Are Earthquakes Encouraged by High Tides?

Brian Handwerk
for National Geographic News

October 22, 2004

Scientists have long suspected a relationship between tides and earthquakes but have reached little consensus. Now a new study reveals that very high tides might indeed be linked with seismic activity along coasts.

Elizabeth Cochran, a graduate student at the University of California, Los Angeles, is one of the study's authors.

Cochran and colleagues focused on the magnitude of high tides that they found to be associated with earthquakes. They determined that very high tides, rather than a normal tide cycle, seemed to coincide with seismic activity.

"For each [earthquake] event, we calculated the tides around the time of the occurrence. We calculated the stress [produced by tides] on the fault, and we're basically seeing a very good correlation," said Cochran, who is with UCLA's Department of Earth and Space Sciences. "We see more events occurring when tidal stress is high—as long as the amplitude of that stress is large enough."

The team's findings are published in this week's issue of Science.

Very High Tides

The team used the Harvard Centroid-Moment Tensor (CMT) catalog to examine over 2,000 seismic events at shallow thrust faults around the world. The team researched quakes above 5.5 on the Richter magnitude scale that occurred from 1977 through 2000.

A distinct correlation between tides and earthquakes appeared. Some 75 percent of the events they studied happened when tidal stresses were high, as opposed to only 25 percent when they were low.

"If you randomly scatter quakes throughout this [tidal] cycle, they shouldn't relate to the tidal phase at all," Cochran said. "But we see 75 percent for the largest amplitude, and [that percentage] decreases as the tidal stress decreases."

The team focused on activity at shallow thrust faults. The faults were an ideal test location, because their spatial orientation is generally better known, making it possible to more accurately calculate how stresses act on the fault.

The faults also occur in areas where larger ocean tides occur more frequently. In study areas along the continental margins of Japan, New Zealand, Alaska, and the west coast of South America, very large tides coincide with thrust subduction-zone, or faults. Thrust faults are cracks in Earth's crust where one continental plate is rising up and over the other.

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.

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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