The effect of tides on earthquakes

Dear Answer Man: My question has to do with earthquakes that seem to occur around five or six or seven in the morning. An equal tug upon the earth’s crust, as with the tides, could trigger earthquakes, seems to me. Have the earthquake folks done their calculations for any correlation with that lunar and solar tug? — Wes Farmer, San Diego

This seductive line of reasoning has been scientifically poked and prodded for decades. Many astrophysicists and geophysicists have cranked historical earthquake data through their formulas, but what’s come out the other end has been, at best, inconclusive, at worst, junk. Scripps Institution’s Duncan Agnew is as knowledgeable as anyone on this topic, and he says that virtually every study that has claimed to find some hint of a correlation between ocean tides or lunar gravitational forces and the onset of earthquakes has proven to be statistically flawed. That is, there have been problems with methodology or assumptions about the data that have made the studies’ results invalid or at least suspect. The few that have passed methodological muster show no correlation at all between gravitational forces and earthquakes.

The net gravitational force generated between Earth and the moon (the so-called tidal force) does two things. It pulls water around, creating ocean tides, and it deforms Earth’s elastic crust, creating what are called solid Earth tides. Relative to Earth’s core, when we’re just hanging out on a street corner somewhere, we bob up and down about a foot twice each day as a result of these tidal forces. All this bulging and stretching sets up underground stresses, and these stresses, some figured, might trigger earthquakes. The final straw for some fault zone on the brink of busting loose.

Enticing as the logic is, says Agnew, it doesn’t seem to work that way. At least on Earth. Surprisingly, it does work that way on the moon. It’s scientifically well established that the Earth raises (geologic) tides on the moon (there’s no water up there), and these tides generate clusters of tiny moonquakes at more or less predictable times of the month. That Earth doesn’t respond the same way may have something to do with microscopic differences in rock physics between the two bodies.

Even if there actually is some tidal-force influence on earthquakes, says Agnew, the effect may be so marginal that it would be nearly useless as a predictive measure. Though they’re certainly inevitable, earthquakes are also unlikely at any particular time or place. To be able to say quakes are 1 or 3 or 5 percent more likely at a given time of day (high tide, for instance) doesn’t tell us much. If you get my (continental) drift.