Tilt measurements still vital to volcano monitoring after more than a century

USGS-HVO — Measuring how a volcano deforms in response to moving magma is one of the cornerstones of volcano monitoring. Our instrumentation includes a number of newer, satellite-based methods but another important instrument has been around a lot longer: the tiltmeter. Tilt data was the first geodetic data collected by the Hawaiian Volcano Observatory (HVO) and continues to be incredibly important both for monitoring and basic research in volcano behavior. In the early 20th century, scientists were only beginning to recognize that volcanic eruptions (and earthquakes) were accompanied by “topographic changes” (as they were called at the time). In Japan, one of the forebearers of seismology, Dr. Fusakichi Omori, realized that the boom arm on his seismometer affected by ground tilt and that this annoyance was important data. In 1917, Dr. Thomas Jaggar began tracking tilt changes at Hawaiian volcanoes. At first, by looking at the deflections of the HVO’s Omori and Bosch-Omori seismometers. Later, by constructing specially designed “clinoscopes.” These provided a rough idea of the amount of tilt over the span of a day to a week. In the 1950s, HVO scientist Dr. Jerry Eaton designed an improved tiltmeter using water. This apparatus provided a precise tilt measurement to be used to track daily or even hourly tilt. The “water tube tiltmeters” allowed HVO scientists to track the movement of Kīlauea’s summit through several eruptions, providing a continuous record of inflation and deflation. The water tube tiltmeter consists of three “pots” of water connected by tubes such that the water can freely flow between them. One pot is placed in the center, and the other two are placed east and north of the center pot. The water in the tubes will always seek to be level, but when the ground tilts and moves the pots up or down, it will look like the water level is moving down or up. By reading the depth of the water in each pot, one can work out how much the ground has tilted since the last reading. Water tube tilt data was especially valuable because it could be collected regularly to form a continuous time series. This means data was collected whether it seemed important in the moment or not. This kind of continuous record facilitates discoveries that wouldn’t be possible if data was only collected during eruptions. In particular, the water tube tiltmeters showed cycles of steep inflationary tilt between eruptions and then sudden deflation as eruptions drained magma chambers. However, once Puʻuʻōʻō began erupting on Kīlauea’s middle East Rift Zone in 1983, these cycles became much more subdued. In the early 1970s, electronic tiltmeters began to be installed around Kīlauea. These instruments are installed in boreholes about 16 feet (5 meters) deep, such that they are protected from the weather, and can provide tilt measurements down to a fraction of a microradian every minute. After the end of the Puʻuʻōʻō eruption in 2018, the character of tilt at Kīlauea’s summit changed again. Electronic tiltmeters began to record steep inflationary tilt, not too different from the rates observed by the water tube tiltmeter in the 1950s–1970s. Major eruptions, similar to Kīlauea Iki or Maunaulu—which resulted in large summit deflations—have not occurred recently, but the similarity of the current records to those from the 1950s-1970s is yet another sign that Kīlauea is not too different now from how it was before the 2018 eruption. This means that the data collected, and the lessons learned about Kīlauea’s plumbing, are still applicable today. Scientists can still use Kīlauea’s past behavior to make forecasts about future behavior and test out new ideas for what may have happened in the past. For example, currently the tiltmeter at Uēkahuna bluff (UWE) is not showing major tilt changes, but another tiltmeter south of Kīlauea’s caldera (SDH) has shown a steep increase over the past couple weeks. By looking back at past records, scientists recognize that this is likely due to magma accumulation in a South Caldera reservoir, which occurs intermittently; most recently in 2015 and 2021. The long and faithful recording of tilt at Kīlauea gives scientists a boost to understanding current processes and could facilitate new discoveries about the past.­ ---- Images and captions from USGS: [1] Graph of tilting at Uēkahuna bluff—at the summit of Kīlauea—in a direction toward and away from Halemʻaumʻau, collected by a water tube tiltmeter (black) and an electronic tiltmeter (grey). Major intrusions and eruptions are marked by blue and pink arrows respectively. USGS plot. [2] A water tube tiltmeter. The tiltmeter is read by looking into the portal on each pot and adjusting the sight until two pins appear to touch. In fact, one of the pins is a reflection, so having the pins almost touch establishes the exact water level within the pot. The difference in water level gives the tilt, and repeated measurements give changes in tilt 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 HVO research geophysicist Ingrid Johanson.