For a long-dormant volcano, Hawaii’s Mauna Kea is pretty active.

According to a new study, more than a million deep, low-frequency earthquakes have rumbled beneath it over the past two decades in what the researchers describe as a record of seismic activity unlike any other on Earth.

These tiny tremblors have occurred with remarkable regularity – once every seven to 12 minutes – and respond to the ocean’s tides and other seismic events with combined energy equating roughly to a magnitude 3.0 earthquake each day for decades. 

But there is no need for the many daily visitors to the highest point in Hawaii or those who work at the many astronomical observatories to be nervous, Aaron Wech from the US Geological Survey and colleagues suggest in a paper in the journal Science Advances.

While earthquakes near volcanos often herald future eruption, Mauna Kea’s unique seismicity is more likely tied to the “second boiling” of cooling magma deep below the dead volcano.

Deep long-period earthquakes (DLPs) are a curious type of volcanic tremblor tied most often too quiet or inactive volcanos. However, the processes responsible for triggering DLPs are poorly understood and often attributed to the ascent or intrusion of deep magma, Wech says. 

He and his colleagues decided to make a careful and systematic scan of seismic archives of the Island of Hawai’i, revealing Mauna Kea’s “subtle yet persistent” activity. 

They report that they can trace the origins of the volcano’s near-periodic pulses to the abandoned plumbing deep below the dormant volcano. 

“Our model provides a new framework for interpreting DLPs that does not imply an increased volcanic hazard. DLP swarms or DLPs at persistently erupting volcanoes still likely signal magmatic intrusions,” they write. 

“But at volcanoes with low magma supply rates and/or long repose times, the occurrence of DLPs, in the absence of other anomalies, is likely just a reminder of the continuous, active processes associated with deep, cooling magma.” 

The authors suggest that DLP activity worldwide is likely underreported. 

In a related Perspective in the journal, Robin Matoza, from the University of California, Santa Barbara, notes the “surprising” results from an “impressive” study but suggests the hypothesis that many DLPs are related to second cooling “should be tested at other volcanoes in different tectonic settings worldwide”.