Volcano Watch: How measuring gravity on Maunakea helps us monitor Mauna Loa

  • A gravimeter measuring the force of gravity on Maunakea. These measurements are used to calibrate the instruments so they can precisely monitor changes in gravity from magma accumulation at Mauna Loa (background). (USGS/Special to West Hawaii Today)

It’s 5 a.m. and a somewhat sleepy scientist is getting ready to leave his home in Honomu and head to the USGS Hawaiian Volcano Observatory (HVO) office on Kamehameha Avenue in Hilo. Yawning — and not quite fully caffeinated — he says goodbye to the dog following him around the kitchen who’s wondering why they’re up so early.

At the HVO office, he picks up two identical instruments the size of a shoebox — gravimeters — and loads them into a USGS four-wheel-drive vehicle. Today will be a long day driving from Hilo to the summit of Maunakea and back — twice.


But these days are necessary, and any day spent on the mountain is a good day, especially if the sky is clear and views stretch to the horizon. Between Hilo and the summit of Maunakea, the HVO scientist will stop approximately half a dozen times at a series of locations (benchmarks) established beginning in the 1960s. At these benchmarks, the two gravimeters will be used to measure the variation of the force in gravity.

Days like this one are not particularly eventful; they consist of driving from one measurement site to another and waiting for the gravimeters to stabilize at each new site. They’re filled with audiobooks, Hawaii Public Radio, and listening to the sounds of the wind. These are not the exciting days trekking on the floor of Kilauea caldera or flying by helicopter to the summit of Mauna Loa.

Gravimeters, essentially extremely precise pendulums, can measure a change in the force of gravity to one-in-one billionth of the force you feel every day. This force varies based on the distance and the amount of mass between the instrument (or you) and the center of the Earth.

Just like atmospheric pressure, the force of gravity changes depending on your altitude. For example, the higher in elevation you go (like driving up a mountain), the farther away you are from the center of the Earth (and its mass), and the weaker the force of gravity. This elevation effect is the primary contribution to changes in gravity measured on Maunakea. The changes in gravity are not as noticeable as the change in the atmosphere (it’s hard to breathe at the summit), but the average person also weighs about one-third of a pound less — equivalent to the weight of an orange — at the summit of Maunakea than they do in Hilo!

Since the 1970s, small changes in time-varying gravity (microgravity) have been measured on the active volcanoes Mauna Loa and Kilauea to determine whether magma is accumulating in their magma reservoirs. This intruding magma often opens and fills cracks and/or empty spaces, causing a net increase in the volcano’s mass that can be measured with a gravimeter. Measuring the gravity is an independent way to confirm whether ongoing uplift, like that occurring at Mauna Loa since 2014, is from new magma intruding into the volcano.

The precision and sensitivity of the gravimeters makes them extremely delicate, and they require regular calibration. As the dominant effect we measure is from changes in elevation, our ability to measure volcanic changes on the high elevations of Mauna Loa (13,680 ft or 4,170 m) requires us to calibrate the instruments over similar elevations on Maunakea where there is currently no influence from volcanic activity (the last eruption was more than 4,500 years ago). Without Maunakea, we would have to send the gravimeters back to California to be calibrated, making them susceptible to damage on their long journey.

The opportunity to calibrate HVO gravimeters on Maunakea provides the ability to design a gravity monitoring program to help understand volcanic unrest at Mauna Loa. Along with ground deformation and seismicity, future gravity surveys could help detect how much magma is slowly being supplied to Mauna Loa’s shallow magma storage system. On the Island of Hawaii, Maunakea and Mauna Loa have played important roles in the past; today, they are acting together to help inform us about future volcanic activity.

Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Volcano Activity Updates

Kilauea Volcano is erupting. Its USGS Volcano Alert level is at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kilauea updates are issued daily.

Lava activity is confined to Halema‘uma‘u with lava erupting from a vent on the northwest side of the crater. Laser rangefinder measurements April 22 indicate that the lava in the western (active) portion of the lake is 741 feet deep, with the eastern portion of the lava lake solidified at the surface. The summit tiltmeters recorded minor change over the past 24 hours. Sulfur dioxide emission rates measured on April 14 were 950 t/d. Seismicity remains stable, with elevated tremor. For the most current information on the eruption, see https://www.usgs.gov/volcanoes/kilauea/current-eruption.

Mauna Loa is not erupting and remains at Volcano Alert Level ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption from the current level of unrest is certain. Mauna Loa updates are issued weekly.


This past week, about 175 small-magnitude earthquakes were recorded below Mauna Loa, 188 of which were beneath the summit and upper-elevations; most of these occurred at depths of less than 5 miles. GPS measurements have recently shown variability in summit deformation patterns, moving from contractional to slightly extensional over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone remain stable. Webcams show no changes to the landscape. For more information on current monitoring of Mauna Loa Volcano, visit https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There were three events with three or more felt reports in the Hawaiian Islands during the past week: a magnitude-3.0 earthquake less than a mile east-northeast of Pahala on April 21 at 2:09 a.m.; a magnitude-2.8 earthquake 3 miles east of Pahala on April 20 at 11:08 a.m., and a magnitude-3.2 earthquake 14 miles east of Honaunau-Napoopoo on April 16 at 7:46 p.m.

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