Non-Traditional Earthquakes on Hawai‘i Island

Jun 14, 2018

Events at the summit of Kīlauea over the past few weeks have dramatically reshaped Halema‘uma‘u, shown here in this aerial view, which looks west across the crater. The obvious flat surface (photo center) is the former Halema‘uma‘u crater floor, which has subsided at least 100 m (about 300 ft) during the past couple weeks
Credit U.S. Geological Survey

The ongoing eruption at Kīlauea summit continues to produce ash and gas explosions around once a day. Each is preceded by dozens of relatively small seismic events, but they are not what you might think of as traditional earthquakes.

When most people think of an earthquake, they imagine a shift in the earth’s tectonic plates, or along a fault. What is being recorded now as “seismic activity” at Kīlauea Summit is slightly different

Tina Neal is Hawaiian Volcano Observatory’s Scientist in Charge:

“They’re not earthquakes in the classical sense, they’re kind of a hybrid process creating a lot of ground shaking  measured the same way we try to measure earthquakes. There’s a lot of energy being released, the combination of an explosion process, which has something to do with a sudden release of pressure, we think, from the upper magma reservoir. The bottom is filled with rubble, probably many hundreds of meters thick and those explosions are moving a lot of earth. All of these things are contributing to what we are measuring as a single earthquake.”

Most of the dozens of seismic events every day, in the 1 to 3 magnitude range, are too small to feel outside of Hawai'i Volcanoes National Park and the Volcano Village area. Around once a day, a gas and ash explosion at the summit registers as around 5.0 to 5.5.  They have caused damage inside the Park, but so far only minimal damage has been reported elsewhere.

Tina Neal, talking with HPR reporter Sherry Bracken.
Credit Janet Babb

Hawaiian Volcano Observatory Geophysicist Brian Shiro:

“These large explosive events are what’s driving the seismic observations  we record. But this is not a typical earthquake where it’s slippage along a fault. The mechanism of the movement is different. To distinguish the source type from what people expect as an earthquake, these are referred to as eruptions. The USGS earthquake program often puts out magnitudes for explosions at nuclear test sites and mining blasts, things that are not earthquakes also.”