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Policemen search for survivors at a landslide site after earthquakes in Minami-Aso, Kumamoto prefecture, Japan, on April 17, 2016. KAZUHIRO NOGI/AFP/Getty Images[/caption] Over the weekend, two massive earthquakes in two days hit on opposite sides of the Pacific. First a M7 earthquake hit the Japanese island of Kyushu, then a little more than day later, a M7.8 earthquake hit Ecuador. The destruction from these earthquakes have killed hundreds and wrought massive amounts of damage to the areas near the temblors. Of course, whenever closely spaced earthquakes like this occur, people start getting worried that they are related somehow---that the big Japanese earthquake directly caused the Ecuadoran earthquake. I'll make it simple: There is no geologic evidence that big earthquakes that are far away (1,000-10,000s kilometers) are directly triggering each other. Sure, they are both part of the grand system of plate tectonics, but one earthquakes does not lead to the other. Both Japan and Ecuador share the same type of tectonic setting: a subduction zone. This is where one plate slides underneath another plate, causing earthquakes and volcanoes. However, although they are both on the Pacific rim, the forces causing the earthquakes are not connected, so although rocks on a fault in one place may move, they are not going to necessitate an adjustment across all the plates, especially not in the order of days. Remember, for every pair of earthquakes as we just experienced, there are massive earthquakes that exists all by themselves (the "recency effect" is powerful). Large earthquakes do trigger smaller earthquakes nearby, called aftershocks, but those are typically limited to 100s of kilometers from the epicenter of the big earthquake. So, what about volcanoes and earthquakes? Surely a big earthquake can cause volcanoes to erupt? Here, the evidence is sparse at best, even for volcanoes right next to the earthquake. There was news after the Japanese earthquake that there was an eruption at the Aso caldera in Kyushu---but vents in the Aso caldera were already erupting before the earthquake. (By the way, anything you've read that tried to connect the earthquakes to eruptions at places like Colima in Mexico or Villarrica in Chile are completely bogus---both were already erupting and too far away.) The key is that for an earthquake to directly lead to an eruption, that volcano likely needs to be ready to erupt already. That is, a quiet volcano (with no signs of magma moving in underneath) is not going to spontaneously explode just because an earthquake occurred. This is because most volcanoes need eruptible magma near the surface (likely under pressure) to erupt, so without that reservoir of magma, that change in stress isn't going to cause an eruption. If the volcano wasn't experiencing earthquake swarms and the other telltale signs of magma rising up underneath, an eruption is highly unlikely ... and even if the signs were there, an eruption directly associated with the earthquake isn't likely. One report on Facebook from Japan mentioned that a surface rupture (a crack) formed after the earthquake near the Aso caldera. Could this cause an eruption? Again, without magma under pressure, probably not. The fact that eruptions were already occurring means that magma has a path to the surface, so the pressure might be relatively low. A crack could conceivably intersect a pressurized magma body and release that pressure, causing an eruption. That's what happened at Mount St. Helens in 1980, when an earthquake caused a landslide (a really big crack) that released pressure on the magma near the surface, triggering the big blast. This is not likely to be the case at the Aso caldera in Japan (or most any volcano on Kyushu for that matter). There is some evidence that very large earthquakes, such as the M9 earthquakes that occur off of Chile, can cause an increase in eruptions over the years following an earthquake. This is in interesting (and tenuous) connection where maybe those big events can "get the ball rolling." This might be caused by that slight change in stress allowing magma to work its way through the crust to the volcano. However, the mechanism is clearly not well understood. Overall, very large earthquakes (usually over M8) seem to cause changes in volcano behavior or start new earthquake swarms, but they never cause an instant increase in eruptions. Of course, you can find interesting examples that suggest a connection between some larger earthquakes and subsequent eruptions. The June 1991 eruption of Pinatubo in the Philippines, which hadn't erupted in over 500 years, was preceded by a nearby M7.8 earthquake in June 1990. Could this temblor have started Pinatubo on its way to an eruption? It is hard to make that connection explicit without more evidence, but it does mean that volcanoes near the epicenters of the Japanese and Ecuadoran earthquakes should be closely watched as the year moves on. However, the timing of these earthquakes is caused by the random distribution of large earthquakes over time. Just because we've seen large earthquakes in Japan and Ecuador doesn't mean that people in other parts of the Pacific rim, like Oregon or Mexico or Alaska, need to be worried that a large earthquake will occur there soon. Plate tectonics doesn't appear to work that way. Instead, these earthquakes should remind us to be prepared for the next big one---because we don't know when it will come. Therein lies the real danger, not a dubious connection between earthquakes and eruption.
