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Deformation measurements at Long Valley Caldera

When magma moves into a volcanic system, and closer to the surface of the earth, the area surrounding the volcano may move upward and outward. This swelling is typically measured using the Global Positioning System (GPS) and tiltmeters (tilt).

Networks of deformation instruments recorded significant caldera uplift at Long Valley in the years 1978-1983, 1990-1995, 1996, and 1997-1998.


Continuously recording GPS instruments are the most used type of volcanic deformation-monitoring equipment in the world. 24 satellites, orbiting the Earth twice each day, transmit their position in orbit to receivers on the surface of the Earth. The receivers record vertical and horizontal position, and by comparing these measurements through time, it is possible to determine the amount of movement for a specific location on the surface of the earth.

There are currently 46 GPS receivers that make up the network at Long Valley Caldera. The first instrument was installed in 1995 and the network was updated and modernized between 2006 and 2008.

Data for Long Valley is available from the USGS Earthquake Hazards Program website. Additional information can be found on their Continuous GPS at Long Valley information page or via the links in the list below. 


Like a carpenter's level, an electronic tiltmeter uses a small container filled with a conducting fluid and a “bubble” to measure a change in slope. Tiltmeters measure the amount of tilt in microradians, which is the angle that would be created by placing a flat dime underneath one end of a one-kilometer long beam (equivalent to 0.00006 degree). Volcanologists use tiltmeters with ranges of between 100 and 10,000 microradians depending on the volcano and expected degree of tilt.

At Long Valley, there are currently 9 tiltmeters that measure movement around the caldera. Most of the instruments were installed in the early 1980's, with additional instruments added in 2009.

  • Longbase tilt data for past 10, 30, and 100 days (+NS=South down tilt; +EW=East down tilt)