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Merged 8.8 quake in Sendai, Japan

NoahFence said:
I think we're getting confused between a Tsunami and a Tidal Wave.

If I can recall, a tidal wave is a singular event, while a tsunami has multiple waves associated with it?
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A tidal wave happens every day, in most places twice a day! Many people erroneously refer to Tsunamis as "tidal waves" but this is an incorrect use of the term.
 
William Parcher said:
This is the wall of water I have been asking about. Of course this illustration shows a wall of water much higher than 13 meters, but it could be imagined as being that height.

Did Japan experience one of these things that was 13 meters?
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There's a couple of problems with the above picture. It depicts a breaking wave, and as you'll probably recall from childhood science classes a wave breaks when it enters water ~half the depth of the wave height (I think, I don't recall the general ratio). Obviously that means a really tall wave isn't going to break right on shore like that picture - it would break miles out to sea and what you'd get on shore is the run up - think the surge of waves up a beach, only enlarge in scale appropriately.

Of course the reality is Tsunamis don't generally break like this at all, except for the very largest. That's because they have such long wavelengths - hundreds of km when out to sea. While the wave will "stack up" as it comes into shore (a phenomena called "wave shoaling") it takes a long time to reach it's maximum height. Meanwhile it's also losing speed dramatically, which also interferes with any chance of the wave forming a breaker.

This is mostly because of the slope of the seafloor and how rapidly it shallows. As any surfer will be able to tell you, steeper beaches form cleaner breakers, technically known as plunging waves. This is because wave amplitude increases rapidly, much faster than the wave can decelerate, thus the top of the wave is trying to move forward faster than the bottom of the wave (which is experiencing friction from the sea floor). Thus the wave curls over. This is the "classic" surfing wave, which is also much more dangerous to surf as a plunging wave is much more destructive (it it plunges onto a board it can quite happily snap it in half).

On a gentler slope, you don't get plunging waves, but instead spilling waves. Instead of curling over, these grow to height more gradually and slow at a more similar pace, just the top part of the wave, instead of curling over, "spills" down the front of the wave as whitewater. This is a much safer wave to surf as you're riding "atop" the wave rather than under the face of it.

For a wave as large as a Tsunami, of course, covering such vast distances and with such a long wavelength, virtually any beach has a gentle slope, relatively speaking, and a Tsunami won't cause a plunging wave. Even if it hit a steep enough continental drop-off that it did form a curling breaker, this would happen miles and miles out to sea, and would still only hit the coast as a giant surge of foam and surf, not as a curling wave.

More normally, the sea floor slopes very gradually (relative to the wavelength of the Tsunami) thus instead of plunging waves, it forms a spilling wave. These will start to "break" miles out to sea (as we see in the video with the coastguard ship) but as a spilling wave breaks over a long period of time, we see them still breaking as white foam when they hit the shore.
 
r-j said:
A really powerful earthquake, right off the coast, couldn't it cause a wave that looked something like that?
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No. At least, not unless you happen to have a deep ocean trench "right off the coast", and I don't think anywhere does, and least not on these sorts of scales. "Right of the coast" in geological terms is more like a hundred miles off the coast.

I suppose technically if you had a rapid enough megathrust rupture in a deep ocean trench as near to the coast as possible it might just cause something like that (though not that big!) but I can't see it realistically happening, and were it to happen I imagine an event that cataclysmic would probably result in more severe events - such as the entire coastline simply collapsing into the fault, or the earth splitting open like an egg, or something... :p
 
gumboot said:
There's a couple of problems with the above picture. It depicts a breaking wave, and as you'll probably recall from childhood science classes a wave breaks when it enters water ~half the depth of the wave height (I think, I don't recall the general ratio). Obviously that means a really tall wave isn't going to break right on shore like that picture - it would break miles out to sea and what you'd get on shore is the run up - think the surge of waves up a beach, only enlarge in scale appropriately.....<snip>
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Sort of like this maybe:



And thanks for your excellent explanation (all of it).
 
I recognize the human toll, but for all of you dog lovers:

Dog rescued after quake going back to its owner

(CNN) -- A dog rescued off the Japanese coast floating on top of a house is on her way back to her owner Monday.
The dog wagged its tail and jumped up to a woman described by local media as a relative of the owner as she collected her to deliver back to her family for what promises to be a warm reunion.
It turns out the lucky dog's name is "Ban," and she was originally living in Kessenuma before being separated from her master after the March 11 earthquake, tsunami and subsequent fire that swept through the coastal village.
It's not clear how the 2-year-old mixed breed Ban and her master were separated, but Kessenuma is located in Miyagi prefecture, which was virtually wiped out by the disaster three weeks ago.
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Link with video.
 
I wish we were hearing more about how the Japanese towns are coping. It's just fallen off the news in favour of Libya, Libya, Libya.

