Wednesday | October 18, 2017
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Mauna Loa is still the largest active volcano on Earth

Despite reports to the contrary, Mauna Loa is still the largest active volcano on Earth. The volcano off the east coast of Japan, which made the news last week — touted as the largest volcano in the solar system — last erupted 146 million years ago, possibly around the time the Pacific Ocean Basin was formed.

Mauna Loa, on the other hand, has erupted 33 times in the past 170 years — most recently in 1984 — and future eruptions are certain. The USGS Hawaiian Volcano Observatory, with recently upgraded monitoring networks, keeps a close watch on any changes beneath the volcano that might signal its next eruption.

Speaking of which, between Sept. 5 and 7 — seemingly timed to respond to the news stories that challenged Mauna Loa’s stature as Earth’s largest volcano — HVO seismic networks recorded a small earthquake swarm just west of its summit. The swarm consisted of more than 350 detected and tightly clustered earthquakes at a depth of about 4 miles, but only about 25 were strong enough to be located. The strongest was a magnitude-2.4 earthquake, and none were reported felt.

The swarm was in the same region where earthquakes began to occur a year or more before Mauna Loa’s 1975 and 1984 eruptions. It’s an area to which HVO pays especially close attention because of this connection.

While last week’s swarm was small in historical terms, it was the first cluster of earthquakes in this region of Mauna Loa since its 1984 eruption. Therefore, it caught HVO’s attention as a possible, but not definite, precursor to the next eruption of Mauna Loa. Only through continued monitoring over the coming weeks-to-months will the true meaning of the recent small earthquake swarm be known.

But this swarm is not the only Mauna Loa activity to have occurred recently. After the eruption in 1984, Mauna Loa immediately began re-inflating as magma once again filled and pressurized storage reservoirs beneath the summit caldera. Inflation waned in 1993, then resumed in May 2002.

In the second half of 2004, there was an intense swarm of about 2,000 long-period earthquakes more than 19 miles below the summit of the volcano. This swarm was possibly part of the deep magma system that fed the ongoing inflation.

The rate of inflation increased in 2004, but started to slow in 2006. These data fit the pattern produced by magma intruding 2.5 to 5 miles beneath the summit area. After 2009, Mauna Loa inflation continued, but very slowly and sporadically, so the volcano is poised for its next eruption.

This is indeed an interesting time for Mauna Loa, and HVO is well-prepared to characterize any future changes on the volcano. Until the 1990s, our deformation measurements were made by timing a laser pulse across Mauna Loa’s summit caldera and by standard surveying techniques. It was hard work, and it couldn’t be done as often as we wished, because of limited personnel and severe winter weather.

By the turn of the century, we were able to make many more deformation measurements automatically, continuously, and in near real-time, using GPS technology. The upgrade of our seismic monitoring network, funded by the American Recovery and Reinvestment Act of 2009, provided a robust and more sensitive monitoring tool. This enhanced seismic network, along with the GPS network, is ideal for tracking the activity of Hawaiian volcanoes.

HVO also offers increased availability of real-time earthquake data to the public — locations and the actual seismometer traces, webicorders, can be viewed at To view webicorder sites closest to the area in which the recent swarm occurred, click “Webicorders” and choose “ALEP” or “TOUO.”

HVO has good geophysical data prior to and during Mauna Loa’s previous two eruptions, and we are comparing those data with the recently observed events on the volcano. For now, there’s no need to worry — but we should never lose sight of the fact that Mauna Loa is an active volcano, the largest on Earth, and it will erupt again.

Kilauea activity update

A lava lake within the Halemaumau Overlook vent produced nighttime glow visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. A deflation-inflation cycle Sept. 6 and 7 caused the summit lava lake level to fluctuate sympathetically.

On Kilauea’s east rift zone, active breakouts on the coastal plain had died out by Sept. 8, and lava is no longer entering the ocean. A small breakout on the Peace Day flow field started this past week, high above the pali. The Kahaualea 2 flow, fed from a spatter cone on the northeast edge of the Puu Oo crater, continues to burn forest north of Puu Oo.

No earthquakes were reported felt on Hawaii Island in the past week.

Visit for Volcano Awareness Month details and Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to

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