The Richter Earthquake Magnitude Scale, which was perfected by Charles Richter of Cal Tech in 1935, has always been misleading for people who are not used to thinking logarithmically. In this case, it's log10: every whole position is 10 times greater than the one before. Therefore, if 4.0 is taken to be the equivalent of 10 units, 5.0 is 100.
But according to a new way of measuring earthquakes, based on the seismic "moment of force," translated to the equivalent energy released by an explosion of TNT, the Richter Scale is converted from a log10 to a 2/3 log10 scale. In this scale, every positional increase, e.g. Richter 4.0 to 5.0, goes up by a factor of 31.62, which we can round to 32. Thus, R 2.0 corresponds to the detonation of 1 ton of TNT, R 3.0 = 32t, R 4.0 = 1000t, R 5.0 = 32,000t, 6.0 = 1,000,000t, 7.0 = 32,000,000t, and so on. This "Moment Magnitude" is represented by M_{w}. The "w", introduced by Hiroo Kanamori in 1977, presumably stands for "work"; it refers to "elastic strain energy." The formula used here is: Tonnage = 10^{(1.5R3)}.
The Kelly Kiloton Index (KKI), formulated in 2006 by H. A. Kelly of UCLA, in consultation with Geoffrey Mess of the UCLA Math Department, aims at giving a "realistic" picture of earthquake energy. It uses the kiloton (= 1000 metric tons = 2,200,000 lbs) as the basic unit. Here is the KKI range for Richter 6.0 to 6.9 and for 7.0 to 7.9:
Richter 
6.0 
6.1 
6.2 
6.3 
6.4 
6.5 
6.6 
6.7 
6.8 
6.9 
KKI 
1000 
1400 
2000 
2800 
4000 
5600 
8000 
11,000 
16,000 
22,000 
Richter 
7.0 
7.1 
7.2 
7.3 
7.4 
7.5 
7.6 
7.7 
7.8 
7.9 
KKI 
32,000 
45,000 
63,000 
90,000 
125,000 
180,000 
250,000 
355,000 
500,000 
710,000 
The increase for every 2 positions, say, from R 6.0 to R 8.0, is not x 100, as it would be on a log10 scale, but x 1000. So, since 6.0 is 1000, 8.0 is KKI 1000 x 1000 = 1,000,000. Similarly, 8.1 is KKI 1400 x 1000 = 1,400,000, and so on. See below for a complete table.
Here is how some past earthquakes register on the Richter Scale and the Kelly Kiloton Index:
Assisi 1997 Northridge (LA) aftershock 1994 Sylmar (LA) 1971 Northridge (LA) 1994 Loma Prieta Peak (SF) 1989 Pakistan 2005 San Francisco 1906 Sumatra 2004 Chile 1960 
Richter 5.6 Richter 5.9 Richter 6.6 Richter 6.7 Richter 7.1 Richter 7.6 Richter 8.3 Richter 9.2 Richter 9.5 
KKI 250 KKI 710 KKI 8000 KKI 11,000 KKI 45,000 KKI 250,000 KKI 2,800,000 KKI 63,000,000 KKI 180,000,000 
So, the Loma Prieta Earthquake (from San Francisco to Santa Cruz) of 1989 was more than 5 times bigger than the Sylmar quake of 1971, and 4 times bigger than the Northridge quake of 1994. But the San Francisco quake of 1906, just over one position larger on the Richter scale, was 62 times bigger than Loma Prieta, and the recent Sumatra quake of 2004, just two Richter positions higher, had 1400 times the energy. It should be clear that small quakes do not do much in the way of defusing or diffusing the pentup energy that the big ones have in store for us.
Complete Conversion Table
Richter Scale

Kelly Kiloton Index


2.0 
.001 (1 ton) 

3.0 
.032 (32 tons) 

4.0 
1 (1000 tons) 

5.0 
32 
Assisi 1997

6.0

10001400 
Sylmar (LA) 1971 
7.0

32,000 

8.0

1,000,000 
San Francisco 1906 
9.0

32,000,000 
Sumatra 2004

10.0 
1,000,000,000 
