The intensity of an earthquake can be expressed in terms of the destruction it causes. Eye witness accounts of earthquake damage at the surface are matched to the Mercalli Intensity Scale which consists of an arbitrary 12 step scale originally designed by Giuseppe Mercalli. The scale relies on subjective information from people who witnessed the earthquake. From their information it is possible to determine the epicenter and the relative earthquake strength.
The intensity of an earthquake at any particular place depends not only on the magnitude of the earthquake, but also on the distance from the epicenter, depth of the focus, and on the surface and sub-surface geological conditions.
The intensity decreases outwards from the focus, forming concentric zones of declining intensity radiating out from the epicenter.
Modified Mercalli Intensity Scale
| Intensity I Instrumental | Not felt by people, only detected by seismographs. |
| Intensity II Feeble | Felt only by a few people at rest, especially on upper floors of buildings. Delicately suspended objects may swing. |
| Intensity III Slight | Felt noticeably indoors; like the vibrations due to a passing truck. Standing motor cars may rock slightly |
| Intensity IV Moderate | Felt indoors by many people, outdoors by few. Dishes, windows, doors rattle. May awaken some sleepers. Standing cars rocked noticeably. |
| Intensity V Rather strong | Felt by nearly everyone, many awakened. Some dishes and windows broken; occasional cracked plaster; unstable objects overturned. Some disturbance of trees, poles and other tall objects |
| Intensity VI Strong | Felt by all; many frightened and run outdoors. Some heavy furniture moved; some falling plaster or damaged chimneys. Damage slight. |
| Intensity VII Very strong | General alarm; people run outside. Walls crack; chimneys fall. Considerable damage in poorly designed structures. Noticed by persons in moving vehicles. |
| Intensity VIII Destructive | Considerable damage in ordinary substantial buildings with partial collapse. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Changes in well water. Car drivers seriously disturbed. |
| Intensity IX Ruinous | Considerable damage with partial collapse of substantial buildings. Buildings moved off foundations; ground cracks conspicuous. Underground pipes broken. |
| Intensity X Disastrous | Ground cracks badly; landslides on river banks and steep slopes; rails bent. Many buildings destroyed. |
| Intensity XI Very disastrous | Broad fissures in ground; major landslides and earth slumps; floods. Few buildings remain standing; bridges destroyed. Nearly all services (railways, underground pipes, cables) out of action. |
| Intensity XII Catastrophic | Total destruction. Ground rises and falls in waves; lines of sight and level distorted. Objects thrown into the air. |
More Info About Earthquakes
BE INFORMED, BE WARNED, BE PREPARED: BEFORE, DURING and AFTER an EARTHQUAKE