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How was last week’s earthquake judged to be an aftershock of the 2011 Tohoku disaster?

Master Blaster Feb 17, 2021
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Earthquakes have always been largely a guessing game, and this one is no different.

At about 11 p.m. on the evening of Saturday, 13 February, a 7.3 Magnitude earthquake hit the Northeast Region (Tohoku) of Japan. It was the most violent quake Japan had seen in a while but the eeriest part is that it was said to be an aftershock of the 2011 Tohoku earthquake that occurred almost exactly ten years earlier.

It doesn’t seem crazy that an aftershock could be so delayed since ten years is literally a blink of the eye in geological terms, and some major quakes around the world have reportedly caused aftershocks that continue for centuries. Still, how was this one determined to be an aftershock so quickly?

The Japan Meteorological Agency (JMA), who issues earthquake warnings and reports, has three criteria when determining which quakes are aftershocks:

1) Earthquakes directly caused by large earthquakes
2) Earthquakes triggered by situations caused by large earthquakes
3) All earthquakes around the epicenter of a large earthquake, 210,000 square-kilometers (81,000 square-miles) in the case of the Tohoku Earthquake

To know more about what that means, we need to understand the basic mechanics behind the the Tohoku earthquake. It was caused by the slow movement of the tectonic plate under the Pacific ocean sliding underneath the plate on whose edge Japan sits. “Sliding” is putting it mildly, however, as massive amounts of pressure are constantly being applied to miles of jagged rock.

▼ A 3-D diagram of the various plates grinding against each other right next to Japan

Every once in a while too much pressure builds up where these plates meet and something’s got to give, resulting in the sudden and powerful movements of earthquakes. However, tectonic plates can be hard to envision, so instead let’s apply the JMA’s criteria to this video of a car ramming its way out of a Tesco parking lot.

▼ The video is cued ahead to the moment of ramming

For illustrative purposes, let’s say that the blue car is the Pacific plate and all the other parked cars are Japan. That car is going to leave Tesco one way or another and, much like with an earthquake, the amount of damage it will cause in doing so is anyone’s guess.

The first strike of the blue car can be considered the main quake which clears a path for it to leave. Had that strike caused one of the other cars to flip over or roll into another car then these incidental impacts would have been considered aftershocks of the initial strike. That’s the easiest to understand in the sense of an aftershock as something that is the direct result of an action as described in the first criterion.

In addition, if long after the blue car drove off, the axel on one of the struck cars suddenly gave out and its wheel fell off, that would be considered an aftershock by the second part of the criteria. This also makes sense because despite the delay and fact that it wasn’t direct, the damage was still very likely caused by the initial attack.

However, if the next day another car is damaged in that same section of the parking lot, it would still be considered an aftershock by the final criterion. Obviously, this is where things get problematic.

The third part sounds ridiculous in the case of the video because we can clearly see what is happening and judge if causality exists. With tectonic plates, however, we are largely going blind and have to make some assumptions based on very limited information, kind of like listening to that video with your eyes closed and no explanation about what’s happening.

That being said, sensing technology is getting better all the time, and we can at least get a good idea of how each each earthquake came about. The main 2011 earthquake originated from movement in the area where the two plates make contact.

▼ A simple visualization of how the 2011 Tohoku Earthquake happened; try to ignore the heavy breathing or whatever that sound is

Other large aftershocks were caused by breaking in the plate that Japan sits on top of. It would be like if too many people used that science center exhibit and the brown plate suddenly cracked in one place. That’s reasonable, but in the case of the 2021 quake, the crack is believed to have occurred within the Pacific plate, or inside the part of the science exhibit’s blue sheet that was underneath the brown. In the case of the Tesco video, it’d be as if the blue car just suddenly blew up a month later.

Since we’re talking about one of the strongest earthquakes the modern world has ever seen, it’s probably a pretty reasonable assumption that the 2011 earthquake indeed had caused the stress and/or damage within the Pacific plate that led to the 2021 quake. But that is still just an assumption which means that the first of second parts of the criteria are probable but uncertain, and the only one we know for sure is the very vague third criterion.

Since the stakes are always high when dealing with these natural disasters, it really pays to be sure. Even the JMA said on 15 February that they are considering revising their criteria in light of the recent activity, but with current technological limitations there might not be much more they can do right now.

In the meantime, all the rest of us can do is what we’ve always hopefully been doing, which is keeping supplies at the ready and knowing what to do and where to go in the event of an earthquake.

Source: Sankei Biz, Iza, My Game News Flash
Top image: ©SoraNews24
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