Earthquake severity based on intensity levels
The severity of an earthquake is measured in two ways: in terms of intensity and magnitude levels.
Intensity is based on the observed effects of ground shaking on people, buildings, and natural features, according to US Geological Survey.
It varies from place to place within the disturbed region depending on the location of the observer with respect to the earthquake epicentre.
However magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake. It is based on the amplitude of the earthquake waves recorded on instruments which have a common calibration.
The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally--total destruction.
Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale.
It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann.
This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
The following is an abbreviated description of the 12 levels of Modified Mercalli intensity.
- Not felt except by a very few under especially favourable conditions.
- Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
- Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
- Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
- Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
- Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
- Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
- Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
- Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
- Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent.
- Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
- Damage total. Lines of sight and level are distorted. Objects thrown into the air.
The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the non-scientist than the magnitude because intensity refers to the effects actually experienced at that place.
After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned.
The results of this postal canvass and information furnished by other sources are used to assign an intensity value, and to compile isoseismic maps that show the extent of various levels of intensity within the felt area. The maximum observed intensity generally occurs near the epicentre.
The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of Vlll or above.
Comments