New research has revealed the link between deep ocean earthquakes and low tides.
Scientists first realised that quakes along underwater mountain ranges were connected to the tides several years ago, but couldn't figure out why.
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"Everyone was sort of stumped, because according to conventional theory, those earthquakes should occur at high tides," said Christopher Scholz, a seismologist at Columbia University's Lamont-Doherty Earth Observatory.
But a new study published by Scholz in the journal Nature Communications has revealed the answer.
His team studied the Axial Volcano along the Juan de Fuca Ridge in the Pacific Ocean. Mid-ocean ridges are mountain ranges formed when two tectonic plates meet, and feature vertical faults where the plates come together.
During low tide, less water presses down on the soft pockets of molten rock below the ridges. As a result, the magma chamber expands, forcing the fault to slip and causing earthquakes.
"We were able to solve this paradox by including the response of the magma chamber to the tides," Scholz said.
"It's the magma chamber breathing, expanding and contracting due to the tides, that's making the faults move."
The researchers say even the smallest stress on the deep ocean fault could trigger a quake.
"People in the hydrofracking business want to know, is there some safe pressure you can pump and make sure you don't produce any earthquakes?" Scholz said.
"And the answer that we find is that there isn't any - it can happen at any level of stress."