Scientists discover a surprise rumbling beneath a sacred Hawaiian volcano

By the time you finish reading this article, there's a good chance there's been an earthquake beneath Mauna Kea, a dormant shield volcano on the Big Island of Hawaii.

Mauna Kea is a deeply sacred place in Hawaiian culture and, at nearly 14,000 feet above sea level, an ideal location for astronomy. (These two facts underlie the current tension between Hawaiians and the astronomers who want to build a giant telescope there.)

At about a million years old, it's still revealing surprises.

According to a study released Thursday in Science, Mauna Kea experiences a deep, low-frequency earthquake every 7 to 12 minutes. That means the small shakes have happened more than a million times over the past 20 years.

This finding comes from an analysis of the seismic activity that's rumbled underneath Mauna Kea over the past 19 years.

Mauna Kea's uniquely regular earthquakes are called deep long-period earthquakes, or DLPs.

Sometimes, DLPs are a sign that an eruption is coming. But this research suggests that's not always the case. In fact, DLPs may more commonly signal that magma is cooling beneath the Earth's surface.

Beneath Mauna Kea, cooling, stagnant magma begins to crystallize. That releases gases — a phenomenon called "second boiling" — which, in turn, pressurizes and fractures the deep rock. This triggers a seismic tremor.

DLPs are usually associated with dormant volcanoes, like Mauna Kea. The subtle activity is probably underreported because traditional seismic detection equipment overlooks their weak signals, the study team writes.

It's also plausible that, because scientists haven't tracked many DLPs in the past, we might not understand the reason they occur, the team explains. While previously it was theorized that these small quakes predicted that an eruption was coming, now it seems more likely that DLPs are the result of cooling magma.

Not all earthquakes are dramatic — While the word "earthquake" might call to mind trembling buildings and broken dishes, the effects of seismic activity aren't always so serious.

Some seismic tremors are so subtle that they get swallowed up in the noise we create — from driving cars, flying planes, and even tromping about on foot.

With much of the world still under lockdown due to the global pandemic, some of that noise has disappeared over the past few months. As a result, seismologists are able to draw new information from tiny ripples that may have been lost under usual circumstances. It's one of the many effects that global lockdown is having on nature.

Quieting things down to this level is extremely rare, seismologist Thomas Lecocq told Nature. In fact, the recent noise reduction is usually observed only once a year — on Christmas.

With more silent nights (and days), geologists are able to detect more trends in the data. That has improved the sensitivity of seismometers — equipment can now pick up smaller earthquakes as well as other sounds caused by humans. If the global lockdown persists, the technology may become further refined, even picking up aftershock effects normally quieted by bustling cities.

Abstract: Deep long-period earthquakes (DLPs) are an enigmatic type of volcanic seismicity that sometimes precedes eruptions but mostly occurs at quiescent volcanoes. These earthquakes are depleted in high-frequency content and typically occur near the base of the crust. We observed a near-periodic, long-lived sequence of more than one million DLPs in the past 19 years beneath the dormant postshield Mauna Kea volcano in Hawai‘i. We argue that this DLP sequence was caused by repeated pressurization of volatiles exsolved through crystallization of cooling magma stalled beneath the crust. This “second boiling” of magma is a well-known process but has not previously been linked to DLP activity. Our observations suggest that, rather than portending eruptions, global DLP activity may more commonly be indicative of stagnant, cooling magma.