Volcano Watch – Tilting toward lava: How tiltmeters monitor volcano activity

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

Over the past century, technological advancements have vastly improved volcano monitoring. One key innovation was the introduction of modern borehole tiltmeters, devices that measure very small changes in the inclination of the volcano’s surface.

Borehole tiltmeters have been used by the USGS Hawaiian Volcano Observatory (HVO) since the early 1970s and have since become an essential part of HVO’s volcano monitoring program. An older style of instrument called a “water tube tiltmeter” goes back even further to the 1950s. Today the modern tiltmeter network on the Island of Hawaiʻi forms part of a larger array of monitoring tools, including seismic stations, GPS receivers, gas sensors, and webcam/satellite imagery. Together, these tools help scientists keep a close eye on the changing behaviors at volcanoes that may lead to eruptions.

A tiltmeter is a sensitive instrument designed to detect very slight changes in deformation of the ground. They are installed around volcanoes to monitor changes in the Earth’s surface caused by magma moving underground. These movements often precede eruptions, as pressure from magma pushes against the surrounding rock, causing the surface to bulge or shift slightly.

Imagine a carpenter’s level laid on the ground at Kīlauea, oriented radially away from Kaluapele (the summit caldera). As magma enters the magma reservoir beneath the caldera and the volcano inflates, the end of the carpenter’s level closest to the caldera is raised upwards, and the bubble in the level moves inward and upward.

Tiltmeters work in much the same way, only with much higher precision. HVO tiltmeters detect changes as small as five nanoradians—that’s less than one millionth of a degree—or the equivalent the Uēkahuna bluff being raised in elevation about the thickness of a human hair relative to the center of Kaluapele. This level of precision makes them invaluable for tracking subtle changes in volcanic activity and providing early warnings to scientists.

More than a dozen borehole tiltmeters are strategically installed on Kīlauea and Mauna Loa at key locations across the volcano summits and calderas. These areas are of particular interest because they are most likely to experience significant ground deformation during periods of volcanic unrest and before an eruption onset.
These tiltmeters operate continuously and produce one data point every 60 seconds, transmitting data in near real-time to HVO. This data is critical for early detection of volcanic activity. For example, when magma begins to rise toward the surface, it can cause noticeable tilting of the ground, which is recorded by the tiltmeters. By analyzing multiple monitoring datasets, scientists can determine where magma is moving and whether an eruption may be imminent.

Borehole tiltmeters need routine maintenance including changing batteries and upgrading the radio telemetry used to send the data back to HVO. Each tiltmeter also has a limited range of tilt over which it can accurately record deformation. For example, our analog tiltmeters, such as the instrument at Uēkahuna bluff at the summit of Kīlauea, need to be manually leveled in their boreholes if deformation exceeds 300 microradians. Then, the tiltmeter will need time to “settle” from the physical disturbance before the data can be used quantitively—although they remain qualitatively useful, providing indications of short-term volcanic processes. Other digital tiltmeters, however, can be leveled remotely with no interruption in data quality.

Tiltmeters have been particularly useful in tracking changes as Kīlauea’s summit inflates and deflates, and large changes can indicate that magma is migrating. Kīlauea’s tiltmeter network provided valuable information about magma moving from the summit to the middle East Rift Zone during the several intrusions leading up to the most recent eruption in and near Nāpau Crater from September 15–20, 2024.

Mauna Loa has also been under close surveillance by HVO’s tiltmeter network. Although less active than Kīlauea in recent decades, Mauna Loa is still capable of producing hazardous eruptions. In the months leading up to and in the hours during the initial onset of the 2022 Mauna Loa eruption, tiltmeters played a critical role in helping scientists track unrest and heightened summit deformation.

Tiltmeters are a crucial component of the volcanic monitoring network in Hawaii. By detecting subtle changes in ground inclination, they provide early warning signals of volcanic unrest and help scientists to better understand the behavior of Hawaii’s dynamic volcanoes.

Volcano Activity Updates

Kīlauea is not erupting. Its USGS Volcano Alert level is ADVISORY.

Over the past week, earthquake rates beneath Kīlauea summit and upper-to-middle East Rift Zone were on par with the previous week. About 30 earthquakes were located beneath the summit, and about 200 were located in the upper-to-middle East Rift Zone. Ground deformation rates show slow inflation at the summit with continued slow magma accumulation near the September 15-20 middle East Rift Zone eruption site. Future intrusive episodes and eruptions could occur with continued magma supply. The most recent measurement of sulfur dioxide (SO2) emission rate at the summit was 60 tonnes per day on Sept. 17, 2024, and SO2 emissions were not detected at downwind of the eruption site on Sept. 23.

Mauna Loa is not erupting. Its USGS Volcano Alert Level is at NORMAL.

No earthquakes were reported felt in the Hawaiian Islands during the past week.

HVO continues to closely monitor Kīlauea and Mauna Loa.

Visit HVO’s website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information, and more. Email questions to askHVO@usgs.gov.