Geoscience Reference
In-Depth Information
Table 4.1
Earthquake magnitudes and
corresponding descriptors
Earthquake
Earthquake
Approximate number
magnitude
descriptor
per year
>
8.0
Great
1-3
7.0-7.9
Major
<
20
6.0-6.9
Strong
180
5.0-5.9
Moderate
1 800
4.0-4.9
Light
10 000
3.0-3.9
Minor
90 000
2.0-2.9
Very minor
1 000 000
Source
: IRIS.
Japan, on the plate boundary with the subducting Pacific plate, was followed by
ayear during which the amount of displacement resulting from aseismic slip was
comparable to the displacement that occurred during the earthquake. The next
decades will provide much more data on deformation, slip rates and the release
of strain in earthquakes around the world.
4.2.3 Earthquake magnitude and moment
The concept of earthquake magnitude was introduced by C. F. Richter in 1935.
He was studying local earthquakes in southern California and proposed a par-
ticular logarithmic magnitude scale for them that was based on the maximum
amplitude of the first arriving P-wave as measured by a 12-Hz Wood-Anderson
seismometer. His was a local magnitude scale for southern California. Since then,
measurements of magnitude have been extended globally to earthquakes at all
depths and distances and to both body and surface waves. These are quantita-
tive scales that give an estimate of the size of the earthquake. Richter's name is
known around the world because press releases on earthquakes invariably quote
magnitude as being 'on the Richter scale'. The descriptors generally associated
with earthquakes of a particular magnitude are listed in Table 4.1.
Magnitude scales
for earthquakes are all logarithmic and are based on mea-
surements of the amplitude of the seismic waves. In addition there are
intensity
scales
such as the Mercalli scale (Appendix 10), which are subjective and are
based on the shaking and damage to buildings, breaking of glass, ground crack-
ing, people running outside, and so on. Intensity scales must take into account
different building codes and styles around the world and so are necessarily con-
tinent/country specific. Maps showing the extent of earthquake damage would
normally utilize an intensity scale.
