This document discusses earthquakes and seismic activity. It defines key terms like earthquake, seismic waves, focus, and epicenter. Earthquakes are caused by disequilibrium in the earth from factors like plate tectonics and volcanic activity. They are classified as natural or artificial based on their cause. Natural earthquakes result from processes inside the earth, while artificial earthquakes are human-induced. Earthquakes most commonly occur along plate boundaries and zones of geological faulting or folding. When an earthquake occurs, it generates primary, secondary, and surface seismic waves that propagate outward from the focus/epicenter.

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