The
Richter magnitude scale, also known as the
local magnitude (
M_L)
scale, assigns a single number to quantify the
amount of
seismic
energy released by an
earthquake. It
is a base10
logarithmic scale
obtained by calculating the logarithm of the combined horizontal
amplitude of the largest displacement from
zero on a Wood–Anderson torsion
seismometer output. So, for example, an
earthquake that measures 5.0 on the Richter scale has a shaking
amplitude 10 times larger than one that measures 4.0. The effective
limit of measurement for local magnitude M_L is about 6.8.
Though still widely used, the
Richter scale has
been superseded by the
moment
magnitude scale, which gives generally similar values.
The
energy release of an earthquake, which
closely correlates to its destructive power, scales with the power
of the shaking amplitude. Thus, a difference in magnitude of 1.0 is
equivalent to a factor of 31.6 (=({10^{1.0}})^{(3/2)}) in the
energy released; a difference of magnitude of 2.0 is equivalent to
a factor of 1000 (=({10^{2.0}})^{(3/2)} ) in the energy
released.
Development
Developed
in 1935 by Charles
Richter in partnership with Beno
Gutenberg, both of the California
Institute of Technology, the scale was firstly intended to be used only in
a particular study area in California, and on seismograms recorded on a particular
instrument, the WoodAnderson torsion seismometer. Richter originally reported
values to the nearest quarter of a unit, but decimal numbers were
used later. His motivation for creating the local magnitude scale
was to separate the vastly larger number of smaller earthquakes
from the few larger earthquakes observed in California at the
time.
His inspiration was the
apparent
magnitude scale used in astronomy to describe the brightness of
stars and other celestial objects. Richter arbitrarily chose a
magnitude 0 event to be an earthquake that would show a maximum
combined horizontal displacement of one micrometre on a seismograph
recorded using a WoodAnderson torsion seismometer from the
earthquake epicenter. This choice was intended to prevent negative
magnitudes from being assigned. However, the Richter scale has no
upper or lower limit, and sensitive modern seismographs now
routinely record quakes with negative magnitudes.
Because M
_{L} is derived from measurements taken from a
single, bandlimited seismograph, its values saturate when the
earthquake is larger than 6.8.To overcome this shortcoming,
Gutenberg and Richter later developed a magnitude scales based on
surface waves,
surface wave
magnitude M
_{S}, and another based on
body waves,
body wave magnitude
m
_{b}. M
_{S} and m
_{b} can still saturate
when the earthquake is big enough.
These traditional magnitude scales have been superseded by the
implementation of methods for estimating the
seismic moment and its associated
moment magnitude scale, although
still widely used because they can be calculated quickly.
Richter magnitudes
The Richter magnitude of an earthquake is determined from the
logarithm of the
amplitude of waves recorded by seismographs
(adjustments are included to compensate for the variation in the
distance between the various seismographs and the epicenter of the
earthquake). The original formula is:
 M_\mathrm{L} = \log_{10} A  \log_{10}
A_\mathrm{0}(delta),\
where A is the maximum excursion of the WoodAnderson seismograph,
the empirical function A
_{0} depends only on the
epicentral distance of the station,
delta. In practice, readings from all observing stations are
averaged after adjustment with stationspecific corrections to
obtain the M
_{L} value.
Because of the logarithmic basis of the scale, each whole number
increase in magnitude represents a tenfold increase in measured
amplitude; in terms of energy, each whole number increase
corresponds to an increase of about 31.6 times the amount of energy
released.
Events with magnitudes of about 4.6 or greater are strong enough to
be recorded by any of the seismographs in the world, given that the
seismograph's sensors are not located in an earthquake's
shadow.
The following describes the typical effects of earthquakes of
various magnitudes near the epicenter. This table should be taken
with extreme caution, since intensity and thus ground effects
depend not only on the magnitude, but also on the distance to the
epicenter, the depth of the earthquake's focus beneath the
epicenter, and geological conditions (certain terrains can amplify
seismic signals).
Richter magnitudes 
Description 
Earthquake effects 
Frequency of occurrence 
Less than 2.0 
Micro 
Microearthquakes, not felt. 
About 8,000 per day 
2.02.9 
Minor 
Generally not felt, but recorded. 
About 1,000 per day 
3.03.9 
Often felt, but rarely causes damage. 
49,000 per year (est.) 
4.04.9 
Light 
Noticeable shaking of indoor items, rattling noises.
Significant damage unlikely. 
6,200 per year (est.) 
5.05.9 
Moderate 
Can cause major damage to poorly constructed buildings over
small regions. At most slight damage to welldesigned
buildings. 
800 per year 
6.06.9 
Strong 
Can be destructive in areas up to about 160 kilometres
(100 mi) across in populated areas. 
120 per year 
7.07.9 
Major 
Can cause serious damage over larger areas. 
18 per year 
8.08.9 
Great 
Can cause serious damage in areas several hundred miles
across. 
1 per year 
9.09.9 
Devastating in areas several thousand miles across.

