You may have spotted a rather exaggerated image of a catfish on some of the signs hanging over the main roads. They are there to warn that these highways will be closed in the event of a major earthquake.
The catfish wasn’t chosen at random: in Japanese mythology, a giant catfish, Namazu, is believed to live under Japan and cause the earth to move when it trounces. Some say that it starts thrashing when it senses multiple smaller tremors with its ultra-sensitive whiskers (much like actual catfish, apparently), making it a much-used image for earthquake prevention apps, such as Yurekuru and more. The Japan Meteorological Agency, who sends out information based on the earthquake early warning system, is an exception to the rule – we'd like to suggest a cute little fluffy Namazu mascot to drive the point home.
In pop culture, Namazu was the inspiration behind the Pokemon character Whiscash. This blue catfish lookalike was originally named Namazun, and in its Pokedex description, Whiscash is said to trigger a massive earthquake whenever an enemy gets too close.
As for the main roads being closed after a big earthquake, it might sound counter-intuitive, but it's actually quite pertinent: the emergency services need to be able to get through, so to prevent gridlock on main traffic arteries, the catfish guards over them. Pretty smart.
For more information on what to do in the case of a natural disaster, check out our Tokyo disaster survival guide.