Friday, March 11, 2011

Putting magnitude in perspective

How does the Japan earthquake of magnitude 8.9 compare to other recent large quakes?

The news media do a better job than they used to of noting that each magnitude number is 10 times that of the lower number. But most everyone assumes that refers to the relative amount of energy released by the quake - comparable to measuring the power of atomic bombs for instance.

Not true.

The magnitude is a measure of the amplitude of the seismic waves. But each 1.0 magnitude increase is equal to approximately a 32 times increase in energy release. Each increase of magnitude by 2.0 equals 32 x 32 or 1,000 times increase in energy released.

The M8.9 Japan quake released the equivalent of 336 megatons of TNT. In comparison, last months, Christchurch, New Zealand M6.3 quake was equal to 43 kilotons, and last years M7.0 Haiti quake was equal to 474 kilotons.

The Japan quake was about 7814 times bigger than the Christchurch quake and 709 times larger than the Haiti quake.

I've simplified this in regards to Richter magnitude vs moment magnitude but my intent is to emphasize the power of the Japan quake.


  1. Anonymous8:34 AM

    I think you may be missing a decimal when you say "last years M7.0 Haiti quake was equal to 474 kilotons". Otherwise the next sentence makes no sense and is mathematically incorrect.

    Not a geologist, just trying to understand when the math doesn't seem right...


  2. Doug, I just checked the numbers and they look right. What am I missing?

  3. Anonymous11:05 AM


    Sorry, someone pointed out to me that I had missed that you switched from kilotons to megatons. Those 3 orders of magnitude can make a rather large difference. My bad.