On March 25, HVO staff and collaborators conducted the annual gravity survey in Kaluapele (the summit caldera of Kīlauea volcano). Measurements of gravity over time can show how mass is distributed beneath a volcano. At Kīlauea, these routine microgravity surveys help the observatory to monitor volcanic activity and to determine changes in gravity. USGS photo by A. Ellis.

At each gravity survey location, a kinematic GPS station is deployed nearby for precise location data, used to correct for the variety of factors that influence gravity results—especially vertical deformation. This survey location is located near the CALS continuous GPS site on the portion of Kīlauea caldera that collapsed in 2018. The CALS station has not been covered by lava during recent eruptions within Kīlauea caldera, and it is a part of HVO's telemetered monitoring network. Photo by E. van Dalfsen

A gravimeter being deployed on the floor of Kīlauea caldera, with a GPS station located on a tripod nearby. The gravimeter is the small, shoebox-sized instrument, which can measure a change in the force of gravity to one-in-one billionth of the force you feel every day. USGS photo by A. Ellis.

A volcano observatory scientist and a collaborator carry the gravity survey instruments across spatter deposits from the September 2023 Kīlauea summit eruption. Only one benchmark survived the September eruption; comparing data collected there during previous surveys to data collected there during the recent survey will allow the scientists to see changes over time. USGS photo by A. Ellis.

Gravity measurements detect subsurface mass change—for example, magma accumulation or removal beneath the surface. If magma is filling or draining void spaces, it may not be signaled by ground deformation or earthquake activity, but it will be signaled by gravity because of the mass change. In this way, gravity is a way of detecting changes in the magma reservoir that might otherwise be hidden. This photo shows scientists taking gravity measurements on the floor of Kīlauea caldera. Lava flows from Kīlauea summit eruptions in 2023 steam in the background, and the caldera walls rise above. The scientist is straddling the gravimeter (small silver box) to shield it from the wind. USGS photo by A. Ellis.

A scientist takes gravity measurements on the sloping floor of Kaluapele. This part of the caldera floor, referred to as the "down-dropped block" moved downwards during the Kīlauea summit collapse of 2018. The scarp that formed as a result of the collapse is visible in the background of the photo, with the spatter cones that formed during the September 2023 eruption on the caldera floor visibly steaming. Photo by E. van Dalfsen.

During March of 2024, HVO is conducting a GPS survey of Hualālai volcano. This survey, usually done every three years, hasn't been conducted since 2015 due to eruptions at Kīlauea and Mauna Loa, the pandemic, and staff/personnel rotations. In this photo, HVO staff deploy a campaign GPS unit at station KEHU on Ka'upulehu crater (elevation 1894 m or 6213 ft) on Hualālai's northwest rift. This station is near the source vent for Hualālai's most recent eruption, in 1800–1801. USGS photo by A. Flinders.

To conduct the Hualālai campaign survey, HVO staff requested permission from land managers before navigating to benchmarks cemented into the ground surface. Data collected at each location will together provide a picture of how the ground surface of Hualālai volcano has changed since the previous survey in 2015. The survey data will also provide important "background" information, supplementing and extending a several decades-long timeseries for monitoring Hualālai. USGS photo by A. Flinders.

HVO chief field engineer sets up a campaign GPS GNSS unit at station KEHU on Ka'upulehu crater as part of the ongoing Hualālai survey. During the survey, several GPS instruments are deployed at established benchmarks; their recorded positions can be compared with those from previous years to discern subtle patterns of ground deformation since the previous survey in 2015. USGS photo by A. Flinders.

The HVO geodetic network manager sets up a campaign GPS unit at station KIPA on Kipahe'e crater (elevation 2331 m or 7647 ft) of Hualālai. USGS photo by A. Flinders.