The ground beneath long-dormant Hawaii volcano has been regularly rumbling since 1999. On a timescale of 7-12 minutes, Mauna Kea shakes periodically.
The quiet rumbles aren’t any warning of alarm, but it sends the strangest seismic signals. The magnitude of these signal is about 1.5, coming from 25 kilometres deep at the base of Earth’s crust. The unusual highly periodic rhythm of Mauna Kea’s quakes is long-lasting.
Many volcanoes send such signals which are called deep long-period earthquakes. The cause behind these deep long-period (DLPs) quakes remains a mystery.
The depth of DLPs ranges from 10 to 60 kilometres. Scientists think that the movement of magma within a volcano causes DLPs. But in 1991 this prediction proved to be wrong as Philippines’ Mount Pinatubo erupted devastatingly, preceded by hundreds of DLPs pulses in the following week. However, often DLPs don’t appear to forecast an eruption at all. For such eruptions, scientists suggest that the tiny quakes are due to stress and strain which is caused by the hot magma, trying to push its way into cracks and then cooling and contracting.
Some teams suggest that gases, emitted by the pool of cooling magma, cause the quakes. The gases leave, seeping into fractures of rocks, pressurizes it. The built-up pressure causes the rock to rumble and then when the pressure builds again, the grumbling is quiet.
Wech said that the DLP discovery below Mauna Kea was just an accident as he went applying the technique in other volcanoes which he did in just one. In 2013, he was using an algorithm to understand the seismic signals coming from beneath the volcano Kilauea. He came up with several observations.
“When you see deep seismicity, there’s a temptation to assume it’s a sign of unrest,” Wech says. “These signals can still mean magma ascent, but the point here is that doesn’t have to be your first interpretation.”
Vidale says he finds the results intriguing and plausible. “I don’t think it’s proof, but it’s good evidence,” he says. “They identify something that’s clearly an ongoing process that’s been happening for years at regular intervals.”