Volcano Watch — Benchmarks help scientists monitor the Earth

One of the tools we use to determine the likelihood of future eruptions and earthquakes is to measure changes in the shape of the ground surface. As magma accumulates within the magma reservoir located from one to a few miles beneath the summit of Kīlauea and Mauna Loa Volcanoes, the ground surface actually changes shape. The magma chamber acts like a balloon that inflates and uplifts the ground. The flanks of the active volcanoes also slowly slide toward the sea. This gradual movement builds up strain within, or beneath, the volcano that eventually leads to large earthquakes, which occur when the accumulated strain is suddenly released.

Over many years, the Hawaiian Volcano Observatory has developed and employed a number of methods to measure these subtle changes in the shape of the ground surface. Electronic distance measurements are made with a laser to accurately measure the distance between two points. Leveling has been used to accurately measure changes in relative elevation. Tilt, which is a short-baseline form of leveling, can determine relative changes in the elevation of two points. There are a number of types of tilt measurements, including water tubes, electronic device, and two-point leveling (called spirit-level tilt).

Each of these methods of measuring changes in the shape of the ground surface records only relative changes and is limited in that it cannot differentiate vertical changes from horizontal changes. For example, a change in the slope of the ground surface, as measured by tilt changes, does not indicate whether the change is caused by the ground between the end points extending or contracting, or by one end of the line rising or falling relative to the other. It is probable that both horizontal and vertical changes occur at the same time, but we cannot distinguish between them with any of the techniques we have used in the past.

For the last six years, we have been experimenting with a new technology that uses orbitting satellites to precisely locate the horizontal and vertical position of a point. This technology, called the Global Positioning System (GPS), was developed by the military to locate vehicles and ships accurately and quickly. The system has been modified by scientists to allow for determination of extremely accurate positions. During the six-year experimental period, computer techniques and receivers have been developed that allow the data to be post-processed to acquire precision and accuracy of less than a few centimeters.

GPS is a triangulation technique in which the distances from the receiver to each of a group of precisely positioned satellites are measured; those distances are then used to determine the position of the receiver on the Earth's surface. There are many advantages to this technique, including separation of horizontal and vertical movements, determination of absolute movements instead of the relative movements determined by all previous techniques, and the fact that the receivers need only see the sky rather than each other.

The points where we set the receivers are on the same benchmarks that we have used for other distance and elevation measurements for many years. These benchmarks are small, round brass or aluminum plates or stainless steel carriage bolts that are cemented into solid rock. Since the differences in position from year to year determine the rate and direction of movement, the same point must be measured this year that was measured last year. As we returned to some sites this year, we have found that benchmarks have been removed. Benchmarks that are lost to construction or burial by lava flows must be replaced, and the continuity of measurements is lost. If you find these benchmarks, please leave them in place, as they are an essential part of the eruption and earthquake monitoring done by the Hawaiian Volcano Observatory.

During the next few weeks, we will be occupying nearly 100 of these benchmarks with GPS equipment as part of a joint study with a group from Stanford University and the U.S. Geological Survey in Menlo Park, California. The data collected will provide a snapshot of the shape of Kīlauea and Mauna Loa Volcanoes that we will be able to compare to similar, although less accurate, data collected in previous years. These measurements form a part of the database we use to predict future eruptions and to understand the causes of earthquakes here in Hawai`i.