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Modified Mercalli Intensity Scale

Level I Felt only by a few under very favorable circumstances.
Level II Felt by a few at rest, especially those on upper floors. Some suspended objects may swing.
Level III Noticeably felt indoors, especially on upper floors. May not be recognized as an earthquake.
Level IV Felt by many indoors during the day and a few outdoors. Some are awakened at night. Noticable disturbance of dishes, windows and doors. Felt by persons in stationary vehicles.
Level V Felt by nearly everyone during the day. Many are awakened at night. Some damage to items such as dishes and windows. Some noticable disturbance to trees, poles and tall objects. Pendulum clocks may stop.
Level VI Felt by everyone. May cause fright, causing some to run outdoors. Heavy furniture may be moved. Some slight damage to structures such as fallen plaster and chimneys.
Level VII Everyone frightened enough to run outdoors. Slight to moderate damage to well-designed/constructed buildings and considerable damage to poorly-designed/constructed buildings. Felt by persons in moving vehicles.
Level VIII Considerable damage in ordinary buildings with some collapse. Heavy furniture is overturned. Persons in moving vehicles are disturbed.
Level IX Considerable damage to many buildings with some collapse in large buildings. Some buildings are thrown off their foundations. Conspicuous cracks occur in the ground. Ground pipes are broken.
Level X Most masonry buildings are destroyed. The ground is seriously cracked. Rails are bent. Landslides on steep slopes.
Level XI Few masonry structures survive. Bridges are destroyed. Broad ground fissures. All ground pipes are broken.
Level XII Practically every structure is damaged or destroyed. Waves on the ground are seen. Objects are thrown into the air.

Also see:
-- Richter Magnitude Scale
-- Major Earthquakes in Los Angeles County