Foundation, Concrete and Earthquake Engineering

### Intensity of earthquake

The earth suffers a highly variable intensity of earthquake from time to time. Some of them are catastrophic resulting complete destruction of building and ground, while some can’t be felt, only can sensitive instrument can predict. So, once an earthquake struck, it is important to know about severity of it and impact of it on the community to get prepare for aftershocks and future trembling. In order to have a comparative study of earthquakes and to define areas of known intensities, it is essential to establish a measure or a scale of reference.

Several classification of earthquake intensity have been used. To the engineer, a classification based on the maximum acceleration of the ground is of most interest. The acceleration due to gravity is 9.8 m/sec. It should keep in mind that the earthquakes may have different degree of accelerations as well as that different geological conditions produce different effects.

Measure of Intensity

To develop a effective scale to measure intensity, the data required are

a) the acceleration produced

b) the extent of damage caused to buildings and ground surface.

Historical Development of Scale

Initially a scale of earthquake intensity having ten divisions was given by Rossi and Forel, which was based entirely on the sensation of people and damage caused. However it has been modified by mercalli and further by Wood and Neumann in 1931, which will be discuss in this article.

Modified Mercalli Intensity Scale

Collection of Data

It measures the impact of an earthquake by sending out trained observers to look at the damage done to the built environment and the earth (landslides etc.) and at the reaction of people to the event.

Application of Collected Data

Instrumental records of earthquake vibrations are supplemented by information gathered from individuals through such types of questionnaires. Many data that are not brought out by instrumental methods are thus assembled; and information is gathered also from areas where no instruments have been established.

Details of Modified Mercalli Intensity Scale

This scale defines the effects of an earthquake over a limited geographical area. Intensity scales assign whole numbers usually from 1 to 12, expressed in Roman numerals. An intensity I means the earthquake was not felt, while XII means absolute and total destruction. The descriptions used to assign earthquake intensity values on a M.M.I scales are shown in following table :

The advantage of using the M.M.I scale is that it relies on the observations of people experiencing an earthquake instead of scientific instruments. This allows seismologists to assign earthquake intensities to seismic records, an activity that helps them to estimate seismic risk for earthquake sites today.

The fact that the Mercalli scale relies on peoples observations is a disadvantage because this makes a evaluation subjective and dependent upon the social infrastructural conditions of a country. This scale also not very helpful in an area with little human habitation, since no one would be around to experience the earthquake.

Example

In case of intensity value VII on the M.M.I scale not effective or should be rewritten for a country without chimneys and automobiles.

Latest scale of intensity

Now a scale , called European Macro seismic Scale (1992) is widely used which has been developed and tested over period of years by a working group of the European seismological commission. The EMS makes the imprecise and subjective nature of assigning intensities more robust and straight forward with regard to earthquake effects on human, objects and buildings.

Isoseismal lines

The intensity of earthquake decreases with increase in distance from the centre of disturbance. This decrease is inversely proportional to the square of this distance. In an area, subjected to earthquake, the places suffering same intensity can be determined. A line joining points of same intensity is called an ‘Isoseismal Line’. If the focus of earthquake is a point, the area enclosed by an isoseismal is circular whereas an elongated zone or line results a elliptical area.

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