Earthquake Energy

The ultimate source of the energy for earthquakes is the cooling of Earth, which drives plate tectonic processes. How much energy is "released" during a particular earthquake? Well, the total amount of energy released is hard to estimate. But we have reasonable estimates of the amount of energy released as seismic waves during earthquakes. This energy is what matters to those concerned with the effects of earthquake shaking, but ignores the potenailly large amount of energy converted to heat and expended fracturing rocks along the fault as the earthquake processes.

The relationship between magnitude and energy (radiated as seismic waves) is empirical, and was originally derived by Gutenberg and Richter (GR) in the 1940's and 1950's. The increase in energy associated with an increase of one unit in magnitude is a factor of 31.6 (roughly 32). This is 101.5, which is the slope of the trend in the 1956 GR seismic energy-Ms relationship. If we let Es represent the energy radiated from the earthquake in the form of seismic waves and Ms represent the surface wave magnitude, then

Es = 10(11.8+1.5*Ms - 7) (in Joules, J).

Joules are very small quantities of energy. When comparing large energy amounts, the press often presents the numbers in terms of the equivalent kilotons (kt) of TNT.  Early nuclear explosions were measured in kt of equivalent TNT (Trinity was 19 kt) and later the largest exlposions were measured in megatons of equivalent TNT.

Another measure of energy is the kilo-watt hour, which is the amount of energy used in an American home over the time period of one hour (US homes typically consume energy at a rate of 1 kilowatt). The table below lists the earthquake energy equivalents for a range of magnitudes (Ms).

Ms Energy Energy Energy
4 6 x 1010 J 17,500 kWh 0.015 kT  
5 2 x 1012 J 500,000 kWh 0.5 kT  
6 6 x 1013 J 17,500,000 kWh 15 kT  
7 2 x 1015 J 500,000,000 kWh 500 kT  
8 6 x 1016 J 17,500,000,000 kWh 15,000 kT  
9 2 x 1018 J 500,000,000,000 kWh 500,000 kT  

 

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.