Lassen Volcanic Center, which includes Lassen Peak, first underwent leveling surveys by the U.S Coast and Geodetic Survey in 1932 and 1934. These surveys were repeated by the U.S. Geological survey in 1991, who found no changes larger than the uncertainty in the measurements (Dzurisin, 1999). In 1980-1984 the U.S Geological Survey established and remeasured a trilateration network at Lassen Volcanic Center, measuring slope distances with EDM. Repeat measurements of this network were probably within measurement error (Chadwick et al. 1985).
An ERS-1/2 InSAR survey of the volcano in 1996-2000 indicated up to 10 mm/yr subsidence over a ~40 km diameter area centred 5 km southeast of Lassen Peak (Poland et al. 2004). During this time the EDM network was reoccupied with campaign GPS, which indicated shortening of up to 145 mm suggesting that subsidence is a longer lived process. The cause of this deformation is uncertain but the volume loss is modelled at a depth of 11.6 km and rate of 0.007 km³/yr.
Most recently in 2007-2008, 8 continuous GPS receivers were installed to make up the continuous monitoring network at Lassen Volcanic Center (CalVO website) (http://earthquake.usgs.gov/monitoring/gps/NCalifornia/).