A measure of
the strength of an earthquake or strain energy released by it, as determined by
seismographic observations. The amplitude on a seismogram, the magnitude and
the energy released are related through a log- linear relationship, which was
originally defined by Charles Richter in 1935. An increase of one unit of
magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave
amplitude on a seismogram or approximately a 30-fold increase in the energy
released.
In other words, a magnitude 6.7
earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake
- or it takes about 900 M:4.7 earthquakes to equal the energy released in a
single 6.7 earthquake! This is an open-ended scale and hence there is no beginning
or end to this scale.
However, rock mechanics seems to
preclude earthquakes smaller than about -1 or larger than about 9.5. An
earthquake of magnitude -1.0 releases about 900 times less energy than a M:1.0
quake. Except in special circumstances, earthquakes below M:2.5 are not
generally felt by humans. Depending upon the range of magnitude, epicentral
distance and the type of seismic waves considered in the computation, there are
several magnitude scales in use as: Local magnitude (Ml or ML), commonly
referred to as "Richter magnitude", Surface-wave magnitude (Ms),
Body-wave magnitude (mb), and Moment magnitude (Mw).
The first three magnitude scales Ml,
Ms and mb make use of amplitudes and time periods of seismic wave and suffer
from the saturation effect. They have some or other limitation with regard to
their applicability uniformly to all magnitude ranges, epicentral distances and
focal depths. To avoid the saturation effect and standardize the magnitude
scales, a magnitude scale based on seismic moment (Mo) was proposed by Kanamori
(1977).
The moment magnitude (Mw) scale is
estimated using the formula, Mw=(log Mo –16)/1.5, where Mo, is the seismic
moment in dyne-cm. Since seismic moment is a measure of strain energy released
from the entire rupture surface, a magnitude scale based on seismic moment most
accurately describes the size of large earthquakes. Since Mo does not saturate,
so also Mw. The moment magnitude scale is the most preferred magnitude scale in
case of large earthquakes. |