Okay, so you all remember what I wrote regarding the momentary early-morning earthquake in Fukushima prefecture of Japan on November 22. At magnitude 7.4, it was close to crossing over the level of destruction that took place back in 2011, but fortunately only a handful of people were injured, buildings received superficial damage, and aside from a brief cooling unit shutdown no serious problems were detected at the deactivated but still cooled Fukushima Daiichi nuclear plant. Even the precautionary tsunami warnings panned out as the day passed by, and besides, the residents have evacuated without fuss. But then the slightly higher waves that came ashore have caused a very interesting occurrence.
CNN reports that video footage from civilian cameras and news agencies have come up showing what appears to be a series of waves with ripple-like effect travelling up canals, rivers and channels in the Fukushima and Miyagi prefectures, working up against the seaward flow of the many waterways’ currents. This uncommonly spotted phenomenon is what’s known as a “tidal bore”, and has caught the fascination of those who have seen it online.
But what is a tidal bore? Georgia Tech geophysicist associate professor Andy Newman says that they are cause by the sudden large amount of incoming ocean tide that gets funneled into estuaries, or the mouths of waterways from inland. Depending to the geographical features of the estuary’s location, this funneling action of seawater into a smaller area causes the wave to push against the waterways downstream current, to the point that it begins rolling over the seaward flow in order to keep moving upstream. The result then is a wave, slower moving that out at sea, travelling up the river or canal even as the water underneath flows out to sea.
Why is the tsunami labeled a tidal action, in this case? That goes back to decades past when tsunamis (from the Japanese meaning “harbor wave”) were instead known as “tidal waves” due to the mistaken assumption that they were generated by the movement of the tides. Now we know that a tsunami is triggered by seismic activity near the sea, or by geological upheavals at the ocean bottom like undersea volcanoes or landslides.
Although I’ve stated that tidal bores aren’t common, there are certain locations throughout the world that have them occur almost regularly due to geographical placement, such as Canada’s Bay of Fundy or China’s Qiantang River; the Chinese call this particular tidal bore occurrence the Silver Dragon.
Some of them are even good for surfing too. But for now the Japanese can take a collective sigh of relief that the Tohoku Quake hasn’t had an encore that day, as well as wonder at the tidal bores that have appeared at their waterways.
Photo Credit to http://abcnews.go.com/