General Awareness Earthquakes

Earthquakes

`color{blue} ✍️` It refers to the vibration of the Earth's surface caused by the endogenetic forces of Earth. Seismology is the special branch of geology that deals with the study of earthquake.

`color{blue} ✍️` The magnitude of an earthquake; usually expressed by the Richter scale, is a measure of the amplitude of seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released an amount that can be estimated from seismograph reading.

`color{blue} ✍️` The plane of origin of earthquake is called focus. The place on the ground surface, which is perpendicular to the focus or hypocentre is called epicentre.

`color{blue} ✍️` The waves generated by earthquake are called seismic waves and they are classified into 3 types such as :

`color{Green} {►" Primary Wave (P Waves)"}` These are the waves of short wave length and high frequency. They are longitudinal waves and can travel through solid, liquid and gases.

`color{Green} {►" Secondary Wave (S Waves)"}` These are the waves of short wavelength and high frequency. They are transverse waves, which travel through all solid particles.

`color{Green} {►" Surface Waves or Long Waves (L Waves)"}` They are the waves of long wavelength, confined to the skin of the Earth's crust. It causes most of the earthquake's structural damage.

Shadow Zone

`color{purple} {"Shadow zone :"}`
Areas on the surface of the Earth where no P-waves or S-waves from a particular earthquake are received.

`color{blue}✍️` Shadow zones are caused by the refraction (bending) of waves as they pass through the different layers of the Earth. The shadow zones for each earthquake depend on its location.

`color{blue} ►` If no P or S waves - the reason is a shadow zone.

`color{blue} ►` If P-waves but no S-waves - the reason is S-waves were blocked by the outer core of the Earth.

`color{blue} ✍️ ` The traditional zones of earthquake are :

`color{green}{(a)}` Circum Pacific belt
`color{green}{(b)}` Mid Continental belt
`color{green}{(c)}` Mid Atlantic belt

Foreshocks, Mainshocks and Aftershocks

`color{purple}{"Foreshocks, Mainshocks and Aftershocks"}`

`color{blue} •` A major earthquake never occurs alone.

`color{blue} •` The mainshock, which is the earthquake with the highest magnitude, is accompanied by foreshocks and aftershocks.

`color{blue} •` The smaller intensity shocks, which occur before the mainshock is called the foreshock.

`color{blue} •` The fault that moves in the mainshock experiences massive redistribution of stress and this disrupted surface causes most of the aftershocks.

`color{blue} •` The higher the magnitude of the mainshock, the larger the radius in which the aftershocks will be felt. For example, the aftershock zone of magnitude 5 earthquake will be around 5 miles while that of a magnitude 8 will be more than 200 miles.

`color{blue} •` The quantity of aftershocks also depends on the magnitude of the mainshock. For example, a magnitude 5 aftershock will produce in a sequence 10 magnitude 4 aftershocks, 100 magnitude 3 aftershocks and 1000 magnitude 2 aftershocks and so on.

`color{blue} •` Sometimes, there are smaller shocks that occur before (foreshock), after (aftershock) and a main earthquake (mainshock).
At times fore shocks are so big and scientists are unsure if it is the main shock.

`color{blue} •` Foreshocks and aftershocks can occur for days, weeks and months of a main earthquake.

Tsunamis

`color{blue}✎` A tsunami is a series of great sea waves caused by an underwater earthquake, landslide, or volcanic eruption. It is very rare but sometimes a tsunami can be generated by a giant meteor impact with the ocean

`color{blue} ✍️` The seismic waves caused by the earthquakes travelling through sea water, generate high sea waves, this phenomena is known as tsunami. It is generated by the displacement of water.

`color{blue} ✍️` Sometime, tsunamis are referred to as tidal waves. This once popular term derives from the most common appearance of tsunami, which is that of an extraordinarily high tide bore.

`color{blue} ✍️` Since the pacific ocean is girdled by the ring of earthquakes and volcanoes, tsunamis are more common in the pacific with a minimum frequency of two tsunamis per year.