A plate is not exactly like a plate
Geologists describe the motion of continents and oceans around the Earth’s surface using a model called Plate Tectonics. The idea is that solid, rigid plates move across the surface, and occasionally bump into each other at the edges. However, this way of thinking about plates misses some important properties of plates that have been on display this month.
As of last Wednesday, the month of August has seen 8 separate earthquakes with magnitudes over 6.5 on this planet. This is far more quakes than have been seen in any other month this year, leading some, including tabloid press, to suggest that these events are linked even though they are far away. However, seismologist Lucy Jones demonstrated that it isn’t unusual to see a number of strong earthquakes in one month, as some months have had as few as zero earthquakes of that strength.
If you take 2 rigid objects, like well, plates, and move them across your table, they slide and eventually bump into each other. When you’re doing this exercise, the plate is truly rigid. You can’t move one part of the plate without moving another plate, unless you actually break it.
It is that rigidity where the plate metaphor breaks down. If a plate on Earth worked the same way, a huge motion at one edge would cause the entire plate to move, but that’s not what we see. This is a map of earthquake aftershocks in the 10 days after the huge Tohoku Earthquake in 2011; parts of the Pacific and Eurasian plate shifted by more than 10 meters during that monstrous quake. After the quake, there were aftershocks – smaller earthquakes triggered by the shift in stress from the big quake – but those aftershocks didn’t cross the entire Pacific Ocean, they stayed close to the area that ruptured in the big quake.
Plates on Earth are so much bigger than the plate you have sliding across your table they actually can deform, just a little bit, without fully breaking. For the Earth, this is normal behavior, even if it seems odd using the plate metaphor. The energy released in a big quake like Tohoku is stored by bending the tectonic plate a little bit near the fault – the rocks themselves actually bend, and then snap back during an earthquake, releasing elastically stored energy like the snapping of a rubber band. So, tectonic plates, like those on the Earth, are somewhat rigid, but it is ok to bend them (don’t try bending your dinnerware at home).
Overall, the Pacific Plate is gradually marching across that ocean, but 10 meters of motion in Japan cannot trigger large motions on the opposite side of the planet. There are no aftershocks or earthquakes in-between the other quakes this month, showing that they’re not connected. When GPS units show meters of displacement near one fault, they don’t move anywhere else on the plate, just near the fault that broke. All of the bending and release of energy involved in those quakes is happening right at the edge, where the plate is actually bending.
It is possible for one earthquake to trigger another. Obviously, earthquakes can trigger aftershocks nearby, and sometimes one aftershock might even be bigger than the first quake, making the first quake a foreshock. The seismic waves that travel around the Earth have also been suggested as capable of triggering earthquakes on faults that are ready to break, and there is some published evidence of this happening. However, none of those cases means that faults several thousand kilometers away from each other are affected. When one fault breaks, it’s breaking because the stresses in that area overwhelmed what the rocks could store, and that fault breaking is a story of the stresses in that area, no where else.
Image credit: IRIS https://ds.iris.edu/ds/products/aftershocks/ Inspiration for post: https://twitter.com/SeismoSue/status/1032994154492522497
