# Volcano Watch — Monitoring volcanoes in real-time with GPS

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Boaters, hikers, pilots, and other outdoor enthusiasts can use hand-held Global Positioning System (GPS) receivers to locate themselves or to navigate to a point of known position. Volcanologists routinely use hand-held GPS receivers to map lava flows and other volcanic features.

Boaters, hikers, pilots, and other outdoor enthusiasts can use hand-held Global Positioning System (GPS) receivers to locate themselves or to navigate to a point of known position. Volcanologists routinely use hand-held GPS receivers to map lava flows and other volcanic features. They can now use specialized GPS receivers and sophisticated analysis software to continuously track the subtle ground movements that precede an eruption.

First, some background on GPS. The constellation of 24 GPS satellites was launched by the U.S. Department of Defense to locate people, equipment, and targets during military operations. The GPS satellites broadcast accurate classified military signals on two frequencies and less accurate civilian signals on one of those frequencies. These signals along with information about the GPS satellite orbits are encoded as modulations on the GPS broadcast frequencies, in a manner similar to how music is sent on radio waves. The music from your radio is decoded from modulations of radio waves broadcast by radio stations. The GPS "music" broadcast by the satellites provides accurate position and time.

Quality hand-held GPS receivers may be purchased for as little as \$150, but their accuracy is limited to about 300 feet. The military GPS receivers are accurate to about 20 feet. These receivers can track your position as you walk, drive, cruise, or fly. To monitor volcanic movements, we use more capable and expensive GPS receivers that are fixed to the ground and send their data by two-way radio to our computer. With sophisticated data processing software, we can obtain positions accurate to 3 mm (1/8 inch). Unfortunately, these results take an hour or more to compute, and in a volcanic crisis an hour is a long time.

A volcanic crisis begins when magma starts cracking its way to the surface, in a manner similar to the splitting of a log with a wedge. The crack propagates upward at less than 30 cm (1 foot) per second, and the magma takes several hours to reach the surface. This splitting of the ground deep within the volcano causes earthquakes and diffuse ground deformation that become more concentrated and larger as the magma nears the surface. We can use this precursory ground deformation to forecast the place and, with luck, the size of an eruption.

Until recently, tiltmeters provided our only real-time monitor of ground deformation. Last week we began testing real-time GPS monitoring of Mauna Loa. At the same time our mainland counterpart, Cascades Volcano Observatory (CVO), began tests at Long Valley Caldera, California. We are "beta" test sites for software developed by Magellan Corporation's Ashtech GPS subsidiary and distributed by Condor Earth Technologies. The expected primary market for the software, besides volcanologists, is engineers who monitor the stability of dams and bridges.

GPS has an expanding number of applications. HVO and CVO are testing a new capability, real-time volcano monitoring, which we expect will improve our ability to forecast eruptions.

### Volcano Activity Update

Lava continues to erupt from Puu O`o and flow through a network of tubes from the vent to the sea near Kamokuna. A bench collapse on the evening of April 9 or morning of April 10 destroyed a bench nearly 800 m (2,600 ft) long, including one newly formed since March 31 (1.6 ha, 3.5 acres) and a one-year old section (3.5 ha, 8.6 acres). The recently trimmed cliff line is exceptionally unstable, owing to the numerous cracks that occurred as the buttressing bench subsided beneath the waves. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying these frequent collapses of new land. The steam clouds are highly acidic and laced with glass particles.

A magnitude 3.4 earthquake occurred 8 km (5 mi) south southeast of Kawaihae at a depth of 13 km (8 mi). It was felt in Waikoloa.