An earthquake that rattled the Big Island on Tuesday evening had nothing to do with any current or recent volcanic eruptions, experts say.
At about 9:30 p.m. Tuesday, a magnitude-4.8 earthquake shook the island, emanating from a point about seven miles southeast of Pahala and about 20 miles below sea level, according to the Hawaiian Volcano Observatory.
HVO seismologist Ninfa Bennington on Wednesday said the quake was a part of a greater cluster of quakes around Pahala, referred to as the “Pahala swarm,” that has been ongoing since 2015.
More than 400 quakes have been detected in the area within the past four weeks, mostly with magnitudes between 2.0 and 3.0, according to HVO data.
“They aren’t related to any eruptive activity,” Bennington said. “There’s just a lot of these quakes all between 20 and 40 kilometers deep, and they all originate from generally the same area below Pahala.”
Neither the 2018 eruption of Kilauea nor the 2022 Mauna Loa eruption brought any changes to the frequency or intensity of the Pahala quakes, Bennington said, although she added that in 2019, the number of quakes in the swarm increased “about 70-fold” for a reason that remains unknown.
Bennington said Tuesday’s quake was not unusually powerful compared to other quakes within the swarm, explaining that occasional magnitude-5.0 quakes are detected in the area.
As for why there are so many quakes below Pahala, there are fewer answers.
Bennington said HVO is “still studying” the cause of the swarm, but added that a 2022 report by the California Institute of Technology determined that the nature of the quakes suggests they originate from some form of magma storage network connected in some way to other magma supply systems beneath the island.
That paper — published in “Science” magazine in December — concludes that several layered flat sheets or sills of magma beneath Pahala draw material from the Earth’s mantle and ultimately adds it to the magma plumbing of other Big Island volcanoes, possibly both Kilauea and Mauna Loa.
Whether those conclusions are accurate is still up for debate, Bennington said, adding that as far as she knows, the swarm is one of a kind.
“I’m not aware of any analogues to compare the swarm to anywhere else,” Bennington said.
Email Michael Brestovansky at mbrestovansky@hawaiitribune-herald.com.