Earthquake | Definition, Causes, Effects, & Facts, Intensity and magnitude

Earthquake

The physical forces the surfaces are rearranging rock materials by shifting magmas about altering the structures of solid rocks. The adjustment beneath the surface however involve various crystal movements, some of which because of suddenness and intensity produce tremors in the rocks and they are known as earthquake. The science dealing with the study of earthquakes in all their aspects is called seismology.

Focus and epicenter:

The exact spot underneath the earth surface at which an earthquake originates is known as its focus. These waves first reach the point at the surface, which is immediately above the focus or origin of the earthquake. This point is called epicenter. The point which is diametrically opposite to the epicenter is called anticenter.

Intensity and magnitude:

Intensity of an earthquake may be defined as the ratio of an earthquake based on actual effects produced by the quakes on the earth.

Magnitude of a tectonic earthquake may be defined as the rating of an earthquake based on the total amount of energy released when the over strained rocks suddenly rebound, causing the earthquake.

Causes of earthquake:

The earthquake may be caused due to various reasons, depending upon it intensity. Following causes of earthquake are important:

1. Earthquakes due to superficial movements:

The feeble earthquakes are caused due to superficial movements.i.e, dynamic agencies, and operation upon surface of the earth.

The dashing waves cause vibrations along the seashore.

Water descending along high water falls, impinges the valley floor and causes vibrations along the neighbouring areas.

At high altitudes the snow falling down is an avalance.also causes vibrations along the neighbouring areas.

2. Earthquake due to volcanic eruptions:

Most of the volcanoes erupt quietly and as consequence, initiate no vibration on the adjoining area. But a few of them when erupt, cause feeble tremors in the surface of the earth. But there may be still a volcanic eruption may cause a severe vibration on the adjoining area and have really disastrous effects.

3. Earthquake due to folding or faulting:

The earthquakes are also caused due to folding of the layers of the earth’s crust. if the earthquakes are caused due to folding or faulting then such earthquakes are more disastrous and are known as tectonic earthquakes and directly or indirectly change the structural features of the earth crust.

Classification of earthquakes:

Earthquakes are classified on a no. Of basis. Of these the depth of focus, the cause of origin and intensity are important.

a) Depth of focus:

Three classes of earthquakes are recognized on this basis, shallow, intermediate and deep seated. In the shallow earthquakes the depth of focus lies anywhere up to 50 km below the surface. The intermediate earthquakes originate between 50 and 300 km depth below the surface.

b) Cause of origin:

i)       Tectonic earthquakes are originated due to relative movements of crystal block on faulting, commonly, earthquakes are of this type.

ii)  Non tectonic earthquakes: that owes their origin to causes distinctly different from faulting, such as earthquakes arising due to volcanic eruptions or landslides.

C) Intensity as basis:

Initially a scale of earthquakes intensity with ten divisions was given by Rossi and ferel. Which was based on the sensation of the people and the damage caused. However it was modified by Mercalli and later by wood and Neumann.

Engineering considerations:

The time and intensity of the earthquake can never be predicted. The only remedy that can be done at the best, it is provide additional factors in the design of structure to minimize the losses due to shocks of an earthquake. This can be done in the following way:

To collect sufficient data, regarding the previous seismic activity in the area.

To assess the losses, which are likely to take place in furniture due to earthquake shocks To provide factors of safety, to stop or minimize the loss due to sever earth shocks.

Following are the few precautions which make the building sufficiently earthquake proof.

The foundation of a building should rest on a firm rock bed. Grillage foundations should preferably be provided.

Excavation of the foundation should be done up to the same level, throughout the building.

The concrete should be laid in rich mortar and continuously

Masonry should be done with cement mortar of not les than 1:4 max. Flat R.CC slab should be provided.

All the parts of building should be tied firmly with each other. Building should be uniform height.

Cantilivers, projections, parapets, domes etc, should be provided. Best materials should be used.