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Earthquake Intensity

 Prior to the deployment of large-scale seismological instrumentation, the observed effects of earthquakes were used for describing the severity of earthquakes quantitatively.  In the event of an earthquake one or more of the following direct effects can be observed:

 §         Vibrations, sometimes setting of land slides and damage,

§          Cracks and fissure,

§          Local elevations and depressions,

§          Starting and stopping of springs,

§         Movements in surface of water in ponds and lakes,

§         Visible breaks in the Earth’s surface (faulting).

§         Collapse

§         Subsidence,

§         Movements of  animals and birds, and their general behaviour

The indirect effects of a major earthquake result from fire arising from short circuits or dislocation of fire sources and flooding from spilling of water from reservoirs with damaged dams. These have been called the macroseismic effects of earthquakes as against the microseismic effects in the form of small-scale vibrations, which can be detected only by seismographs. Based on the severity of the observed effects an intensity scale value is assigned to an earthquake. Since the effects vary from place to place and, normally, decrease as the distance of the observing station from the earthquake source increases, the intensity scale value of an earthquake varies from place to place. The maximum value of intensity, generally, occurs in the epicentral area

         Several intensity scales were proposed from time to time. The most commonly used is the Modified Mercalli (MM) intensity scale. In this, as well as most other intensity scales, intensity varies between XII and I.  Though the intensity value is assigned after detailed studies (based on the collected information including interviews and answers to questionnaires), a brief description of a 12 point intensity scale may be understood on the following type of classification:

I.                    Detected only by instruments.

II.                 Feeble.

III.               Slight

IV.              Moderate

V.                 Rather Strong.

VI.              Strong.

VII.            Very Strong.

VIII.         Destructive.

IX.              Ruinous.

X.                 Disastrous.

XI.              Very disastrous.

XII.            Catastrophic.

Intensity in the epicentral area is represented by I_0, The other intensity scales are Rossi Forel (RF), Medvedev, Spomheur and Karnik (MSK), Mercali Cancani Sieberg (MCS) and the Japan Meteorological Agency (JMA) scales. Accept the JMA (I to VII) and the RF (I to X) others range between I and XII.

On maps places of the same intensity are joined to draw isoseismal lines. The macroseismic effects of an earthquake, and hence the intensity of an earthquake, at a certain location depend on several factors. These include the total energy release during the earthquake, depth of the earthquake source, epicentral distance, soil conditions in the observation area and nature of the earthquake source (orientation of the fault and type of fault movement during the earthquake) and its energy radiation pattern (directional effects). The area within the high intensity isoseismals is called meizoseismal area of the earthquake.

Modified Mercalli (MM) Intensity Scale

 I.           Not felt. Marginal Detected only by seismographs.

 II.        Felt by persons at rest, upon 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. Vibrations like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls. Standing motorcars rock. Windows, dishes doors rattle. Glasses clink. Crockery clashes. In the upper range of grad IV, wooden walls and frames crack.

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

 VI.         Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books and so on, off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked (classification of masonry follows the scale description). Small bells ring (churches and schools). Trees, bushes shaken visibly, or heard to rustle.

 VII        Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices, unbraced parapets, and architectural ornaments. Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in 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 and 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 temperature of springs . 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. General damage to foundations. Frame structures, if not bolted, shifted of foundations. Frames racked. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, earthquake fountains, and sand craters.

 X.          Most masonry and frame structures destroyed with foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams dikes and embankments. Large landslides. Water thrown on banks of canals, rivers, lakes etc. 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 in the air.

   Classification Of Masonry 

            Masonry A, B, C ad D. are specified as follows:  

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

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

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

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

This page was updated on10-01-11