There is no natural disaster sneakier than an earthquake. Hurricanes can be predicted and tracked weeks in advance, and even tornados, monsoons and blizzards at least have seasons. But earthquakes strike entirely without warning. Now, however, a new studysuggests that we may want to brace for a surge of quakes in the year ahead, and the reason for the danger is an unlikely one: the rotation of the Earth has slowed slightly.
While accurately forecasting earthquakes is impossible, a backward look through the seismic record allows geologists to detect some distinct patterns. In the new study — which was presented at the annual meeting of the Geological Society of America, in Seattle, and published in Geophysical Research Letters — geologists Roger Bilham of the University of Colorado, Boulder, and Rebecca Bendick of the University of Montana, tracked the incidence of magnitude 7 or greater earthquakes worldwide since 1900. While in most years there is an average of just 15 such major shake-ups — already more than enough — there have been evenly spaced intervals in the past 117 years in which the annual total jumped to between 25 and 30.
A little more than a century on a planet that is more than 4 billion years old is not exactly a representative time sample, but Bilham and Bendick noticed something else about these volatile, quake-prone periods. They seem to follow periodic slowdowns in speed of the Earth’s rotation. Our solid planet is a lot less solid than it seems, and that’s true not just of its oceans and air, but of its outer core, which is about 1,200 mi. (2,200 km) thick and is composed mostly of liquid iron and nickel. That molten ooze tends to slosh about, following a pattern that oscillates more or less predictably over time, much the way — on a vastly smaller and more fleeting scale — water sloshing in a bucket will move back and forth in a repeating cycle.