Posted on 11/09/2006 4:44:21 PM PST by blam
Himalaya mega-quakes likely every 1000 years
13:08 09 November 2006
NewScientist.com news service
Catherine Brahic
A colossal reservoir of energy stored up under the Tibetan plateau has been discovered – and it can only be fully released by mega-earthquakes striking about every 1000 years, researchers have found.
The study suggests that earthquakes in the past 200 years in the central Himalaya, while catastrophic, are small in comparison to what the region has seen in the past - and will see again.
The energy builds up as the result of the collision between the Indian subcontinent and Asia, and the movement of the two continental plates was tracked using GPS technology.
The reservoir of energy is so large because of the nature of the two plates. They are both continental, and therefore made of relatively low density rocks. This means that, rather than one heavier, denser plate plunging deep under a lighter plate, as happens when dense oceanic crust plunges under a continental plate, they both strive to float near the surface of the planet. This generate a lot of friction, causing a huge amount of energy to be stored underneath Tibet (see map).
Draining the reservoir
Large earthquakes registering between magnitude 7.5 and 8.2 tap into this reservoir but do not deplete it, the researchers have determined. Instead, every 1000 years or so, a mega-earthquake of magnitude 8.4 to 8.6 "is essential to drain the reservoir", says lead researcher Roger Bilham of the University of Colorado, US.
(Excerpt) Read more at newscientist.com ...
It's always something.
Alaska had the 9.3, so this proposed mega-quake isn't really so extraordinary. Remember the 9.3? It flipped railroad trains and ocean liners over. It flipped Valdez right off the map--gone, just gone.
BTW, Seattle has a 9.3 on tap. One of these days. Seattle could lose its harbor; Anchorage did.
THE 1964 EARTHQUAKE
The great Alaskan earthquake of 1964 was the largest earthquake in North America and the second largest ever recorded (largest occurred in Chile in 1960). The nine deaths that were due to the earthquake occurred in downtown Anchorage (5), Turnagain Heights (3), and at the International Airport (1). The earthquake occurred at 5:36pm on March 27, 1964, Alaska Standard Time (or, at 03:36 Universal Time code on March 28, 1964). The epicenter was in the Northern Prince William Sound (61.1N 147.5W) about 75 miles E of Anchorage, or about 55 miles west of Valdez. The reported Richter magnitudes (Ms) for this earthquake ranged from 8.4 to 8.6. The moment magnitude (Mw) is reported as 9.2. The depth, or point where the rupture began was about 14 miles within the earth's crust.
The strong ground motion reported in the Anchorage area lasted about 4-5 minutes which triggered many avalanches and landslides - some being tsunamigenic. Ground deformations were extensive with some areas east of Kodiak being raised by 30 feet and areas about Portage being dropped by 8 feet (Pflaker, 1964). The rise is estimated to come in two thrusts of about 5 meters each. The maximum intensity reported was XI on the modified Mercalli Intensity scale, indicating major structural damage, and ground fissures and failures. This scale is a 12-point one usually given in roman numerals ranging from I, (not felt/no damage) to XII (total destruction many lives lost). From this event, significant damage covered an area of about 50,000 square miles. Intensities of IV-V (felt by most people/minor damage) were reported as far away as Cold Bay, Bethel, McGrath, Kotzebue, Deadhorse, Ft. Yukon, Eagle and Skagway.
Anchorage does not have a harbor - it has a dock. The waterfront areas in Seward, Valdez, Kodiak, and Whittier got hammered by the tsunami. Valdez was rebuilt out of the path of the NEXT tsunami. You can still see remnants of the original townsite at the northern edge of Valdez Arm on the delta of all the rivers that join up there.
Anchorage is built on clay and silt. If there's a whole lot of shaking going on, that stuff liquifies. The next big one will see most of downtown Anchorage go slippin' and slidin' into the west and I would not want to be in the Turnagain area either.
Wait a minute! Did Anchorage have a harbor before the '64 quake? Down by the rail yards? I got there in 1976 and the Ship Creek area was just mud (which it still is).
I know a fellow in Alaska that lived just inland from all the fancy homes in Turnagain that went over the bluff. He was 16 at the time. He was outside and layed down. As he described it, the ground waves were like a four foot chop on the ocean. Now THAT would have been something!
I'd probably need to change my undies after...
Yes, it did. It was called Anchorage when the Feds built the railroad (completed to Fairbanks in '23), and it still is although there isn't an anchorage anymore. Nenana was also a railroad construction town. The anchorage is mud now, which makes Seward and Valdez the main deep water ports.
The quake lasted for four minutes. That is really a long time and indicates a lot of movement. The 7.3 a few years ago lasted about half a minute. In Fairbanks they thought it was a massively heavy quake and when they learned it was epicentered near Anchorage they were stuned.
Thanks.
Tis a region of the world I care a lot about.
I saw that illustrated on one of the documentary channels. Amazing.
Wasn't that the Denali Fault Quake that happened in early November 2002? The epicenter of that one was on the road between Glenallen and Tok Junction. I know. I drove through it 10 days later.
Yes. That nearly knocked the speakers off the top of my PC monitor. Felt like a 5.0 here, but went on for 20 seconds plus, so was clearly some distance away and larger. The pipeline rearranged itself on some of the supports--nearly slipped off the skid pads altogether in one location.
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