An almost-full, half-pie, waxing moon hanging lopsided in the night sky has long been a symbol of things to come. Now scientists have a new symbolism for the lunar phase we call first quarter: a looming risk of earthquakes.
The moon is (mostly) responsible for Earth’s tides, which are strongest when the sun and moon are aligned, during a full moon or a new moon. It’s small, the moon, but so close that its gravity stretches and compresses the water across the globe, into high and low tides called spring and neap tide, respectively. It pulls on the Earth’s crust, too, but only a tiny bit, especially compared to the breath-like rise and fall of an ocean.
Still, scientists have wondered for years whether the moon might play a role in earthquakes, which are essentially movements of the Earth’s crust atop its mantle. It would make sense that the moon’s gravity could tug at a fault in the crust, especially one that is already close to failing and slipping. But going back to the 1800s, nobody had demonstrated firm evidence for this. A new study gets closer to drawing this link.
Studying data from the past two decades, Satoshi Ide and colleagues from the University of Tokyo measured the timing of high tides and reconstructed the amplitude of the moon’s pull at those times, focusing on the two weeks prior to large earthquakes. They measured the amplitude of the tides against the timing of those quakes, and found some of the largest and most devastating earthquakes in recent memory happened when the Earth’s crust was under the highest tidal stress.