Seismic intensity scales

Seismic intensity scales categorize the intensity or severity of ground shaking (quaking) at a given location, such as resulting from an earthquake. They are distinguished from seismic magnitude scales, which measure the magnitude or overall strength of an earthquake, which may, or perhaps may not, cause perceptible shaking. Intensity scales are based on the observed effects of the shaking, such as the degree to which people or animals were alarmed, and the extent and severity of damage to different kinds of structures or natural features. The maximal intensity observed, and the extent of the area where shaking was felt (see isoseismal map, below), can be used to estimate the location and magnitude of the source earthquake; this is especially useful for historical earthquakes where there is no instrumental record. Ground shaking can be caused in various ways (volcanic tremors, avalanches, large explosions, etc.), but shaking intense enough to cause damage is usually due to rupturing of the earth's crust known as earthquakes. The intensity of shaking depends on several factors: The "size" or strength of the source event, such as measured by various seismic magnitude scales. The type of seismic wave generated, and its orientation. The depth of the event. The distance from the source event. Site response due to local geology Site response is especially important as certain conditions, such as unconsolidated sediments in a basin, can amplify ground motions as much as ten times. Where an earthquake is not recorded on seismographs an isoseismal map showing the intensities felt at different areas can be used to estimate the location and magnitude of the quake. Such maps are also useful for estimating the shaking intensity, and thereby the likely level of damage, to be expected from a future earthquake of similar magnitude. In Japan this kind of information is used when an earthquake occurs to anticipate the severity of damage to be expected in different areas.

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