How tides can trigger earthquakes

Tides turn on a quest to solve an earthquake mystery.

Years ago, scientists realized that earthquakes along the mid-ocean ridges – the submarine mountain ranges at the edge of tectonic plates – were tidal-related. But no one can understand why there is a slight rise in tremors at low tide.

"Everyone was puzzled, because according to conventional theory, these earthquakes should occur at high tide," said Christopher Scholz, seismologist at the Lamont-Doherty Earth Observatory of Columbia University.

In a study published today in Nature CommunicationsHe and his colleagues have discovered the mechanism of this apparent paradox, which boils down to the magma below the ocean ridges.

"It's the magmatic chamber that breathes, grows and contracts because of the tides, which moves the cracks," said Scholz, who co-directed the study with Yen Joe Tan, a graduate student of Lamont-Doherty.

Go against the current

The correlation between low tide and low tide is surprising because of the way the fault moves in the open ocean. Scholz described the fault as an inclined plane separating two blocks of land. During movement, the upper block slides downward from the lower block. Scientists therefore expected that at high tide, when there is more water above the fault, the upper block lowers and earthquakes occur. But that's not what happens. Instead, the flaw slips at low tide, as the forces actually go back – "which is the opposite of what you expect," said Scholz.

To unravel the mystery, he studied with Tan, and Fabien Albino from Bristol University, the Axial Volcano located along the Juan de Fuca Ridge in the Pacific Ocean. As the volcano explodes every decade or so, scientists have set up dense networks of ocean floor instruments to monitor it. The team used the data from these instruments to model and explore different ways that low tides could be causing tremors.

In the end, it was an element that no one had ever considered before: the magma chamber of the volcano, a soft, pressurized pocket under the surface. The team realized that when the tide is low, there is less water above the chamber, so it expands. As he swells, he braids the rocks around him, forcing the lower block to slide up the fault and causing earthquakes.

In addition, said Scholz, the tidal earthquakes in this region are "so sensitive that we can see details in the response that no one could see before". When the team mapped the earthquake rate against the stress on the fault, they realized that even the smallest stress could trigger an earthquake. Tidal data was used to calibrate this effect, but the triggering stress could be caused by anything, such as seismic waves from another earthquake or fracking wastewater pumped into the ground.

"People in the hydrofracking industry want to know if there is safe pressure that you can pump and that you do not produce an earthquake." said Scholz. "And the answer we find is that there is none, it can happen at any level of stress."

Of course, a little stress on a small area is not going to cause a devastating earthquake, and the exact amount of stress needed varies from place to place. "Our goal is that there is no intrinsic stress to be overcome to cause an earthquake," Scholz said. "There is no rule of thumb."