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Japan sits at the boundaries of no less than four different plates.
Three of the boundaries are marked by ocean trenches. These mark the zones where one plate is subducted beneath another.
Subduction zones produce volcanoes and earthquakes and can also produce tsunamis.
This earthquake was the sixth largest since 1900, when seismological records began. The most devastating earthquake to strike Japan was in 1923, when a magnitude 7.9 tremor devastated Tokyo and Yokohama and killed an estimated 142,000 people.
Marking the epicentre as a dot on a map is misleading. The zone of fracture, in this case, was a large rectangular slab of ocean floor off the NE coast of Japan. The earthquake occurred at the relativel shallow depth of 15 miles, meaning much of its energy was released at the seafloor.
The Pacific Plates move fast in tectonic terms, at arate of 9cm per annum. This leads to the rapid buildup of huge amounts of energy. As the Pacific Plate moves down or subducts under the Okhotsk Plate, it sticks to the overhead plate and and pulls it down too. At 2.46 on Friday 11th March, 2011 the overhead plate sprang loose, causing the seafloor to spring upwards by several metres.
he scale used to measure the strength of earthquakes is called the Richter Scale.
"The largest wave heights are expected near the earthquake epicenter, off Japan. The wave will decrease in height as it travels across the deep Pacific but grow taller as it nears coastal areas. In general, as the energy of the wave decreases with distance, the near shore heights will also decrease (e.g., coastal Hawaii will not expect heights of that encountered in coastal Japan). The second image shows the depth of the Pacific Ocean floor. Notice the similarity between areas of low wave height and deeper areas of the ocean." NOAA Environmental Visualization Laboratory
Notice how abruptly the Japanese islands rise out of the ocean. Other coastal Asian areas have much more gradual slopes. The islands and mountain ranges throughout the ocean, visible in this imagery, also affect the tsunami travel time and speed. In the open ocean, tsunamis can travel at speeds up to 500 mph (800 kph). This momentum is what creates such a destructive force as the wave moves inland. NOAA
As tsunami waves travel across ocean basins they are only centimeters high, but extend down to the ocean floor (unlike traditional waves which are only surface features). As the tsunami waves approaches the coast, the shallowing ocean floor pushes the water mass upwards. The quicker the ocean floor transitions from deep to shallow, the greater potential for a higher wave height. The epicenter of the earthquake that triggered the recent Japanese tsunami was very close to shore, though, which reduced the amount of the water column that is displaced and the overall tsunami height. NOAA
The Japanese PM did really well. Got his condolences in and then took charge of the whole operation (which runs like clockwork in Japan and doesn't really need anyone extra). Alex Salmond did really well also. Asked the audience to show their support for Japan by applauding. Did his homework as well. Cameron is in Brussels, trying to look like a European leader but only succeeding in looking like a miffed school boy.
As the phone system collapsed under the weight of demand, Skype and Twitter (in the hands of mainly Canadians and Americans) supposedly came to the fore. Except that not one meaningful warning or piece of information came from either of them. Radio and satellite links dominated the real battlefield while the rest just supplied chatter.
The emergency services had a good day 2. The nuclear engineers and technicians did not/
Fires still rage all along the coast.
Four workers were injured on Day 2 when there was an explosion and radiation leak at the 40 year old Fukushima Daiichi nuclear reactor. Workers have been struggling to cool the shut down reactor since the tsunami took out it’s reserve cooling plant. This has the potential to grow into a major catastrophe.