Is Mauna Loa gaining weight?
Posted on August 13, 2015
USGS Hawaiian Volcano Observatory geophysicist Ingrid Johanson measures gravity with a gravimeter on the slope of Mauna Loa with Mauna Kea visible in the background. USGS photo.
As part of our monitoring of subsurface activity at Mauna Loa, we recently conducted a series of measurements of the force of gravity at various locations near the summit caldera and rift zones of the volcano. Did that sentence surprise you? Most of us don't think much about gravity, but when we do (as in, perhaps, cursing it when our cell phone drops to the ground), we don't usually think of it as being different from place to place or from time to time.
Actually, the force of gravity varies along the surface of the Earth due to many factors, all stemming from the fact that the force is stronger with greater mass, and smaller with greater distance from the bulk of the mass. These changes are generally quite small. For example, you will weigh less at the top of Mauna Loa than you do at sea level in Hilo, because Mauna Loa's summit is 4,170 m (13,680 ft) farther from the center of the Earth. But the difference—less than half a pound—is probably not worth the trip as a weight loss measure.
How we use gravity to monitor volcanoes takes advantage of the knowledge that the pull of gravity is stronger when there is more mass beneath the spot where it's measured. So when magma rises to shallower levels and accumulates in a magma reservoir (increasing mass), gravity increases at the nearby surface.
If there is a large enough volume of magma and/or the depth to the reservoir is shallow enough, we can measure that slight change in gravity with a specialized instrument called a gravimeter. We use these measurements to help constrain the amount of magma and possibly the depth at which it is being accumulated.
Usually, when magma rises beneath a volcano, the ground surface also swells (rises) in response to the increased pressure from below. Thus, when we measure changes in gravity with time, we must also measure the change in elevation to correct for its effect on gravity. Nowadays, the easiest way to do that is to measure gravity at stations in the continuously recording Global Positioning System (GPS) network on Mauna Loa. Data from these stations are processed at the USGS Hawaiian Volcano Observatory to track positions for each site with an accuracy of a few centimeters (about an inch).
The position differences over time recorded by the GPS network do more than just provide elevation changes for correcting gravity measurements. The patterns of surface motion are another window to the activity beneath.
For a little more than a year, GPS stations on Mauna Loa have been recording a pattern of motion that indicates influx into magma storage reservoirs beneath the volcano's summit area. The rate of the increase has not been steady, but rather seems to be happening in pulses, with lulls of up to more than a month interspersed with periods of faster inflation.
Micro-seismicity beneath the summit area started picking up even before this most recent episode of inflation became apparent. As we've reported in recent Volcano Watch articles, the number of small earthquakes beneath the summit area has increased since at least late 2013.
The deformation of the surface and the increased seismicity strongly suggest that Mauna Loa is indeed gaining weight or in other words, that magma is accumulating at fairly shallow levels. The recent gravity survey is not easily comparable to measurements made around the time of the most recent eruption in 1984, but it forms an excellent baseline for future assessments.
Gravity measurements complement the numerous other methods we use to track the movement of magma beneath the surface of Hawaiian volcanoes, including monitoring deformation with GPS instruments and tiltmeters, earthquakes with seismometers, volcanic gas fluxes with gas sensors, and temperature anomalies with infrared webcams and thermal sensors in fumaroles. Gravity measurements are unique in that they offer the only method of directly monitoring increases or decreases in subsurface mass.
Our goal is to integrate all these data for a better understanding of the processes happening below the surface that lead to eruptions, with the hope that this will ultimately lead to better forecasts of the time, place and magnitude of eruptions.
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