suggesting that the subsidence has been ongoing since at least 1981. Four mechanisms may account for the deformation observed near Lassen Peak: 1) magma withdrawal from a shallow chamber into a deeper crustal reservoir, 2) cooling and/or crystallization of a subsurface magma body, 3) loss of hydrothermal fluids and subsequent cooling, and 4) tectonic motion that is creating a pull-apart basin or other similar structure. The possible sources are similar to those invoked to explain subsidence at Medicine Lake volcano, located 100 km to the north. Geophysical and petrologic studies have found no evidence for a large magma reservoir ($>$ 5 km diameter) in the upper crust beneath the Lassen Peak area; therefore, the first two mechanisms are unlikely. The area surrounding Lassen Peak is known for numerous hydrothermal features, but the relatively shallow depth of the hydrothermal system is not consistent with the spatial extent of the subsidence. In contrast, geologic studies confirm that Basin and Range extension is occurring in the region and the Walker Lane zone of dextral shear passes through the subsiding area. Interaction between these tectonic regimes may result in the formation of a pull-apart basin or other structure that is responsible for the observed subsidence. We will present results of inverse modeling that should help distinguish between these mechanisms, as well as additional interferograms that will better constrain the behavior of the subsidence over time.
DE: 8424 Hydrothermal systems (8135)
DE: 8434 Magma migration
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting