Tides may stress the faults in two major ways. Solid-earth tides are caused by the pull of the moon and sun's gravity on the Earth. The Earth's solid mass has enough elasticity that it behaves similarly to ocean tides but to a much lesser degree.
The second stress is caused by what is more commonly thought of as tides—ocean loading. That process includes the movement of staggering volumes of water sloshing around in the ocean basins.
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In 2002 Maya Tolstoy, a marine seismologist at Lamont-Doherty Earth Observatory of Columbia University, uncovered some evidence of tidal/earthquake links on the bottom of the ocean—at the geologic hotspot of the Mid-Ocean ridge. The underwater ridge snakes its way between the continents along areas where continental plates are moving apart.
At the ridge an abundance of hydrothermal fluids and magma made the localized tide/earthquake relations more pronounced and easily observed for the team. These fluid substances are more susceptible to tidal forces than solid earth.
"On the seafloor it's obviously a lot harder to make measurements, and we had fewer instruments, so we couldn't constrain the fault plane very well," Tolstoy said. "But the correlation was just striking. Plotting data against the tides it became obvious."
"This research is exciting," Tolstoy said of Cochran's new work, which tackles the issue on a more global level. "Because these ideas sometimes tie into earthquake prediction, and are so hard to show, people have sometimes been very skeptical."
Indeed, as with all earthquake science, questions have inevitably arisen about the study's possible usefulness in predicting devastating quakes. "We're working on a better understanding of what forces are needed to trigger an earthquake," Cochran said.
"We're looking at how [earthquakes] correlate with diurnal, or daily, tides. But all we're really saying is that if earthquakes are gong to occur anyway, [high tides] may be adding a little extra force. So we might [someday] be able to say that a quake could be more likely during a morning tidal period, for example, but not be able to know which specific morning."
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