BBC Horizon - The Next Megaquake (2005) PDTV
English | AVI | XVID 1004 kbps 25 fps | 560x320 | MP3 160 kbps 48 KHz | 48mns | 349 MB
Genre: Documentary

On March 11, 2011, a magnitude 9.0 earthquake struck just off the eastern shores of Japan. Though there was only half an hours notice, an efficient tsunami warning system meant loss of life was relatively low, less than 20,000, even though the tsunami reached 10 km inshore in places. The worst disaster of this quake was economic and ecological -- $220 billion in property damage (making it the most economically damaging earthquake of all time), including the meltdown of 3 of 6 reactors at the Fukushima I nuclear power facility. Six other Japanese nuclear power facilities also recorded nuclear incidents, with significant damage occurring at Fukushima II.
The most powerful earthquake ever recorded, at magnitude 9.5, struck southern Chile on May 22, 1960. It triggered a tsunami up to 25m in height, which raced across the Pacific Ocean, reaching nearly 11m height 10,000 km away. Due to the local geography, which focused the incoming wave, Hilo, Hawaii was devastated. Loss of life in southern Chile was light, less than 6000, but as the region is quite poor, the economic impact was enormous.
On Good Friday, March 27, 1964, a magnitude 9.2 earthquake struck Prince William Sound, Alaska, the second strongest earthquake ever recorded. Many coastal villages were inundated (though with low loss of life) by the resultant tsunami, which caused moderate damage along the coast from British Columbia to California, and as far away as Hawaii. While the quake devastated the Alaskan economy, the most significant impact was geological: near Kodiak, 200 km to the southwest, some areas were uplifted by as much as 10m, while southeast of Anchorage, the land dropped 3m, putting parts of the Seward Highway under the new high tide mark.
These "great earthquakes" have one common feature: They are all megathrust quakes, which occur only along subduction zones, where one tectonic plate rides up and over another, forcing the latter back down into the mantle. These are the most powerful earthquakes of all, typically having magnitude 8 or greater. They typically involve slippage of a fault line along hundreds of kilometers, producing long-lasting quakes with durations of several minutes. The geology of subduction zones means that most of the ground motion is vertical (10s of meters); when this occurs under an ocean (almost always), the immense amount of displaced water is manifest as a tsunami.
Of all the worlds subduction zones, the ones which form the "Ring of Fire" surrounding the Pacific Ocean are perhaps the most active. However, there is one such zone which has not seen any major tectonic activity in recorded history. This is the Cascadia zone stretching from northern California roughly to the midpoint of Vancouver Island. Until recently, the only evidence of tectonic activity came from local native oral traditions, which were usually discounted as myth or legend. The general consensus was that the risk of a major earthquake in the region was very low.
This program shows how a handful of investigators defied the accepted view to turn that myth into historical truth. Through an ingenious combination of the natural sciences (geology, biology and the physics of water waves) with historical tsunami records in Japan, they showed that a megathrust earthquake, of magnitude 9.0 or greater, struck the region at approximately 9 p.m. on January 26, 1700.
It is now known that, for the past several thousand years, a significant earthquake has occurred along the Cascadia subduction zone on average every 500 to 600 years (there are periods when the interval between quakes as as low as 200 to 300 years). The entire fault is locked from the ocean floor to a depth of at least 60 km. At present, the Juan de Fuca plate is being subducted at a rate of 4 cm annually. There is evidence that quakes on the Cascadia subduction zone may have triggered most of the major quakes along the northern San Andreas fault at least for the past 3000 years (1906 San Francisco apparently being an exception).
Recent findings conclude the risk of a major earthquake in the next 50 years is significant: as high as 37% for magnitude 8 or higher, and 10% to 14% for magnitude 9 or greater. While a tsunami warning system is now being put in place, and many buildings are being re-engineered to withstand such an event, British Columbia and the US Pacific Northwest are still fully unprepared for a disaster of this magnitude.



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