Volcano Watch — New tools help volcano scientists

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Not only does Hawaii move up and down, as I described last week, it also moves horizontally. There are several ways to measure these movements, and a new, exciting technique has recently been added to the repertoire.
 

Not only does Hawaii move up and down, as I described last week, it also moves horizontally. There are several ways to measure these movements, and a new, exciting technique has recently been added to the repertoire.

In the past we used mainly laser-ranging and triangulation to measure changes in the position of survey points. The laser-ranging technique measures the distance between two points by determining the time it takes for a laser beam to travel to a reflector and back again. If either of the two points has moved, the distance will change. Triangulation requires three points that form a triangle and measures the angle between sides of the triangle. If the angle changes, the points have moved relative to one another.

Each of these techniques can only determine that the measurement points have moved relative to one another, but cannot determine which point actually moved, nor can they distinguish between horizontal and vertical motions. To determine the actual movement, we need to include measurements to a point that we know is not moving. Each technique requires line-of-sight between the measurement points; weather also affects the results.

The line-of-sight requirement means that we were unable to measure between points where we might have wanted a measurement, simply because we could not see one point from the other; our measurement networks were therefore set up as much to measure what we could measure as it was to measure what we wanted to know.

More recent technology overcomes these weaknesses. We now employ GPS, the Global Positioning System, for most of our distance measurements. This technique measures distances to a constellation of orbiting satellites and uses these distances to locate the point on the ground. The only requirement is that we must have a clear view of the sky to see the satellites. It is therefore possible to determine line lengths across ridges (such as the rift zones on the volcano), and to include reference points outside the active zone. The data can be computer-processed to yield locations accurate to a few millimeters horizontally, and a few centimeters vertically.

Application of GPS to ground motions in Hawaii began in 1987; we have now progressed to the point where we measure more than 100 points each year and a small subset once a month. The rates of horizontal movement are generally quite low, except on the south flank of Kīlauea Volcano, where we commonly measured rates of horizontal movement as great as 10 centimeters (four inches) per year. However, these high rates have slowed somewhat in the last year and the maximum rates, at a location near Apua Point south of Kīlauea caldera, are about six centimeters per year towards the southeast.

The entire south flank of Kīlauea from Naliikakani Point to Kalapana, and bounded inland by the rift zones, is moving towards the ocean at rates exceeding several centimeters per year, with the most rapid movement occurring seaward of the summit caldera and the southwest rift zone.

In early October, NASA's shuttle mission, as part of their Mission to Planet Earth, collected new data that should again revolutionize our ability to measure changes on the volcanoes in Hawaii and around the world.The instrument used is called the Spaceborne Imaging Radar-C and X-band Synthetic Aperature Radar. It collects detailed observations of the Earth at three different microwave wavelengths.

These images can be superposed with similar images collected previously to determine changes that have occurred. NASA's Jet Propulsion Laboratory at Pasadena, California, processed data collected last April and again during the recent Shuttle flight to create a preliminary map of motions for the south flank of Kīlauea Volcano. Their map differs from our GPS data in that it collects data for the entire surface, as opposed to our selected point locations, and should allow for a more detailed characterization of the motions and the forces that cause the measured changes. However, the rates of motion they determined were significantly higher (10 centimeters in less than six months) than those we have obtained using GPS for the same time period, and the distribution of motions does not match what we have measured.

We suspect that these inconsistencies reflect the preliminary analysis that the scientists at the Jet Propulsion Laboratory have had time to perform, and that with more detailed analysis of the data, and the use of our GPS data to provide ground truth of their map, a powerful new tool will be available to understand the ground motions at active volcanoes around the world. Such a technology will not replace the types of measurements we now perform at the Hawaiian Volcano Observatory because of the infrequency of the space-borne measurements and the need to have a rapid, accurate response to eruptive activity.