Energy is surely one of the best arguments for adopting SI. Clearly, it
would be easy to specify all earthquakes in joules, with a note if necessary
indicating which guy's formula was used to compute them. All quakes big
enough to get on the news would be in TJ or PJ, with maybe a very few
monsters in EJ. If they wanted too, they could give everything in TJ. Then
nearly all newsworthy quakes would have a simple value from 1 to 900 000,
with no decimals, and folks could see directly how they really compared.
Baron Carter found another site on earthquake energy vs. magnitude
(www.aeic.alaska.edu/Input/lahr/magnitude/energy_table.html) with a
mind-boggling mishmash of energy units, including:
"inch-pounds" [meaning poundforce inch, 113 mJ]
"foot-pounds" [meaning poundforce foot, 1.356 J]
"ounces TNT" [1.97 kJ]
"pounds TNT" [31.5 kJ]
"tons TNT" [63.1 MJ]
"ktons TNT" [63.1 GJ]
"mtons TNT" [63.1 TJ]
Worse yet, these "Energy equivalent weights of TNT" are completely different
(66 times smaller) than the "tons TNT" used used for nuclear devices and
used in other tables comparing earthquake magnitiude and energy! Those
latter tons of TNT are a selling device like horsepower, supposedly equal to
the explosive energy of a short ton of TNT (even though they are symbolized
t, which is the metric ton), but in reality are arbitrarily defined as
exactly 10^9 thermochemical calories or exactly 4.184 GJ. Of course, few
people, unless they are in the bombing business, have any idea of the
explosive energy of a ton (long, short, or otherwise) of TNT.
But all these sites seem to agree that the energy of a quake is
E = (k) 10^(1.5 M)
where E is the energy in joules, M is one of the innumerable magnitude
scales, and k is a constant. The formulas only disagree on the value of the
constant, which seems to range from 22 000 up to 63 000, depending on which
"Richter scale" you use. For example, the big 1964 Alaska quake is rated
mag. 8.5 on one scale, 8.9 on another, and 9.5 on another. We may not be
able to figure the energy precisely, but at least a joule is a joule.
