Earthquake, causes and distribution seismic waves and their intensity

1. Earthquake

2. Terminologies
• Earthquake – it is a motion of ground surface
ranging from a faint tremor to a wild motion
capable of shaking buildings apart and causing
gaping fissures to open in the ground.
• Seismic waves – waves generated by the
earthquake
• Focus – place of origin of the earthquake
• Epicentre – place on the surface of the earth,
perpendicular to focus
• Richter scale is used to measure the intensity of
earthquake

3. Cause of earthquake
• Due to disequilibrium caused by different
factors like
• Vulcanicity, folding, faulting, gaseous
expansion &contraction inside the earth etc
• Plate movements
• Hydrostatic pressure of manmade
waterbodies like dams, lakes etc
• Weaker zones on earth are more prone to
seismic activity

4. Classification of earthquakes
• On the basis of causative factors
1. NATURAL- caused by natural processes like
endogenetic forces.
Volcanic earthquake – caused by volcanic
eruptions- fissure or explosive. And this is
dependent on intensity &magnitude of the
earthquake
Tectonic earthquake – caused by dislocation
of rockblocks during faulting

5. • Isostatic earthquake – due to sudden
disturbances in the isostatic balance at regional
level
• Plutonic earthquake – focus at greater depths
ranging from 240 km to 670 km
2.ARTIFICIAL
Man induced – caused by human activities like
pumping of water &mineral oil from underground
Deep ground mining, nuclear explosion, storage
of he volume of water in reservoirs etc

6. Distribution of earthquakes
• Mostly occur in:
• Zones of young folded mountains
• Zones of faulting and fracturing
• At the junction of continental and oceanic margins
• Zones of active volcanoes
• Along different plate boundaries
• Major regions are:
• Circum-Pacific belt surrounding the pacific ocean
• Mid continental belt- Alpine – Himalayan chain,Northern
Africa &epicentre of East African fault zones
• Mid Atlantic belt

8. Seismic waves and tremors
• Primary waves- P waves, longitudinal waves are
analogous to sound waves. faster peed in solid &
in liquid at slower speed
• Secondary waves – transverse waves, S waves are
analogous to light waves. Cannot pass through
liquid matter
• Surface waves – long period waves, L waves.
Affects only the surface of the earth & covers
longest distances. Speed is lower than S&P waves
but more destructive & violent.

9. Seismic waves
seismograph Path of seismic waves