Scientists have figured out why earthquakes along mid-ocean ridges occur during low tides.

For 20 years, scientists have known about the link between earthquakes and tides. But because most mid-ocean ridges feature vertical faults, or faults featuring steeply inclined planes, researchers assumed earthquake-generating slips would be more likely to occur at high tide. The seismic data showed the opposite was happening.

"Low tides would reduce the vertical force, which should inhibit sliding and hence earthquakes," Christopher Scholz, a seismologist at Columbia University's Lamont-Doherty Earth Observatory, told UPI.

Researchers were stumped by the paradox, but Scholz and his colleagues discovered an explanation.

The earthquakes along mid-ocean ridges are triggered by the expansion and contraction of the magma chambers below. The discovery -- described this week in the journal Nature Communications -- was made possible by a network of seafloor instruments along the Pacific's Juan de Fuca ridge, which helped scientists measure the fault's movements and model potential explanations for the seismic activity.

"We were able to solve this paradox by including the response of the magma chamber to the tides," Scholz said.

Scholz and his colleagues realized that during low tide, less water mass is pressing down on the soft pocket of molten rock below the volcanic ridge. As a result, the magma chamber expands. When it does, the bottom fault block is pushed upwards, sliding along the top block. The movement triggers earthquakes.

According to the new analysis, it doesn't take much to trigger an earthquake along the mid-ocean ridges.

"The main reason these faults are so sensitive to stress is that they are very shallow," Scholz told UPI. "Their average depth is about 1 kilometer as opposed to typical earthquake depths of continental earthquakes of about 10 kilometers. This makes the faults much weaker, and the sensitivity is given by the ratio of the triggering stress divided by the fault strength."

Researchers estimate small quakes on the mid-ocean ridge could be triggered by other mechanisms, too, and the team of scientists plan to investigate other possible seismic patterns.

"Our next project related to this problem is to study the relationship of the earthquakes with the fortnightly tides," Scholz said. "That may reveal some new behavior related to the flow of water in and out of the faults which might occur at the longer time periods."

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