Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 223

7.2.3 Radiated Energy and Magnitude

One the most fundamental parameters for describing an earthquake is the radiated
seismic energy. The total energy from an earthquake includes energy required to
create new cracks in rocks, energy dissipated as heat through friction and energy
elastically radiated through the Earth in the form of seismic waves. The only
quantity of these energies which can be measured is the radiated one, because it
shakes buildings and can be recorded by seismographs. Seismologists use a
magnitude scale to express the seismic energy released by each earthquake,
independent of the place of observation. Each earthquake has a unique amount of
energy, but the magnitude values given by different seismological observatories for
the same event may vary, due to the fact that seismologists use several different
methods to estimate the magnitude. Therefore, there are a lot of definitions of
magnitude: Richter magnitude or local magnitude, energy magnitude, moment
magnitude, body-wave magnitude, etc. Therefore, one must be very careful in
using the data of the earthquake magnitude, when the used scale is not mentioned.
Among these magnitude types, the technical literature is very rich in correlation
formulae (Bergman, 2000).
The scale of magnitude commonly used to express the seismic energy released
by each earthquake is the Richter scale. The typical effects of earthquakes for
various magnitude ranges are listed below:

Richter magnitude Earthquake effects

a) Micro earthquakes

Less than 3.0 M Generally not felt, but recorded

b) Low earthquakes

3.0 - 4.5 M Often felt, but it rarely causes damage, only for poorly

constructed buildings.

c) Moderate earthquakes

4.6 - 6.5 M Major damage to poorly constructed buildings,

slight damage to well-constructed buildings