Featured image: A GPS station in the Sawtooth National Forest near Ketchum, Idaho. Photo by Scott Haefner (USGS).
Paper: Months-long thousand-kilometre-scale wobbling before great subduction earthquakes
Authors: J. R. Bedford, M. Moreno, Z. Deng, O. Oncken, B. Schurr, T. John, J. C. Báez, M. Bevis
We’re always on the lookout for earthquake precursors, indicators that the Earth might be gearing up for some shaking, and geophysicists think they might have found a new one: a small but measurable back-and-forth “wobble” of the land starting several months before very big earthquakes hit.
Now, let me say this upfront: we cannot predict when earthquakes are going to happen. What we can do, though, is assess risk based on past earthquakes, and try to identify those precursory signals – signs that an earthquake might happen in the near future. Precursors aren’t sure things, but just like dark clouds on the horizon might prompt you to carry an umbrella on a walk, our past experiences tell us that certain things often happen right before big earthquakes. For example, perhaps the best-known earthquake precursors are foreshocks, smaller earthquakes that indicate tectonic plates are on the move and a bigger quake might be on the way. In the case of the 2011 Tohoku earthquake in Japan, for example, seismometers recorded an increase in seismic activity during the preceding 1-3 months.
In a new study, Jonathan Bedford and co-authors found that large expanses of land shifted around in distinctive ways before two of the largest earthquakes in recent years: the 2011 Tohoku and 2010 Maule, Chile events. The authors measured surface motion using GPS stations, instruments that use GPS satellites to continuously record their location and measure how Earth’s surface moves. After accounting for non-tectonic signals such as seasonal motion, the GPS data show an East-West oscillation in both Japan and Chile in the 5-7 months leading up to these major earthquakes. The “wobbles,” as the authors call them, are only a few millimeters back and forth, but occur coherently across thousands of kilometers of land.
But what could cause this wobbling? For the Tohoku earthquake, the authors used computer models to track the surface motion back to its source, and found that a relatively sudden pull on the Philippine Sea tectonic plate sinking into the mantle beneath Japan could have kicked off the wobbles. The pull might have been caused by a speed-up in the chemical reactions that occur between water and minerals in the descending plate. These reactions form denser minerals which pull the plate down into the mantle faster.
While the wobbling itself may not have caused the Tohoku and Maule earthquakes, the same shifts in stresses that set off the wobbles may also have triggered the subsequent seismic slip. It’s hard to say if this is a universal or even a common precursor to large earthquakes since the multi-year GPS records needed to pick out this kind of surface motion aren’t available for most places on Earth, and earthquakes on the scale of Tohoku and Maule are, fortunately, rare. But this research suggests that long-term GPS records could be a fruitful place to look for earthquake precursors, and probing the tectonic shimmies associated with seismic activity may lead us to a better understanding of when and how big earthquakes happen.
Tiny wobbles foreshadow big earthquakes by Hannah Mark is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.