‘A‘ā lava flows erupt from the Northeast Rift Zone of Mauna Loa on March 25, 1984—the first day of the volcano’s most recent eruption. USGS photo.

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Over the past few months, Mauna Loa, Hawai‘i Island’s largest volcano, has shown subtle signs of stirring from its 31-year-long slumber (its most recent eruption began on March 25, 1984). The U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO) has recorded numerous small earthquakes beneath Mauna Loa’s summit and western flank, and has detected slight expansion across Moku‘āweoweo, the volcano’s summit caldera—signals that Mauna Loa should not be forgotten!

What can we expect as this great volcano reawakens and builds toward its next eruption?

Generally, as magma rises and eventually infiltrates and fills Mauna Loa’s summit magma reservoir, pressure builds within the volcano. When sufficient pressure is achieved, the volcano expands to accommodate the additional volume of molten rock within it. During this expansion, the flanks of the volcano move apart and the summit moves upward. The increased pressure also generates stresses that result in earthquakes, which accounts for the increased numbers of earthquakes recently recorded by HVO’s seismic network. Historical accounts of the effects and felt reports of earthquakes suggest that many of Mauna Loa’s 33 eruptions since 1843 were preceded by precursory earthquake activity.

HVO tracks activity at Mauna Loa with a variety of tools including Global Positioning System (GPS) receivers, tiltmeters, radar satellites, gas sensors, remote cameras, and seismometers. We use patterns seen in all of these data to forecast when the volcano might erupt.

Instrument-based volcano monitoring in Hawai‘i began when HVO was established in 1912. The emergence of electronics in the 1950s and 1960s gave rise to networks of seismographs and laser-ranging Electronic Distance Measurement (EDM) instruments that greatly improved our ability to monitor and forecast volcanic activity.

Mauna Loa’s two most recent eruptions, in 1975 and 1984, occurred as HVO expanded and modernized its seismic and geodetic monitoring networks. Examining these past eruptions can give us insight into how Mauna Loa will behave in future eruptions.

Prior to the 1975 eruption, the length of an EDM line spanning Moku‘āweoweo lengthened as magma accumulated within the volcano and caused it to inflate. In addition, more than a year before lava erupted in July 1975, HVO recorded a significant increase in earthquakes, with the seismicity concentrated in two distinct regions within the volcano. Earthquakes initially clustered northwest of Mauna Loa’s summit at depths between 5 and 10 km (3–6 mi) were followed by shallow earthquakes that were concentrated less than 5 km (3 mi) beneath the summit. In the months leading to the 1975 eruption, the number of earthquakes dramatically increased, reaching levels of several hundred events per day.

The sequence of ground deformation and seismicity followed a similar pattern prior to Mauna Loa’s 1984 eruption. Increased numbers of earthquakes were again seen in the two areas where seismicity was concentrated before the 1975 eruption, and EDM measurements across Moku‘āweoweo showed extension (inflation) months before the 1984 eruption.

How do current observations of Mauna Loa compare to the previous two eruptions?

The recent swelling of the volcano is small compared to that observed in 1975 and 1984. Earthquake activity, while notable, is also modest. In terms of magnitude, recent earthquakes beneath the volcano’s northwest flank have not yet reached levels recorded before the 1975 or 1984 eruptions. Additionally, the number of earthquakes beneath the summit is not yet significant. Overall, we expect more persistent and heightened rates of both ground deformation and seismicity as the volcano nears its next eruption.

Recent improvements in HVO’s monitoring capabilities enhance our ability to watch for and track changes on Mauna Loa. With upgrades to and expansion of our seismic network and the installation of additional of GPS stations, tiltmeters, gas sensors, and webcams, we can better monitor Mauna Loa and other active Hawaiian volcanoes. This, in turn, helps improve our understanding of how these volcanoes work and our ability to forecast eruptions.

The take-home message today is two-fold: (1) Mauna Loa is an active volcano, but an eruption is not imminent, and (2) HVO closely monitors Mauna Loa and will immediately inform authorities and the public if significant changes in activity are detected. For now, monthly reports on the status of Mauna Loa are posted on HVO’s website at http://hvo.wr.usgs.gov/activity/maunaloastatus.php

Time-lapse multi-image movie of Mokuʻāweoweo Caldera from the Northwest Rim on Mauna Loa. March 13-20, 2015. Images courtesy of USGS/HVO