Mercalli Scale
Earthquakes are measured in terms of their magnitude and intensity using the Richter Scale and Modified Mercalli Scale of Earthquake Intensity.
The Richter magnitude scale measures an earthquake’s magnitude using an open-ended logarithmic scale that describes the energy release of an earthquake through a measure of shock wave amplitude. The earthquake’s magnitude is expressed in whole numbers and decimal fractions. Each whole number increase in magnitude represents a 10-fold increase in measured wave amplitude, or a release of 32 times more energy than the preceding whole number value.
The Modified Mercalli Scale measures the effect of an earthquake on the Earth’s surface. Composed of 12 increasing levels of intensity that range from unnoticeable shaking to catastrophic destruction, the scale is designated by Roman numerals. There is no mathematical basis to the scale; rather, it is an arbitrary ranking based on observed events. The lower values of the scale detail the manner in which the earthquake is felt by people, while the increasing values are based on observed structural damage. The intensity values are assigned after gathering responses to questionnaires administered to postmasters in affected areas in the aftermath of the earthquake.
 |
Mercalli Scale Measurements |
 |
|
The Modified Mercalli Intensity Scale |
|
Scale |
Intensity |
Description of Effects |
Maximum
Acceleration
(mm/sec) |
Corresponding Richter Scale |
|
I |
Instrumental |
Detectable only on seismographs |
<10 |
|
|
II |
Feeble |
Some people feel it |
<25 |
<4.2 |
|
III |
Slight |
Felt by people resting (like a truck rumbling by) |
<50 |
|
|
IV |
Moderate |
Felt by people walking |
<100 |
|
|
V |
Slightly Strong |
Sleepers awake; church bells ring |
<250 |
<4.8 |
|
VI |
Strong |
Trees sway; suspended objects swing; objects fall off shelves |
<500 |
<5.4 |
|
VII |
Very Strong |
Mild alarm; walls crack; plaster falls |
<1000 |
<6.1 |
|
VIII |
Destructive |
Moving cars uncontrollable; masonry fractures; poorly constructed buildings damaged |
<2500 |
|
|
IX |
Ruinous |
Some houses collapse; ground cracks; pipes break open |
<5000 |
<6.9 |
|
X |
Disastrous |
Ground cracks profusely; many buildings destroyed; liquefaction and landslides widespread |
<7500 |
<7.3 |
|
XI |
Very Disastrous |
Most buildings and bridges collapse; roads, railways, pipes and cables destroyed; general triggering of other hazards |
<9800 |
<8.1 |
|
XII |
Catastrophic |
Total destruction; trees fall; ground rises and falls in waves |
>9800 |
>8.1 |
|