Volcano Watch — Measuring how volcanoes move with satellites

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Last week we discussed the different land surveying techniques HVO scientists use to monitor the swelling and movement of active volcanoes.


Measuring how volcanoes move with satellites...

Map showing average annual horizontal motion of benchmarks on the Island of Hawai‘i between 1990 and 1995; note vector scale of 3 inches per year.

(Public domain.)

Last week we discussed the different land surveying techniques HVO scientists use to monitor the swelling and movement of active volcanoes.

This week we focus on a new technology, the Global Positioning System (GPS), that is revolutionizing how we navigate on land, sea, and in the air. GPS is a group of 24 navigation satellites orbiting the Earth twice a day at an altitude of about 12,000 miles. We use GPS to obtain a detailed and accurate measure of how volcanoes move.

GPS was designed by the military to locate tanks, planes, and ships. The system has been adopted by the public for many navigational, surveying, and scientific applications. With the proper equipment, you can determine your latitude, longitude, and elevation at any time and at any place on Earth.

How does GPS work? GPS satellites continuously broadcast messages on two radio frequencies. These messages contain a very accurate time signal, the satellite's position in space, and coded information that a GPS receiver can decipher to determine the distance to the satellite.

The receiver uses its internal clock and the coded information from each GPS satellite to determine the amount of time it takes the signals to reach the receiver. This amount of time, multiplied by the speed of light, provides the distance to each satellite. Once the distances to four satellites are known, the receiver can determine its position on the Earth. The accuracy of the position depends on which of the broadcasted signals are recorded, where and how many satellites are in the sky, the accuracy of the satellite orbits, the method used to correct clock errors in the satellites and receivers, and the methods used to correct path delays introduced by the Earth's atmosphere and ionosphere.

Pilots, boaters, drivers, and hikers can use a GPS receiver to navigate. All they need is a clear view of the sky (this could be a problem in the woods or in the city). This type of measurement with a single receiver is called a point position. Even in the worst weather conditions, they can determine their location to within 300 feet. That accuracy is fine for most navigational purposes, but we need greater accuracy to measure how volcanoes move.

We use a method called "relative point positioning," in which we set out many receivers at the same time to simultaneously record the signals broadcast by the satellites. We then calculate the difference between the signals recorded at each site. With this method, errors (uncorrected path delays, clock errors) that are common to the signals recorded at each site are canceled and we can determine the relative positions of the sites within one third of an inch, even over distances of several hundred miles.

Months or years later, we measure the relative positions of the same sites again and calculate the accumulated movement between the surveys. We use the distribution and rate of movement to estimate where earthquakesmay occur and where magma is moving within the volcano. For example, if a magma chamber is inflated by a fresh charge of magma, the radial pattern of surface displacements will allow us to estimate its location, depth, and amount of expansion.

Results from our annual island-wide GPS surveys are summarized on the map. The vectors show the average velocity of the ground relative to a point on Mauna Kea Volcano, which is assumed to be stable. Between 1990 and 1995, the south flank of Kīlauea Volcano and the southeast flank of Mauna Loa Volcano moved seaward at a rate from 1 to more than 3 inches per year. Most of the rest of the island appears to be stable.

The movements may seem slow, but a tremendous amount of energy is involved in moving the whole side of a volcano. The areas moving now have had moderate to large earthquakes in the past. If the current rate of motion continues for decades or centuries, it may take another large earthquake to relieve the strain accumulated in the Earth's crust.