Earthquakes are among the least of their worries.
Posted on 02/07/2005 4:47:42 PM PST by Strategerist
EVANSTON, Ill. --- Northwestern University seismologists have determined that the Dec. 26 Sumatra earthquake that set off a deadly tsunami throughout the Indian Ocean was three times larger than originally thought, making it the second largest earthquake ever instrumentally recorded and explaining why the tsunami was so destructive.
By analyzing seismograms from the earthquake, Seth Stein and Emile Okal, both professors of geological sciences in Northwestern's Weinberg College of Arts and Sciences, calculated that the earthquake's magnitude measured 9.3, not 9.0, and thus was three times larger. These results have implications for why Sri Lanka suffered such a great impact and also indicate that the chances of similar large tsumanis occurring in the same area are reduced.
"The rupture zone was much larger than previously thought," said Stein. "The initial calculations that it was a 9.0 earthquake did not take into account what we call slow slip, where the fault, delineated by aftershocks, shifted more slowly. The additional energy released by slow slip along the 1,200-kilometer long fault played a key role in generating the devastating tsunami."
The large tsunami amplitudes that occurred in Sri Lanka and India, said tsunami expert Okal, result from rupture on the northern, north-trending segment of the fault -- the area of slow slip -- because tsunami amplitudes are largest perpendicular to the fault.
Because the entire rupture zone slipped (both fast and slow slip fault areas), strain accumulated from subduction of the Indian plate beneath the Burma microplate has been released, leaving no immediate danger of a comparable ocean-wide tsunami being generated on this segment of the plate boundary. However, the danger of a local tsunami due to a powerful aftershock or a large tsunami resulting from a great earthquake on segments to the south remains.
The analysis technique used by Stein and Okal to extract these data from the earth's longest period vibrations (normal modes) relied on results developed by them and colleague Robert Geller (now at the University of Tokyo) in their graduate studies almost 30 years ago. However, because such gigantic earthquakes are rare, these methods had been essentially unused until records of the Sumatra earthquake on modern seismometers became available.
The largest earthquake ever recorded, which measured 9.5, was in Chile on May 22, 1960.
One of the most idiotic elements of the imbecilic made-for-TV movie "10.5" was a seismologist sitting at a screen calling out the magnitude rising as the quake was happening. Doesn't work that way :-)
Well, I guess the good news is that at least the Earth got a really good CAT scan out of it, right? Can't all those readings also tell us something about the interior structure of the earth?
"By analyzing seismograms from the earthquake, Seth Stein and Emile Okal, both professors of geological sciences in Northwestern's Weinberg College of Arts and Sciences, calculated that the earthquake's magnitude measured 9.3, not 9.0, and thus was three times larger. These results have implications for why Sri Lanka suffered such a great impact and also indicate that the chances of similar large tsumanis occurring in the same area are reduced."
The author of the news article doesn't understand the Richter Scale. A 10.0 earthquake is 10 times as large as 9.0 quake. The Richter Scale is logarithmic. A 9.3 quake is about 2 times as large as a 9.0 quake.
Technically, the Richter scale is not currently used for anything.
It's the Moment Magnitude scale I'm sure they're referring to in the press release.
http://wwwneic.cr.usgs.gov/neis/eqlists/eqstats.html
And the article isn't incorrect, just unclear. a 9.3 has twice the ground motion of a 9.0, but does in fact release THREE times the energy of a 9.0, as you can see from the link above.
Earthquakes are among the least of their worries.
I don't believe that the author of the article is correct, but I also don't think you are correct.
Since I am no expert, I would have to defer to someone that is.
You are correct that it is logarithmic, but the magnitude factor you state I am not sure of.
I will find out.
Well, except for confusing the public.
And of all the conclusions, your statement concerning the length of time it would take to accurately evaluate this large scale slippage, will likely be the most accurate.
I predicted 250,000 dead in two months from the incident. I was really hoping to be wrong.
That's because it's a really stupid scale. Nobody understands it except the scientists who study earthquakes. It should be dumped for a simple graph from one to one hundred. It would be good for the public to understand magnitudes and they NEVER will given the perversity of the Richter obfuscation.
The problem is compressing all quakes into a 1 to 100 non-logarithmic scale. You really can't.
>>The problem is compressing all quakes into a 1 to 100 non-logarithmic scale. You really can't. <<
Lets see.
Richter = linear scale
1 = 1
2 = 10
3 = 100
Yeah; that "put them all on a 1-100 scale" idea runs out of room real fast. Maybe that is why Charles Richter used a logarithmic scale in the first place? So much of physics is log rather than scalar, maybe the pubic schools should take some time to demystify it.
In my case, learning to use a slip-stick (back when a "calculator" had a power cord, said 'Marchant' on it, was a bit bigger than a typewriter, and went, "Ka-chunka-ta, ka-chunka-ta, ka-chunka-ta, ka-chunk-a-ding!") drove home the realities of log scales.
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