Volcano Watch — Watching volcanoes move, one survey at a time
The USGS Hawaiian Volcano Observatory (HVO) relies heavily on Global Positioning System (GPS) data to monitor volcanic activity in Hawaii. Data are collected by a combination of permanent stations and annual surveys, which give scientists a picture of Mauna Loa’s changing shape over time.
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. Today’s article is by Research Corporation of the University of Hawaiʻi geophysicist Brianna Corsa.
In addition to a dense network of permanent GPS stations which provide daily positions, HVO also conducts annual campaign GPS surveys to collect high-precision GPS data at benchmark locations across the island. The 2026 annual campaign over Mauna Loa was just recently completed. HVO has been measuring some of these benchmark locations for decades, first with survey techniques like leveling and Electronic Distance Measurement (EDM) and then, starting in 1994, with GPS instruments.
Mauna Loa is the largest active volcano on Earth and has erupted 34 times since 1843. The most recent eruption occurred from November 27th-December 10th, 2022; initial fissures opened within the summit caldera, Mokuʻāweoweo, and later Northeast Rift Zone vents produced lava flows that extended close to 12 miles (19 kilometers) and covered an area of about 16.5 square miles (43 square kilometers). Given Mauna Loa’s history of producing large, fast-moving lava flows, it is important that HVO continues to monitor ground deformation on various time scales to understand its behavior.
Data from the campaign survey increases the spatial coverage of the HVO geodetic network and improves our understanding of the long-term deformation field of Mauna Loa, including how the surface changes in response to magma accumulation or migration below the ground surface. For example, motions away from a volcano’s summit might indicate inflation, growth, or rise of the magma reservoir beneath the volcano. GPS positions pointing inwards suggest deflation or migration of magma away from a region.
Patterns of deformation help prepare scientists for potential hazards, which in turn allows for effective communication of hazards to nearby communities and time for safety efforts to take place. Benchmarks that remain relatively stable through time provide important and reliable ground control points for mapping, calibrating imagery, validating models, and a variety of other research endeavors.
Twenty-two benchmarks on Mauna Loa were surveyed this year, requiring a staged deployment over several days. Each survey set-up consists of a GPS antenna that receives the signals from the satellite, a GPS receiver that records the incoming data, and a tripod, tribrach with an optical plummet, and mounting puck that positions the antenna precisely over the benchmark.
Because we are trying to measure positions with accuracy of a fraction of an inch, it is crucial that the equipment is carefully set up, which can be challenging over varied terrain like lava flows! This means it is important that the antenna is precisely aligned over the benchmark, and that it is level and oriented towards north. The optical plummet in the tribrach allows us to sight down to the benchmark and make the adjustments needed to center the antenna.
The instruments record data for 3-5 days before the team returns to collect the equipment and then redeploy them at the next benchmark locations with fresh batteries. This window of time is needed for the specialized data processing that we use to produce precise positions. It allows for certain measurement uncertainties to be resolved and reduces the errors due to atmospheric effects.
This year’s survey results align well with trends from the past 10+ years. All benchmarks around the summit moved up and away from their original locations, while several stations on the volcano’s flanks moved slightly down and away. As described earlier, these patterns indicate continued inflation directly beneath Mokuʻāweoweo caldera. Average rate of ground motion at the summit from 2025-2026 is about 2.9 inches per year (7.4 centimeters per year), consistent with rates from past campaign surveys and those measured at permanent GPS stations from after the eruption in 2022 (see Volcano Watch article from February 26, 2026).
While Mauna Loa continues to slowly accumulate magma below the surface, seismicity and gas emissions at Mauna Loa’s summit have remained at low, steady levels over the same period, indicating that an eruption is not expected in the immediate future. Nevertheless, HVO scientists continuously monitor deformation to detect subtle changes in activity to provide Hawaiʻi residents with the earliest possible warning of renewed unrest.
