The intensity of an earthquake becomes weaker outward from the epicenter. However, various types of ground respond differently to earthquake vibrations. Buildings on filled ground are damaged more than structures built on solid rock even though both may be at the same distance from the epicenter.
The magnitude of a particular earthquake is a single number which does not vary from place to place. Magnitude is the total energy released by an earthquake at its focus. Earthquakes of large magnitude are stronger and generally more destructive than those of small magnitude. The amount of destruction depends not only on the magnitude but on the kind of ground and types of buildings thereon, and on the location of the focus in relation to heavily populated areas.
Large earthquakes are preceded by many aftershocks, which may persist for days or weeks. The first shock is the most damaging. However, sometimes an aftershock may be even more powerful than the original shock.
The intensity of an earthquake is measured in terms of its geological effects and the overall damage it brings. There are two major scales in which earthquakes are measured. These two scales are the Mercalli Scale and the Richter Scale.
The Mercalli scale was introduced at the turn of the 20th century by the Italian seismologist Giuseppe Mercalli. This scale measures the intensity of shaking with numbers from I to XII. Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale correspond with magnitudes 8 to 9 on the Richter scale.
I. Hardly felt
II. Felt only by a few persons at rest, especially on upper floors of buildings.
III. Can be felt by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake.
IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed. Standing motor cars rocked noticeably.
V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned.
VI. Felt by all
VII. Considerable damage in poorly built or badly designed structures.
VIII. Damage slight in specially designed structures. Damage great in poorly built structures. Heavy furniture overturned.
IX. Damage considerable in specially designed structures. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Many objects destroyed, buildings collapse.
XI. Few structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Total Damage.
The Richter scale was named after the American seismologist Charles Francis Richter. This scale measures the motion of the land surface 60 mi from the epicenter, or focus, of the earthquake. An estimated 800 quakes of magnitudes 5 to 6 occur worldwide each year. About 50,000 quakes of magnitudes 3 to 4 occur each year, and only about one of magnitude 8 to 9 each year.
Between to 0-4.3 on the Richter scale,
People at rest upstairs notice shaking.
Shaking felt indoors; hanging objects swing.
Sleeping people are awakened.
Dishes, doors and trees shake and rock.
Difficult to stand; people walk unsteadily.
Windows break; plaster,bricks, and tiles fall.
Damage to foundations; buildings destoyed.
Water thrown out of river.
Total destruction; roads break up, rocks fall.
Large cracks appear in ground.