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Sunday, September 14, 2008

HISTORIC MEGA EARTHQUAKES AND TSUNAMIS (Part 3)

Most major tsunamis, those that are triggered by subsea earthquakes, top out at 10 meters (33 feet) in the far field (thousands of miles away). Nearer the event, run-ups can be much higher. The report is that this maximum is due to the underground vertical separation generally being limited to this upheaval. For example, although 35 meter (115 feet) eyewitness guesses were reported in the near field (less than 30 minutes from the event), the December 26, 2004, Sumatra tsunami apparently had a maximum height of 10.04 meters in the farther field, and traveled around the world, even reaching Point Reyes, California, with an amplitude of 15.6 inches, Atlantic City, New Jersey (9 inches) and Brest, France (3.2 inches).

However, the bathymetry (physical underwater conditions at the coastline) can complicate things, as the 8.3 magnitude earthquake in the Aleutian islands of March 9, 1957 produced a 16 meter (52.5 feet) tsunami on the North Coast of Oahu. There is an amplification factor, sometimes, caused by the bathymetry, when the wave hits. Then there is the confusion between the actual amplitude of the wave relative to sea level, and the so-called run-up, which can be higher, caused by the momentum of the wave.

The Great Kamchatka Earthquake of October 17, 1737, supposedly produced a tsunami greater than 50 meters on the North Kurile Islands, but this was still generally in the basic neighborhood. The Santorini eruption of 1638 BC produced a tsunami also estimated at 50 meters and the 1490 BC eruption might have caused the end of the Minoan civilization in Greece, but there were no long-distance catastrophes. 50,000 died from the tsunami that hit Taiwan on May 22, 1782 and 36,417 from the Krakatoa explosion on August 27, 1883. The bottom line is that a mega-event in or near the ocean can cause great damage in the near field, but generally does not exceed 10 meters thousands of kilometers away. Of course, a height of ten meters, or 33 feet, is a heck of a large wave, and considerable damages do occur. Also, there is something about volcano flank failures and landsides that seem to magnify the effect, at least in the vicinity of the event.

There is no scientific definition for a mega tsunami, but anything over 40 meters (130 feet) should qualify, and Charles Mader has suggested 100 meters. Gary McMurtry of the University of Hawaii has told me that a 1,600 foot (nearly 500 meters) wave appeared on the Big Island of Hawaii 110,000 years ago, probably caused by a submarine landslide from the undersea flank of Mauna Loa Volcano. This remains a controversial incident.
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Tropical Depression Ike is now at 35 MPH, but, interestingly enough, could further strengthen to gale force winds (45 MPH) and leave the U.S. through Michigan into Canada. There will continue to be huge personal inconveniences, as electricity will be out in large portions of the Galveston-Houston area into southwest Louisiana for several weeks. Search and recovery operations are just beginning, but it is not anticipated that the current number of deaths (20) will match the 80 in the Caribbean. The feared tens of billions in damages might settle at, perhaps, $10 billion.
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Typhoon Sinlaku is continuing to dump considerable rain on northern Taiwan (up to 6 feet), and, now at 80 MPH, should weaken, but head north towards Kyushu, Japan, largely missing Okinawa. However, this has been an eratic and unpredictable storm.
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Crude oil is now selling for just UNDER $100/barrel.
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