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.

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