Earth's internal tides can set off weak earthquakes

When a lot of tectonic pressure builds up near a fault line, it only takes a little more stress to push the system over the edge, resulting in an earthquake.

In a recent paper, earth scientists including Laurent Metivier at Hopkins and colleagues at other institutions described their findings of just how important these small earth stresses are to setting off earthquakes.

They focused on the solid Earth tides. These are not the ocean tides caused by the moon, but rather tides inside the planet's caused by the core's movement.

The team used a database of seismic information from the National Earthquake Information Center (NEIC) to look for tidal correlations in the timing of earthquakes around the globe that occurred between 1973 and 2007.

Earth's solid tidal movement occurs in both diurnal (daily) and semi-diurnal phases. Compiling the phase data allowed the scientists to see if the tides were affecting earthquake triggering.

If they were not, then the earthquake activity would appear to be uniformly distributed across the tidal phases. The result however, was a difference in earthquake activity found for both diurnal and semi-durnal tides: solid tidal earthquake triggering in play.

The next step was figuring out where the tidal effects had the most force in earthquake triggering. Division of the seismic information database into sub-categories allowed for this analysis.

The results showed that earthquakes less than magnitude 4.0 and less than 20 kilometers deep - small and shallow - were more prone to tidal triggering.

Oceanic tides certainly have an effect on earthquake triggering too, considering how much of the planet is water. The scientists took this into consideration, only considering earthquakes that occurred far inland - more than 200 kilometers from the nearest coastline.

Because the anomaly in earthquake triggering was still present after taking oceanic tidal effects into consideration, the scientists were able to conclude that, at least for small and shallow earthquakes, solid earth tides have a significant triggering effect of their own.

This information may help us to better understand the development and onset of earthquakes, and particularly whether an initiation difference exists for small versus large earthquakes.

Perhaps in the future when an earthquake is predicted to strike, geoscientists will be able to tell us just how big of a disaster we are in for.