The volcano is monitored by the California Volcano Observatory, which has a sensor network that can measure increased seismicity, ground deformation, or gas emissions suggesting movement of magma towards the surface near the volcano. The United States Geological Survey, in cooperation with the National Park Service, has been monitoring Lassen Peak and other volcanic areas in the park with tiltmeters, seismometers, and inclinometers. Prior to 1996, geodetic surveys at Lassen Peak did not detect ground deformation, but Interferometric synthetic-aperture radar (InSAR) surveys between 1996 and 2000 suggested that downward subsidence was occurring at a rate of each year within a circular area with a diameter of centered just of the volcano. As a result, additional surveys using the Global Positioning System took place in 2004, and further InSAR surveys showed that subsidence continued through 2010. Lassen Peak is one of four Cascade volcanoes that has undergone subsidence since 1990, with Medicine Lake Volcano, Mount Baker, and Mount St. Helens. Though not conclusively linked to a possible eruption, this subsidence may offer insight into how magma is stored within the region, tectonic setting, and how hydrothermal systems evolve over long periods of time. GPS receivers have been in place to monitor deformation within the Lassen volcanic center since 2008, and 13 seismometers in the vicinity, first installed in 1976 and since updated each decade, continually survey earthquakes within the locale.