Richter
magnitude
less than
3.5
3.5-5.4
Under 6.0
earthquake effects
Generally not felt, but recorded.
Often felt, but rarely causes damage.
At most slight damage to well-designed
buildings. Can cause major damage to
poorly constructed buildings over small
regions.
6.1-6.9
7.0-7.9
8 or
greater
Can be destructive in areas up to about
100 kilometers across where people live.
Major earthquake. Can cause serious
damage over larger areas.
Great earthquake. Can cause serious
damage in areas several hundred
kilometers across.
Charles F. Richter developed the Richter Magnitude Scale in 1934. Richter
defined the scale as “The logarithm to base 10 of the maximum seismic wave
amplitude recorded on a standard seismograph at a distance of 100 kilometers
from the earthquake epicenter.” The seismic wave used in the calculation is not
specified (Bolt 104).
Because it is not specified, S waves or P waves can be
used. The Richter scale was originally designed by Richter to differentiate
between earthquakes with a low focus point in southern California. The Richter
scale is referred to ML, with “L” for local. After many seismograph stations were
established, it became clear the formula was only valid for a certain frequency
and distance ranges (“Measuring Earthquakes”). The original scale was modified
to measure earthquakes at any distance, focal depth and compensate for
geological variations from place to place (Lutgens and Edwards). Adjustments
were also included into the formula to compensate for the variation of the
distance between different seismographs (“Severity”).
The moment magnitude is based on the amount of displacement that occurred
along a fault zone rather than the measurement of ground motion at a given point
(Lutgens and Edwards 314). The Moment magnitude measures energy released
by the earthquake more accurately than the Richter scale. The amount of energy
released is dependent of a rock’s properties, area of the fault surface, and
amount of movement along the fault zone (McConnel). Seismologists calculate
Moment magnitude from seismograms after long-period waves are examined
(Lutgens and Edwards 314). Moment magnitude is calculated with more accuracy
with large earthquakes (McConnel). Moment magnitude gained support and
acceptance among many seismologists and engineers. Seismologists and
engineers accepted Moment Magnitude because:
1) It is the only magnitude scale that adequately measures the size of large
earthquakes
2) It is a measure established from the size of the rupture surface and the
amount of displacement, which better determines the amount of energy
released
3) It can be verified by two different methods, field studies that are based of
measurements of fault displacement and by seismograph methods that
uses long-period waves (Lutgens and Edwards 314).
Displacement is the measurement of the actual change of location of the ground
due to shaking (“Measuring Earthquakes”).
Modified Mercalli Scale
I
instrumental
II
lightest
III
light
People do not feel any Earth movement.
A few people might notice movement if they are at rest and/or on the upper floors
of tall buildings.
Many people indoors feel movement. Hanging objects swing back and forth.
People outdoors might not realize that an earthquake is occurring.
Most people indoors feel movement. Hanging objects swing. Dishes, windows, and
IV
mediocre
doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people
outdoors may feel movement. Parked cars rock.
Almost everyone feels movement. Sleeping people are awakened. Doors swing
V
strongly
open or close. Dishes are broken. Pictures on the wall move. Small objects move
or are turned over. Trees might shake. Liquids might spill out of open containers.
Everyone feels movement. People have trouble walking. Objects fall from shelves.
VI
much fort
Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and
bushes shake. Damage is slight in poorly built buildings. No structural damage.
People have difficulty standing. Drivers feel their cars shaking. Some furniture
VII
strong
breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built
buildings; considerable in poorly built buildings.
Drivers have trouble steering. Houses that are not bolted down might shift on their
foundations. Tall structures such as towers and chimneys might twist and fall. Well-
VIII
violent
built buildings suffer slight damage. Poorly built structures suffer severe damage.
Tree branches break. Hillsides might crack if the ground is wet. Water levels in
wells might change.
Well-built buildings suffer considerable damage. Houses that are not bolted down
IX
disastrous
move off their foundations. Some underground pipes are broken. The ground
cracks. Reservoirs suffer serious damage.
X
most
disastrous
Most buildings and their foundations are destroyed. Some bridges are destroyed.
Dams are seriously damaged. Large landslides occur. Water is thrown on the
banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks
are bent slightly.
XI
XII
catastrophic
Most buildings collapse. Some bridges are destroyed. Large cracks appear in the
ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
great
Almost everything is destroyed. Objects are thrown into the air. The ground moves
catastrophe
in waves or ripples. Large amounts of rock may move.