Levelling lines were first established at Medicine Lake in 1954. These lines were resurveyed in 1988 – 1990 (Dzurisin et al. 1991) revealing volcano wide subsidence of over 30 cm at an average rate of −0.86 ± 0.09 cm/yr (Dzurisin et al. 2002). In order to constrain horizontal displacements at the volcano, campaign GPS surveys were carried out in 1996, 1999, 2000, 2003, and 2004. These measurements show that vertical displacements are ~3 times larger than horizontal displacements (Poland et al. 2006). InSAR measurements made between 1993 – 2000 support the observation of subsidence at a rate of ~ 1 cm/yr.
In 2005-2006 3 GPS receivers were installed to make up the continuous monitoring network at Medicine Lake (CalVO website) (http://earthquake.usgs.gov/monitoring/gps/NCalifornia/). Further InSAR measurements spanning 2003 – 2011 show that deformation continues at a constant rate (Parker et al. 2014).
The best fitting analytical models to all datasets suggest that the source is a horizontal sill located at 9 – 11 km depth, although whether this circular or tabular is not constrained. The rate of volume change is inferred to be -0.001 – -0.003 km³/yr. The exact cause of deformation is uncertain, but is suggested to be a combination of tectonic extension, gravitational loading, plus volume loss at depth, most likely due to cooling and crystallisation of cooling magma.