Hey Dosto, Today In this article of Geomorphology we are going to discuss about what is earthquake, earthquake causes and effects, Terminology Used in the Study of Earthquakes, Measurement, Seismic Waves and much more.
So Let’s dive in on the topic.
- An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth‘s lithosphere that creates seismic waves.
- Earthquake is the form of energy of wave motion transmitted through the surface layer of the earth.
- It may be due to faulting , folding, plate movement, volcaninc eruptions and anthropogenic factors like dams and reservoirs.
- Earthquake are by far the most unpredictable and highly distructive of all the natural disasters.
- Minor earth tremors caused by gentle waves of vibration within the earth’s crust occur every few minutes while Major earthquakes usually caused by movement along faults, can be very disastrous particularly in densely populated areas.
Terminology Used in the Study of Earthquakes
- Earthquake intensity
- Earthquake magnitude
- Richter Scale
- Mercalli Scale
- Seismic wave
Focus and Epicenter
The point within Earth where faulting begins is the focus, or hypocenter.
The point directly above the focus on the surface is the epicenter.
Richter magnitude scale is the scale to measure the magnitude of energy released by an earthquake.
This scale was devised by Charles. F. Richter in the year 1935.
The number indicating magnitude ranges between 0 to 9
An earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times that of an earthquake that registered 4.0, and thus corresponds to a release of energy 31.6 times that released by the lesser earthquake.
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake.
It measures the effects of an earthquake
The number indicating intensity ranges between 1 to 12
- Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth.
- They are the energy that travels through the earth and is recorded on seismographs.
- The two main types of waves are body waves and surface waves.
- Primary waves ( P-waves)
- Secondary waves ( S-waves)
- Love Waves (L-waves)
- Rayleigh waves
Primary waves (longitudinal wave)-
- The first kind of body wave is the P wave or primary wave.
- This is the fastest kind of seismic wave.
- The P wave can move through gaseous, solid rock and fluids, like water or the liquid layers of the earth.
- It pushes and pulls the rock, it moves through just like sound waves push and pull the air.
Secondary waves (transverse wave)
- The second type of body wave is the S wave or secondary wave.
- An S wave is slower than P wave and can only move through solid rock.
- This wave moves rock up and down, or side-to-side.
- S-waves arrive at the surface with some time Lag.
- The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British
- It’s the fastest surface wave and moves the ground from side-to-side.
- The other kind of surface wave is the Rayleigh wave, named after Lord Rayleigh.
- A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
- Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.
- Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.
Classification of earthquake
- On basis of causative factors
- On basis of depth of focus
- Deep focus( 250-700km)
- On basis of human casualities
- Moderate (deaths<50,oo)
- Highly hazardous(51,000-1,00,00)
- Most hazardous(>1,00,00)
World Distribution of Earthquakes
- The world’s distribution of earthquakes coincides very closely with that of volcanoes.
- Region of greatest seismicity are Circum-Pacific areas, with the epicentres and the most frequent occurrences along the ‘Pacific Ring of Fire’.
- It is said that as many as 70% of earthquakes occur in the Circum-Pacific belt.
- Another 20% of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China.
- Remaining occur in the interiors of plates and on spreading ridge centers.
Earthquakes are caused mainly due to dis-equilibrium in any part of the crust of the earth.
A number of causes have been assigned to caused dis-equilibrium or isostatic imbalence in the earth’s crust such as-
- Volcanic eruption
- Faulting and folding
- Upwarping and downwarping
- Gaseous expansion and contraction inside the earth.
- Hydrostatic pressure of man-made water bodies like reservoirs and lakes.
- Plate Movement
Plate tectonics provides the most logical explanation of volcanoes and earthquakes.
There are 3 types of plate boundaries along which earthquake occurs
Damage caused by earthquakes
- Slope instability and landslides
- Damage to human structures
- Damage to towns and cities
- Loss of human lives
- Deformation of ground surface
- Flash floods