Measurements of gravity can be used to determine how mass is distributed beneath a volcano. At Kilauea, the Hawaiian Volcano Observatory performs routine microgravity surveys to monitor volcanic activity and to determine changes in gravity. Those changes can indicate whether magma is accumulating in a volcano’s magma reservoir. The surveys measure small gravity changes over time at precisely controlled locations (“benchmarks”) spread across the volcano’s summit region.
The Kilauea microgravity monitoring network is comprised of about 50 benchmarks. Annual microgravity surveys are crucial in confirming whether ongoing uplift or subsidence is caused by magma intrusion or withdrawal. HVO conducts these surveys using small, shoebox-sized instruments called relative gravimeters that can measure a change in the force of gravity to one-in-one billionth of the force you feel every day.
A single gravity measurement consists of leveling a small baseplate (less than 1 foot or 30 cm diameter and 3 inches or 8 cm tall) on the ground, placing the gravimeter on the baseplate, and making a five-minute measurement. Along with gravity, high-precision positions are also collected using GPS.
Gravimeters are extremely susceptible to vibration, so hard and stable surfaces, like solid rock outcroppings, are required to take a measurement. The measurements are non-invasive and do not disturb the ground.
In addition to tracking changes over time, gravity surveys can be used to map the density characteristics of the ground beneath the surface. These “Bouguer” surveys, named after an 18th-century French geophysicist, measure the gravity at hundreds to thousands of locations at a single point in time and do not need the repeatable location benchmarking or precision of microgravity surveys.
Bouguer surveys use the same relative gravimeters that are used for microgravity surveys, but measurements are tied to a reference “base station,” where the actual value of gravity has been determined absolutely.
While both microgravity and Bouguer surveys are used to determine how mass is distributed beneath a volcano, microgravity surveys are used to model changes in these parameters, whereas Bouguer surveys can reveal the overall characteristics of the materials at depth.
Two-and three-dimensional Bouguer models can provide insights into the geologic structure of volcanoes including identifying magma reservoirs, intrusions, landslide and collapse piles, and unexposed faults. At Kilauea, they’ve also been used to define likely areas of hydrothermal fluid circulation. Together, microgravity and Bouguer data can see not only subsurface structure, but also changes within that structure.
Bouguer surveys have been a routine tool at Kilauea for more than seven decades, with the two most recent summit surveys performed in 2009 and 2000.
Over the month of January, a three-person team comprised of HVO geophysicist Ashton Flinders, University of Wisconsin (UW) Madison PhD candidate Claire Ruggles, and UW student Sophia Thompson will measure gravity at over 400 locations around Kilauea’s summit. Their Bouguer gravity survey will be the first to address significant large-scale changes associated with the 2018 caldera collapse.
Seismic velocity and density are fundamentally related and results from this gravity survey will be used to help refine the model developed from the anticipated summer 2023 Kilauea summit seismic study.
If you see three orange-shirted scientists hiking around Kilauea summit this month, carrying a shoebox-sized case and a long pole with a GPS antenna on the top, that’s us! Feel free to stop and say “aloha,” we love to talk about what we’re doing and why.
Volcano activity updates
Kilauea is erupting. Its USGS Volcano Alert level is at WATCH. Kilauea updates are issued daily.
Kilauea volcano’s summit eruption in Halema‘uma‘u crater within Hawai‘i Volcanoes National Park continues. Activity is concentrated in a large lava lake in the eastern half of the crater, as well as a smaller lake to the west, in the basin of the 2021–2022 lava lake. Summit tilt has shown several deflation/inflation trends over the past week. Summit earthquake activity remains low and eruptive tremor (a signal associated with fluid movement) is present. A sulfur dioxide emission rate of 3,000 tonnes per day was measured on January 20. For Kilauea monitoring data, see https://www.usgs.gov/volcanoes/Kilauea/past-week-monitoring-data-Kilauea.
Mauna Loa is not erupting. Its USGS Volcano Alert Level is at ADVISORY. Mauna Loa updates are issued weekly on Thursdays.
Webcams show no signs of activity on Mauna Loa. Seismicity remains low. Deformation rates show inflation somewhat above background levels, but this is not uncommon following eruptions. Sulfur dioxide (SO2) emission rates are at background levels. For Mauna Loa monitoring data, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring-data.
There were four earthquakes with 3 or more felt reports in the Hawaiian Islands during the past week: a M3.6 earthquake 13 km (8 mi) ESE of Pahala at 30 km (18 mi) depth on Jan. 24 at 10:35 p.m. HST, a M3.1 earthquake 17 km (10 mi) ESE of Naalehu at 35 km (21 mi) depth on Jan. 23 at 11:58 p.m. HST, a M3.1 earthquake 16 km (9 mi) E of Naalehu at 35 km (22 mi) depth on Jan. 21 at 10:27 p.m. HST, and a M3.0 earthquake 8 km (4 mi) E of Pahala at 31 km (19 mi) depth on Jan. 20 at 12:32 a.m. HST.
HVO continues to closely monitor the ongoing eruption at Kilauea, and Mauna Loa.
Please visit HVO’s website 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.