Volcano Monitoring: How do inclinometers monitor volcano activity?

Over the past century, technological advances have greatly improved volcano monitoring. One of the major innovations was the introduction of modern borehole inclinometers, which measure very small changes in the slope of the volcano’s surface.

Borehole inclinometers have been used by the USGS Hawaiian Volcano Observatory since the early 1970s and have since become an important part of HVO’s volcano monitoring program. An older style of instrument called the “water tube inclinometer” dates back to the 1950s. Today, the modern network of inclinometers on Hawaii Island forms part of a wider 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 changing behavior in volcanoes that could lead to eruptions.

An inclinometer is a sensitive instrument designed to detect very small changes in the deformation of the ground. They are installed around volcanoes to monitor changes on the Earth’s surface caused by magma moving underground. These movements often occur before eruptions, as the pressure of the magma pushes against the surrounding rock, causing the surface to swell or shift slightly.

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

Inclinometers work much the same way, but with much higher precision. HVO inclinometers detect changes as small as five nanoradians (that’s less than one millionth of a degree), or the equivalent of the Uekahuna bluff, which rises about the thickness of a human hair above the center of Kaluapele. This level of sensitivity makes them invaluable for monitoring subtle changes in volcanic activity and providing scientists with early warning.

More than a dozen borehole inclinometers were strategically placed at key points along volcano summits and calderas on Kilauea and Mauna Loa. These areas are of particular interest because they are likely to undergo significant ground deformation during periods of volcanic turmoil and before the onset of an eruption.

These inclinometers operate continuously, producing a data point every 60 seconds, transmitting data to HVO in near real time. These data are critical for early detection of volcanic activity. For example, when magma begins to rise towards the surface, it can cause the ground to visibly tilt, which is recorded by inclinometers. By analyzing multiple monitoring datasets, scientists can determine where magma is moving and whether an eruption may be imminent.

Borehole inclinometers need routine maintenance, including replacing batteries and upgrading the radio telemetry used to send data back to the HVO. Each inclinometer also has a limited range of inclinations over which it can accurately record deformation. For example, our analog inclinometers, such as the device located on the Uekahuna bluff at Kilauea summit, must be manually balanced in boreholes if deformation exceeds 300 microradians. The inclinometer will then need time for the physical discomfort to “settle” before the data can be used quantitatively; however, they remain qualitatively useful and provide indications of short-term volcanic processes. However, other digital inclinometers can be leveled remotely without any interruption to data quality.

Inclinometers have been particularly useful in tracking changes as Kilauea’s summit swells and deflates, and large changes can indicate magma is migrating. Kilauea’s tiltmeter network provided valuable information about magma moving from the summit to the Middle East Rift Zone during various intrusions that led to the most recent eruption in and near Napau Crater between September 15 and 20.

Mauna Loa is also under close surveillance by HVO’s network of inclinometers. Although less active than Kilauea in recent years, Mauna Loa is still capable of producing dangerous eruptions. In the months before and during the early hours of the 2022 Mauna Loa eruption, inclinometers played a critical role in helping scientists track unrest and increasing summit deformation.

Inclinometers are a crucial component of the volcanic monitoring network in Hawaii. By detecting subtle changes in ground slope, it provides early warning signals of volcanic unrest and helps scientists better understand the behavior of Hawaii’s dynamic volcanoes.

Volcano activity updates

Kilauea is not erupting. The USGS Volcano Alert level is ADVISORY.

Last week, earthquake rates below Kilauea’s summit and the upper-central East Rift Zone were at the same level as the previous week. Approximately 30 earthquakes were detected below the summit, and approximately 200 were located in the Upper-Middle East Rift Zone. Ground deformation rates indicate slow inflation at the peak, with continued slow magma accumulation near the September 15-20 Middle East Rift Zone eruption area. With continued magma supply, intrusive events and eruptions may occur in the future. The last measurement of the sulfur dioxide (SO2) emission rate at the peak was 60 tonnes per day on September 17, and no SO2 emissions were detected downwind of the eruption site on Monday, September 23.

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

No earthquakes were felt in the Hawaiian Islands last week.

HVO continues to monitor Kilauea and Mauna Loa closely.

Please visit HVO’s website for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email your questions to [email protected].