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How big is an Earthquake?

Geological Survey of Denmark and Greenland (GEUS)
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We can describe this in two different ways:

  • The magnitude on the Richter scale; this measure relates to the energy released by the earthquake, and it is independent of possible damages on the surface of the earth. See below
  • The intensity, which is a measure of the shaking at the earth's surface. Here the Modified Mercalli scale is used.
A relatively small magnitude earthquake that happens near the surface can cause shaking of great intensity, whereas a large magnitude earthquake that happens in a depth of several hundred kilometers will not necessarily produce intense shaking at the surface.

The magnitude on the Richter scale is calculated based on many registrations of one earthquake on different seismographs. The Richter scale is logarithmic, so that an increase of 1 in magnitude means that 10 times larger waves were generated. In other words a magnitude 7 earthquake is 10 times larger than a magnitude 6 earthquake.

Normally it is only possible to determine the intensity of an earthquake in populated areas. The intensity is determined from observations of the tremor and the damage it causes. The most commonly used intensity scale is the Modified Mercalli scale

The Modified Mercalli Scale

The intensity scale is used to describe the observations of tremors and damage in the earthquake area. One of the most frequently used scales is the Mercalli scale, which has 12 levels. The intensity will, of course, vary across the area and be highest near the epicenter.

The modified Mercalli scale of 1931:

I. Not felt. Marginal and long-period effects of large earthquakes

II. Felt by persons at rest, on upper floors, or favorably placed.

III. Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake.

IV. Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of IV wooden walls and frame creak.

V. Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate.

VI. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc., off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked. small bells ring (church, school). Trees, bushes shaken.

VII. Difficult to stand. Noticed by drivers of cars. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices. Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.

VIII. Steering of motor cars affected. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperatures of springs and wells. Cracks in wet ground and on steep slopes.

IX. General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluvitated areas sand and mud ejected, earthquake foutains, sand craters.

X. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.

XI. Rails bent greatly. Underground pipelines completely out of service.

XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air.

Masonry A
Definition: Good workmanship, mortar, and design; reinforced, especially lateraly, and bound together by using steel, concrete, etc.; designed to resist lateral forces.

Masonry B
Definition: Good workmanship and mortar; reinforced but not designed in detail to resist lateral forces.

Masonry C
Definition: Ordinary workmanship and mortar; no extreme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces.

Masonry D
Definition: Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally.

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