Maybe it's just voyeurism, but I feel a certain connection with these places whose names we've learned and whose destruction we've witnessed, and I'd be glad to know how they're all getting on.

Rolfe.
 
Rolfe said:
I wish we were hearing more about how the Japanese towns are coping. It's just fallen off the news in favour of Libya, Libya, Libya.

Maybe it's just voyeurism, but I feel a certain connection with these places whose names we've learned and whose destruction we've witnessed, and I'd be glad to know how they're all getting on.

Rolfe.
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Lots of local news here: http://www.japantimes.co.jp/


I'm in constant contact with friends in Tokyo, one of them from Iwate prefecture. She's a nurse and will be soon posted in the North of Japan to help with the needed care for stranded elderly people.

I was due to visit her at the end of the month, she asked me to postpone since there are problems with transportation (trains are less frequent and reliable than usual), electric power (scheduled cuts, non necessary items like sport facilities and public tv screens aren't lit at night), shorter opening hours for shops, museums, etc. Celebrations for Hanami (cherry trees viewing) have been mostly cancelled, as are many sports competitions, shows, etc.

From our conversations, I can feel the Japanese are deeply affected, even grieving, by the disaster. The events at Fukushima are on the background, of course, but it's mostly the fact that almost everybody has lost someone, or knows someone (family, friends, colleagues, ...) who has, and the expectation of all the work has to be done ...

She told me they were all very surprised, grateful and comforted to know that so many people cared about them.
 
Some (a lot) of that is very sad.

While the bodies so far found in the cold northeast were relatively clean after mud and dried grass were wiped away by police with water, quite a few were overdressed or had bulky luggage, he said.

The little girl had a bag full of retort pouch foods that may have been too heavy for her to run with, while an elderly person wearing a layer of five sweaters and people with backpacks containing passbooks, personal seals and other valuables were found under collapsed houses.

"They may have been accustomed to quakes or tsunami, but that seems to have worked negatively. I couldn't help but think what if they had fled without any belongings," he said.
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Rolfe.
 
I'm still undecided whether the tsunami walls at northern fishing ports were a liability, providing false security and allowing people to go back, or never leaving in the first place.

On the other hand, there are videos where people manage to escape just in time as the water is piling up and over the walls. And, in any case, some of the walls did make a difference between towns getting completely wiped off the face of the planet and "merely" getting the houses ripped off of their bases and dumped into some valley uphill.

Village Tarou had two walls. The destruction behind the first one is complete, there's nothing left but splinters and even those appear to be washed away. Behind the second one, there are still some larger pieces of houses left.

http://maps.google.com/maps?f=q&sou...178,141.973035&spn=0.004893,0.013808&t=h&z=17

I imagine it's more survivable when you've still got some walls or chunks of roof around you. Or is that a river in Egypt and I'm just comforting myself?

Rolfe, I did find an extended Rikuzentakata shot, but still haven't spotted the original. Even this one cuts off for a second after the house gets crushed. As the shot goes on, there's nothing left but dust...

Here's the other one, I was mentioning, showing the aftermath.
 
Thanks, Celestrin, I'll watch these later. I did watch a fairly long segment earlier where a reporter and his cameraman climbed on foot into a bay that had been cut off by the road being washed away, and where other locals believed everyone had died. When they got there, there was no sign of life, with mud and fresh snow undisturbed, and some rather worrying odours coming from the wrecked houses. Apparently the residents had felt so secure behind the tsunami wall, they had ignored the evacuation sirens.

Rolfe.
 
Rolfe said:
I wish we were hearing more about how the Japanese towns are coping. It's just fallen off the news in favour of Libya, Libya, Libya.

Maybe it's just voyeurism, but I feel a certain connection with these places whose names we've learned and whose destruction we've witnessed, and I'd be glad to know how they're all getting on.
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Don't beat yourself up Rolfe, it just shows you care, nowt wrong with that.


BTW UK Channel 5 broacast a programme last night Tsunami: British Search & Rescue. (Sorry I think it may be available only for British-based internet users again)

Some insightful comments from the participants in this documentary.

One of the rescuers commented that, during other disasters he had attended, there were usually lot's of local people about the area picking through ruins, looking for relatives etc but that the affected zones seemed to be largely deserted in this case. The inference being of course that not many survived this terrible event there.

A good programme about another angle of the disaster and it's aftermath. Don't expect any happy endings though.


Compus
 
Thanks, CM. I'm on a relay transmitter that doesn't broadcast Channel 5, and to be honest I don't usually miss it. That looks interesting though, I'll watch it.

I watched a report from a correspondent who had been to Rikuzentakata a year or two ago, and the report included film from before the disaster. I went looking for the full version on the internet, with no success, but I found a series of clips from the town's gala day in August last year. Decorated floats (just like Lanimer Day here), and everybody having a great time.

It really brings the tragedy home, to see carefree footage like that.

Rolfe.
 
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