How big are
earthquakes?
The size of an earthquake can be measured in two ways. The intensity
measures the destructiveness of the quake while the magnitude measures the energy released
by the quake.
Earthquake Intensities
The intensity of an earthquake is a measure of the destructive effects
of the quake at the surface. It is measured on an arbitrary scale of 12 degrees modified
from an original scale devised by the Italian seismologist Giuseppe Mercalli. The scale
uses information supplied by people living in the area of the quake.
The Modified Mercalli Intensity Scale
| Intensity |
Description |
Characteristic effects |
| I |
Instrumental |
Not felt by people, only detected by seismographs. |
| II |
Feeble |
Felt only by a few people at rest, especially on upper
floors of buildings. Delicately suspended objects may swing. |
| III |
Slight |
Felt noticeably indoors; like the vibrations due to a
passing truck. Standing motor cars may rock slightly. |
| IV |
Moderate |
Felt indoors by many people, outdoors by few. Dishes,
windows, doors rattle. May awaken some sleepers. Standing cars rocked noticeably. |
| V |
Rather strong |
Felt by nearly everyone, many awakened. Some dishes and
windows broken; occasional cracked plaster; unstable objects overturned. Some disturbance
of trees, poles and other tall objects. |
| VI |
Strong |
Felt by all; many frightened and run outdoors. Some heavy
furniture moved; some falling plaster or damaged chimneys. Damage slight. |
| VII |
Very strong |
General alarm; people run outside. Walls crack; chimneys
fall. Considerable damage in poorly designed structures. Noticed by persons in moving
vehicles. |
| VIII |
Destructive |
Considerable damage in ordinary substantial buildings with
partial collapse. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
furniture overturned. Changes in well water. Car drivers seriously disturbed. |
| IX |
Ruinous |
Considerable damage with partial collapse of substantial
buildings. Buildings moved off foundations; ground cracks conspicuous. Underground pipes
broken. |
| X |
Disastrous |
Ground cracks badly; landslides on river banks and steep
slopes; rails bent; many buildings destroyed. |
| XI |
Very disastrous |
Broad fissures in ground; major landslides and earth
slumps; floods. Few buildings remain standing; bridges destroyed; nearly all services
(railways, underground pipes, cables) out of action. |
| XII |
Catastrophic |
Total destruction. Ground rises and falls in waves; lines
of sight and level distorted. Objects thrown into the air. |
The earthquake intensity felt at a location depends not
only on the magnitude of the quake but also on the distance from the epicentre, depth of
the focus, and on local surface and subsurface geological conditions. The intensity
decreases outwards from the source, areas of similar intensity forming a roughly circular
pattern aroung the epicentre.
Earthquake Magnitudes
Calculating the energy released by an earthquake can be a long,
complicated process involving exact measurements of the fault dimensions, amount of slip,
and other factors. A simpler method was devised by the American seismologist Charles
Richter based on the amplitude of the seismic wave recorded by seismographs. This method can be used by seismologists
all over the world who can come up with a magnitude measurement within minutes of the
earthquake being recorded.
The Richter scale is logarithmic. An increase in magnitude of one unit
corresponds to a tenfold increase in the size of an earthquake. Thus an earthquake of
magnitude 6 is ten times larger than one of magnitude 5, and a hundred times larger than
one of magnitude 4.
Richter Magnitude Scale
| Magnitude |
Number of earthquakes
per year globally |
Typical intensity
at epicentre |
| >8.0 |
0.1 - 0.2 |
XII |
| 7.4 - 8.0 |
4 |
XI |
| 7.0 - 7.3 |
15 |
X |
| 6.2 - 6.9 |
100 |
VIII - IX |
| 5.5 - 6.1 |
500 |
VII |
| 4.9 - 5.4 |
1,400 |
VI |
| 4.3 - 4.8 |
4,800 |
IV - V |
| 3.5 - 4.2 |
30,000 |
II - III |
| 2.0 - 3.4 |
800,000 |
I - II |
Earthquakes in B.C. Home Page.
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This page was last updated
Wednesday, March 08, 2006
