Basic Concepts of Earthquakes

Earthquakes

Earthquake may be defined as Vibrations induced in the earth’ crust due to external or internal causes, ranging from a faint tremor to a wild motion capable of shaking buildings apart and causing gaping fissures to open up in the ground. It is a form of energy of wave motion transmitted through the surface layer of the earth in widening circles from a point of sudden energy release--“the earthquake focus”. It is, however, evident that no earthquake possibly originates at a mere point alone.

Terminology associated with earthquakes.

1) Seismology:-The science dealing with the study of earthquakes in all their aspects is called seismology. It is an interdisciplinary science which is partly Geology & partly Physics .

2) Focus:-It is the exact spot underneath the earth’s surface at which an earthquake originates. It is also known as Hypocentre.


3) Epicentre:-The point or place on the surface vertically above the focus of a particular earthquake is termed as Epicentre. It is that (Geographical)place on the surface of the earth where vibrations from a particular earthquake reaches first of all. Generally it is the location of maximum damage due to earthquakes.

4) Isoseismal line or Isoseist:-Line joining all points at which the intensity of the earthquake is same is known as isoseismal or isoseist. It is infact, an isodiastrophic line of equal damage.


5) Homoseismal or Coseismal or Homoseists: - Lines joining places where shocks arrive at the same time.

Types of Earthquakes:-

Based on the depth of origin , there are 3 types of earthquakes :-

a) Shallow focus earthquake:- In this case, the seismic shocks originate at a depth upto 60 kms, below the earth surface. A great majority of the earthquakes in the past had been of this type.
b) Intermediate focus earthquake :-In this case the shock waves originate at a depth between 60-300 kms. They are rare in occurrence.
c) Deep focus earthquake :- Here the point of origin of the shock is at a depth between 300-700 km and is a very rare phenomena.

Based on the cause of origin: There are 3 types of earthquake based on origin.
a) Tectonic earthquake :- Caused due to faulting or relative displacement of blocks of the earth crust along rupture planes. Most commonly earthquakes are tectonic is origin.
b) Non Tectonic earthquake :- that owes their origin to causes distinctly different from faulting such as volcanic eruptions, atomic explosions or due to landslides and subsidence.
c) Submarine earthquake:-They of the generates vary large waves on the surface of the seas and destroy the coastal tracts. These submarine earthquakes are known as Tsunamis

Based on Magnitude:- Earthquakes are grouped into five classes on the basis of magnitude as follows:-

a) Class A→ 7.8 and above
b) Class B→ 7.0 – 7.7
c) Class C→ 6.0-7.0
d) Class D→ 5.3-6.0
e) Class E→ Less than 5.3


Types of seismic waves: - during each earthquake, elastic waves are generated at the focus. Theses waves are called seismic waves.They travel in all directions with their characteristic velocities.

There are 3 types of seismic waves-
1. The P-waves: - These are also called Primary waves, Push and pull waves, Longitudinal and compressional waves. These are fastest of the seismic waves (similar to sound waves). They travel in all solid, liquid and gaseous media. They have short wave length & high frequency.
2. The Secondary or S-waves: - These are transverse waves also known as sheer waves. They travel only in solid media. They are slow in comparison to P- waves. They are also having short wavelength and high frequency.
3. L-waves:-These are transverse vibrations and are confined to the outer skin of the crust. They are also known as surface waves because their journey is confined mainly to the surface layer of the earth crust
This is of two types based on their characteristics :-

a. The Rayleigh waves- In this case displacement of the particles is of a complex nature, partly being in the direction of propagation and partly at right angles to it.
b. The Love waves - In this case the displacement of the particles is practically horizontal, that is in the direction of propagation.
In terms of their effects on solid material of the earth crust, the Rayleigh waves tend to distort the horizontal surface into a wavy, Zigzag shape. The Love waves however tend to create shearing (breaking) ruptures.
Causes of earthquakes:-
There are two main causes of earthquakes occurrence:-Tectonic & Volcanic causes.

1. Tectonic earthquake:-Earthquake are produced by sudden movements along faults and are mostly, therefore of tectonic origin. The concept of possible mode of origin of tectonic earthquakes is known as Elastic Rebound theorySuch earthquake generally result from sudden yielding to strain produced on the rocks by accumulating stresses. This causes the breaking of rocks and produces relative displacements of rocks such faulting causes shaking because displacement of rocks can only be possible by overcoming frictional resistance against the walls of the fault plane. The association of earthquakes with fault lines is an established fact.

2. Volcanic earthquake:-Usually earthquakes with volcanoes are more localized both in extent of damage and in intensity of the waves produced in comparison to those which are associated with faulting motions A shock may be produced by any of the following mechanisms.

a. Explosion of the volcano upon the release and expansion of gases and lava ash.
b. Faulting within the volcano resulting from premises in the chamber of molten rock &
c. Collapse of the centre of volcano into the sace formed by the extrusion of gases and molten matters

Scale of intensity :-

Various scales have been proposed to the estimate the intensity of earthquake from the amount of damage caused. Theses scales are

1. Rosiforrel Scale:- For measuring Intensity
2. Mercalli Scale :- For measuring Intensity
3. Richter Scale :- For measuring Magnitude

1. In the Rosi Forrel Scale the intensity has been clarified into- SEVERE CATASTROPHIC & DISASTROUS.
2. In the Mercali scale this is divided into 12 numbers with increase of intensity. In this case number 1 is detected only by seismographs. Gradually the number increase when the earthquake intensity becomes disastrous at number 10 and at number 12 the effect is totally catastrophic where there is total destruction and objects thrown into air.
3. In the Richter scale the scale number ranges from 0 to 9. Here it is particularly important to notice that a magnitude & earthquake is 10 times larger than a magnitude 7 earthquake, 100 times larger than a magnitude 6 earthquake an 1000 times larger than a magnitude 5 earthquakes
Effects:-

Richter classified the effects of earthquakes into 2 main categories:-

1. Primary effects & 2. Secondary effects.

1. Primary effects:- Theses includes all such effects which are related to the cause of origin of an earthquake. The tectonic earthquake are often responsible for producing many important changes in the geological structure of an areas, creation of slaps or scales, figures, warping of strata, emergence or substance of coastlines changes in the causes of stream, origin of new springs and creation of sand dykes in which saturated layers of sand may be forced up into existing cracks & crevices.

2. Secondary effects:- All those effects which are related to passage of seismic waves and associated shaking motion of the ground during an earthquake are broadly grouped as secondary effects.

The instrument use for recording of seismic shocks is known as Seismograph and the records of seismic shocks prepared and presented seismographs are known as Seismograms.

Distribution of earthquakes

A study of occurrence of earthquakes in the past reveals at once a striking fact that earthquake are very common features in some regions of the world whereas certain other parts have remained virtually stable since times immemorial proudly speaking shocks of the past had been largely confined to only two large geographical; belts. The circumparific belts and the Mediterranean belt.

a) The circumparific belt forms a ring closing North America most of Asia and Europe and accounts for more than 75.6 % of world’s seism city.
b) The Mediterranean belt also called the Alp rive-Asiatic belt accounts for 22.1% of the world seism city and includes areas of India, Arafic Africa, S-America and Australia.
c) Another belt extends over Himalayas, KUNLUN, Tien Shan and Altai Range upto Rake Baikal.
d) Another belt extends from the gult of Aden Between Seychelles and Maldives Islands turns to the west soutgh of Africa and goes upto the fallan Island.