Posts Tagged Moment magnitude Scale

Earthquake Analysis and Measurement

What is Earthquake Magnitude?

Magnitude is a measure of the strength of an earthquake, or the amount of strain that rocks in Earth’s crust release when an earthquake occurs. The Richter scale and the moment magnitude scale are used to measure the magnitude of earthquakes.

What is Earthquake Intensity?

Earthquake intensity is a measure of the effects of an earthquake in a particular place. Earthquake intensity is the severity of shaking felt due to an Earthquake. It decreases with its distance from epicentre.

Recording an Earthquake

The vibrations produced by earthquakes are detected and recorded by instruments called seismographs. The time of occurrence, the duration of shaking, the locations of the epicenter and focus, and estimates of the energy released can be obtained from data from seismographs set up around the world

Richter Scale

Richter Scale, method of ranking the strength or size of an earthquake. The Richter scale, also known as the local magnitude scale, was devised in 1935 by the American seismologist Charles F. Richter to rank earthquakes occurring in California. Richter and his associates later modified it to apply to earthquakes anywhere in the world.

The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 mi) from the earthquake’s epicenter, the site on the earth’s surface directly above the earthquake’s origin. The amount of ground movement is measured by an instrument called a seismograph. Seismographs can detect movements as small as about 0.00001 mm (about 0.000004 in) to movements as large as about 1 m (about 40 in). In order to deal with numbers in such a broad range, the Richter scale is a logarithmic scale—each increase of 1 on the Richter scale represents a tenfold increase in movement. Thus, an earthquake registering 7 on the scale is 10 times as strong as an earthquake registering 6, and the earth moves 10 times as far.

Moment magnitude Scale (Mw)

Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1995 Kobe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitude was 6.8 for that tremor.

Modified Mercalli Scale

Modified Mercalli Scale, scale for measuring the intensity of earthquakes, adapted from the original Mercalli scale. The Mercalli scale was devised in 1902 by Italian seismologist Giuseppe Mercalli. American seismologists Harry O. Wood and Frank Neumann created the Modified Mercalli scale in 1931 to measure the intensity of earthquakes that occur in California. The Modified Mercalli scale, or a scale similar to it, is now used worldwide. The scale has 12 levels of intensity. Each level is defined by a group of observable earthquake effects, such as shaking of the ground and damage to structures such as buildings, roads, and bridges.

The levels are designated by the Roman numerals I to XII. Levels I through VI are used to describe what people see and feel during a small to moderate earthquake. Levels VII through XII are used to describe damage to structures during a moderate to catastrophic earthquake. On average, about one earthquake of level X to XII occurs worldwide every year; 10 to 20 earthquakes of level VII through IX occur each year; and over 500 earthquakes of level I to VI occur every year. Each year over 100,000 earthquakes occur that are not noticed by the human population and therefore are not rated on the Modified Mercalli scale.

The Modified Mercalli Intensity Scale of 1931

I

Not felt except by very few people under especially favorable conditions.

II

Felt by a few people, especially those on upper floors of buildings. Suspended objects may swing.

III

Felt quite noticeably indoors. Many do not recognize it as an earthquake. Standing motorcars may rock slightly.

IV

Felt by many who are indoors; felt by a few outdoors. At night, some awakened. Dishes, windows and doors rattle.

V

Felt by nearly everyone; many awakened. Some dishes and windows broken; some cracked plaster; unstable objects overturned.

VI

Felt by everyone; many frightened and run outdoors. Some heavy furniture moved; some fallen plaster or damaged chimneys.

VII

Most people alarmed and run outside. Damage negligible in well constructed buildings; considerable damage in poorly constructed buildings.

VIII

Damage slight in special designed structures; considerable in ordinary buildings; great in poorly built structures. Heavy furniture overturned. Chimneys, monuments, etc. may topple.

IX

Damage considerable in specially designed structures. Buildings shift from foundations and collapse. Ground cracked. Underground pipes broken.

X

Some well-built wooden structures destroyed. Most masonry structures destroyed. Ground badly cracked. Landslides on steep slopes.

XI

Few, if any, masonry structures remain standing. Railroad rails bent; bridges destroyed. Broad fissure in ground.

XII

Virtually total destruction. Waves seen on ground; objects thrown into the air.

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