That 7.1 earthquake hammering Yushu County yesterday, where the death toll is up to 400, reminded me that I wanted to share some information I gleaned from those major recent tremors. This is only April and Planet Earth has experienced five 7.0 and higher earthquakes on the Moment Magnitude Scale. It turns out that we annually experience one 8 and larger quake and 17 in the 7's. Thus, if anything, the shakes are only just above average.
Many of us grew up with the Richter Scale, which was devised mostly for California, and measures the seismic energy released. A 6.8 quake is about the largest measurable with this system, a difference of 1.0 (comparing a 5.0 to a 6.0 for example), would result in an earthquake ten times more powerful. Forget this scale!
The currently accepted tool is the Moment Magnitude Scale, which measures energy released. Without going into the shaking amplitude mathematics (where the Richter difference is multiplied by 1.5 in the exponent), a 6.0 is 31.6 times more destructive than a 5.0, while a 7.0 is a thousand times more powerful.
The major earthquakes this year have been:
Country.........Date........Moment Magnitude................Deaths
Haiti..........January 12.................7.0..........................230,000
Leogane
Chile.......February 27.................8.8.................................486
Maule
Mexico............April 4..................7.7...................................2
Baja
Indonesia.......April 6..................7.7....................................0
Sumatra
China.............April 14.................7.1................................400
Yushu
The Chile 8.8 earthquake was 500 times stronger than the Haiti 7.0, but many more died in the latter. The reasons are many, but quality of construction, depth of quake, and so on determine the lethality.
The following examples and table can be kept and used if you ever in the future want to determine the energy release (powerfulness) difference between earthquakes.
Example 1: Say you want to compare a 6.0 M_{w} earthquake with a 7.0 M_{w}. The difference is 1.0, so using a calculator with a X^{Y} key, place 10 in X and 1.5 (difference of one times 1.5) in Y, press the equal sign to get 31.6, so a 7.0 earthquake is 31.6 times more powerful than a 6.0.
Example 2: Using the table below, the Chile 8.8 M_{w} earthquake is thus 354 times more powerful than the China (Yushu County) 7.1 M_{w}, because the difference is 1.7.
Thus, to compare two earthquakes, subtract the smaller one from the larger, go to the number in the left column. The right column provides the power difference (ratio):
0.1..........1.4
0.2..........2.0
0.3..........2.8
0.4..........4.0
0.5..........5.6
0.6..........7.9
0.7........11.2
0.8........15.8
0.9........22.4
1.0.......31.6
1.1........44.7
1.2........62.8
1.3........89.4
1.4......125
1.5......169
1.6......253
1.7.....354
1.8......500
1.9......702
2.0....1000
2.1....1414
2.2....1986
2.3....2828
2.4....3977
2.5....5617
2.6....7940
2.7..11,213
2.8..15,851
2.9..22,373
3.0..31,622 4.0 1 million

Spent the end of the day at House Without a Key with Connie and Harry Olsen. That background is the very same Diamond Head at the top of this blog.

The Dow Jones Industrials jumped 104 to 11,123, with world markets all also up. Gold increased $5/toz to $1156 and crude oil surging to $86/barrel.

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