1 per 20 years 
10.0+ 
Epic 
Never recorded; see below for equivalent seismic energy
yield.

Extremely rare (Unknown) 
(
Based on U.S. Geological Survey
documents.)
Great earthquakes occur once a year, on average.
The largest recorded
earthquake was the Great Chilean Earthquake of May 22, 1960 which had a magnitude (M_{W}) of
9.5.
The following table lists the approximate
energy equivalents in terms of
TNT explosive force  though note that the
energy here is that of the
underground energy release
(i.e. a small atomic bomb blast will not simply cause light shaking
of indoor items) rather than the overground energy release; the
majority of energy transmission of an earthquake is not transmitted
to and through the surface, but is instead dissipated into the
crust and other subsurface structures.
Richter
Approximate Magnitude 
Approximate TNT for
Seismic Energy Yield 
Joule equivalent 
Example 
0.0 
1 kg (2.2 lb) 
4.2 MJ 

0.5 
5.6 kg (12.4 lb) 
23.5 MJ 
Large hand grenade 
1.0 
32 kg (70 lb) 
134.4 MJ 
Construction site blast 
1.5 
178 kg (392 lb) 
747.6 MJ 
WWII conventional bombs 
2.0 
1 metric ton 
4.2 GJ 
Late WWII conventional bombs 
2.5 
5.6 metric tons 
23.5 GJ 
WWII blockbuster bomb 
3.0 
32 metric tons 
134.4 GJ 
Massive Ordnance Air
Blast bomb 
3.5 
178 metric tons 
747.6 GJ 
Chernobyl nuclear disaster, 1986 
4.0 
1 kiloton 
4.2 TJ 
Small atomic bomb 
4.5 
5.6 kilotons 
23.5 TJ 

5.0 
32 kilotons 
134.4 TJ 
Nagasaki atomic
bomb (actual seismic yield was negligible since it
detonated in the atmosphere)
Lincolnshire
earthquake , 2008 
5.4 
150 kilotons 
625 TJ 
2008 Chino Hills earthquake (Los Angeles, United States) 
5.5 
178 kilotons 
747.6 TJ 
Little
Skull Mtn. earthquake (NV, USA), 1992
Alum Rock
earthquake , 2007 
6.0 
1 megaton 
4.2 PJ 
Double Spring Flat earthquake (NV, USA), 1994 
6.5 
5.6 megatons 
23.5 PJ 
Rhodes ,
2008 
6.7 
16.2 megatons 
67.9 PJ 
Northridge earthquake ,
1994 
6.9 
26.8 megatons 
112.2 PJ 
San Francisco Bay Area earthquake ,
1989 
7.0 
32 megatons 
134.4 PJ 
Java earthquake , 2009 
7.1 
50 megatons 
210 PJ 
Energy released is equivalent to that of
Tsar
Bomba, the largest thermonuclear weapon ever
tested. 
7.5 
178 megatons 
747.6 PJ 
Kashmir earthquake ,
2005
Antofagasta earthquake ,
2007 
7.8 
600 megatons 
2.4 EJ 
Tangshan earthquake ,
1976 
8.0 
1 gigaton 
4.2 EJ 
Toba
eruption 75,000 years ago; which, according to the Toba catastrophe theory, affected
modern human evolution
San Francisco earthquake ,
1906
Queen
Charlotte earthquake , 1949
México City
earthquake , 1985
Gujarat
earthquake , 2001
Chincha Alta
earthquake , 2007
Sichuan
earthquake , 2008 (initial estimate: 7.8) 
8.5 
5.6 gigatons 
23.5 EJ 
Sumatra
earthquake , 2007 
9.0 
32 gigatons 
134.4 EJ 
Lisbon Earthquake , All Saints Day,
1755 
9.2 
90.7 gigatons 
379.7 EJ 
Anchorage earthquake ,
1964 
9.3 
114 gigatons 
477 EJ 
Indian Ocean earthquake, 2004 (40 ZJ in this case) 
9.5 
178 gigatons 
747.6 EJ 
Valdivia earthquake , 1960 (251 ZJ in this case) 
10.0 
1 teraton 
4.2 ZJ 
Never recorded by humans. 
13.0 
10^{8} megatons = 100 teratons 
5x10^{30} ergs = 500 ZJ 
Yucatán Peninsula impact (causing Chicxulub crater) 65 Ma ago. 
See also
References
 USGS: The Richter Magnitude Scale
 USGS: FAQ Measuring Earthquakes
 USGS: List of World's Largest Earthquakes
 What is Richter Magnitude?, with mathematic
equations
External links