Volcano Watch: How tiltmeters track volcanic activity

Over the past century, technological advances have greatly improved volcano monitoring. One of the key innovations was the introduction of modern borehole tiltmeters—devices that measure very small changes in the slope of a volcano’s surface.

Downhole tiltmeters have been used by the USGS Hawaiian Volcano Observatory since the early 1970s and have since become an important part of the HVO volcano monitoring program. An older type of instrument called a “water tube tiltmeter” dates back to the 1950s. Today, the state-of-the-art network of tiltmeters on the island of Hawaii is part of a broader suite of monitoring tools, including seismic stations, GPS receivers, gas sensors, and webcams/satellite imagery. Together, these instruments help scientists closely monitor changes in volcanic behavior that can lead to eruptions.

A tiltmeter is a sensitive instrument designed to detect very small changes in soil deformation. They are installed around volcanoes to monitor changes on the Earth’s surface caused by the movement of magma underground. These movements often precede eruptions as magma pressure pushes down on the surrounding rock, causing the surface to bulge or move slightly.

Imagine a carpenter’s level laid on the ground at Kilauea and oriented radially from Kaluapele (summit caldera). When magma enters the magma reservoir beneath the caldera and the volcano swells, the end of the carpenter’s level closest to the caldera rises upward, and the bubble on the level moves inward and upward.

Inclinometers work in much the same way, only with much greater accuracy. HVO’s tiltmeters detect changes of just five nanoradians—less than one millionth of a degree—or the equivalent as the Huekahuna cliff rises to a height of about the width of a human hair relative to the center of Kaluapele. This level of accuracy makes them invaluable for tracking subtle changes in volcanic activity and providing early warning to scientists.

More than a dozen borehole tiltmeters are strategically installed on Kilauea and Mauna Loa at key points on the volcanoes’ 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 begins.

These tiltmeters operate continuously and produce one data point every 60 seconds, transmitting data to the HVO in near real time. This data is critical for early detection of volcanic activity. For example, when magma begins to rise toward the surface, it can cause a noticeable tilt of the ground, which is detected by tiltmeters. By analyzing multiple monitoring data sets, scientists can determine where magma is moving and whether an eruption may be imminent.

Downhole tiltmeters require regular maintenance, including replacing batteries and upgrading the radio telemetry used to send data back to the HVO. Each tiltmeter also has a limited tilt range over which it can accurately record deformation. For example, our analog tiltmeters, such as the one on the Huekahuna cliff at the summit of Kilauea, must be manually aligned in boreholes if deformation exceeds 300 microradians. The tiltmeter will then need time to “stabilise” from the physical disturbance before the data can be used quantitatively – although qualitatively it remains useful, providing indications of short-term volcanic processes. However, other digital tiltmeters can be leveled remotely without compromising data quality.

Tilt meters have proven particularly useful in tracking changes as Kilauea’s summit inflates and deflates, and large changes can indicate magma migration. Kīlauea’s network of tilt meters provided valuable information about the movement of magma from the summit to the near eastern rift zone during several intrusions leading to the most recent eruption in and near Napau Crater, which occurred September 15–20.

Mauna Loa is also closely monitored by the HVO tiltmeter network. Although Mauna Loa has been less active than Kilauea in recent decades, it is still capable of causing dangerous eruptions. In the months leading up to and in the hours leading up to Mauna Loa’s 2022 eruption, tiltmeters played a critical role in helping scientists track unrest and increasing deformation at the summit.

Inclinometers are a critical component of Hawaii’s volcanic monitoring network. By detecting subtle changes in the earth’s tilt, they provide early warning signals of volcanic unrest and help scientists better understand the behavior of Hawaii’s dynamic volcanoes.

Volcano Activity Updates

Kilauea does not erupt. The USGS Volcano Alert Level is ADVICE.

Over the past week, the frequency of earthquakes beneath Kilauea’s summit and in the upper and middle eastern rift zone has been similar to the previous week. About 30 earthquakes occurred below the summit, and about 200 in the upper and middle eastern rift zone. The rate of ground deformation indicates slow inflation at the summit with continued slow accumulation of magma near the site of the September 15-20 eruption in the Middle East rift zone. Future intrusive episodes and eruptions may occur as magma continues to flow. The last measurement of sulfur dioxide (SO2) emissions at the summit was 60 tons per day on September 17, and no SO2 emissions were detected downwind of the eruption site on Monday, September 23.

Mauna Loa does not erupt. The USGS Volcano Alert Level is at NORMAL.

Over the past week, no earthquakes have been recorded in the Hawaiian Islands.

HVO continues to closely monitor Kilauea and Mauna Loa.

Visit the HVO website for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, information on recent earthquakes and more. Email questions to [email protected].