Earthquakes are the result of crustal structure plates or plates moving through the Earth’s crust. Tensile stress is generated at the boundary of the slip plate, and an earthquake occurs when the fault line is destroyed. In an earthquake, elastic breaking energy is released and waves continue to shake the earth. Scientists can predict where a major earthquake could occur, but research has not yet been able to accurately predict a particular location or weather. The tsunami-causing earthquakes leveled the entire city and affected the entire country. Minor earthquakes can also be caused by human activities, such as extracting minerals from the earth and colliding with large buildings.
An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking buildings apart and causing gaping fissures to open up in the ground. In other words, “The term of Earthquake is a form of energy of wave motion transmitted by the help 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 can originate at a major point alone.
Earthquake = Earth + Quake, it is created by two words, Here is Earth’s term refer as Surface Of The Layer whereas Quake is denoted as Fast Shaking.
Relatives Theories of Earthquake
1. Tectonic Earthquake: Earthquakes are produced by sudden movements along faults, and are mostly, therefore of tectonic origin. The concept of a possible mode of origin of tectonic earthquakes is known as elastic-rebound theory.
Such earthquakes generally result from sudden yielding to strain produced on the rocks and produce a relative displacement of rock. Such faulting causes shaking because the 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 establishing fact.
Elastic-Rebound Theory- According to Prof. H.F Reid, Soil materials, being elastic, can withstand a certain amount of stress without permanently changing shape, but if the stress continues for a long period of time, or if it increases in size, the rock will first undergo permanent deformation or strain. and finally broke. A fault is a break or fracture in the earth’s rock material on one side of the fault that tends to return to its original shape because it is elastic and occurs elastic reflection. It is this reflection that forms seismic waves.
- Volcanic Earthquakes: Usually, earthquakes associated with volcanoes are more localized both extent of damage and in the intensity of the waves produced in comparison to those which are associated with faulting motions. The shock may be generated by any of the following mechanisms:
- Explosion of the volcano upon the release and expansion of gases and laves.
- Faulting within the volcano resulting from presrures in the chamber of molten.
- Collapse of the centre of the volcano into the space formed by the extrusion of gases and molten matter.
Apart from the above, sometimes local reasons can cause damage to a weak dam such as earthquakes under stagnant water pressure, etc.
Important Terminology Used for understanding the Seismology
I. Focus- It is exacted spot underneath the earth’s surface at which an earthquake originates. It is also known as Hypocentres.
II. Epicenter- It is the points on the surface of the earth, above the focus of an earthquake.
III. Isoscismal Line or Isoseists- It is a line joining all points at which the intensity of the earthquake is the same. It is, in fact, an isodiastrophic line of equal damage.
IV. Homoseismals or coseismal or Homoseists- There are lines joining the places where the shock arrives at the same times.
Types Of Earthquakes
Natural earthquakes are of three types; according to the depth of their origin. They are as follows:
a) Shallow-focus Earthquakes:- In this case the seismic shocks originated at a depth between 30 miles or less, below the earth’s surfaces.
b) Intermediate-focus Earthquakes:- In this point of origin of the shock is at a depth between 30 to 150 miles.
c) Deep-focus Earthquakes:- Here the point of origin of the shock is at a depth between 150 to 450 miles.
Types of earthquake Waves
a) Primary or ‘P’ waves-
These are longitudinal waves similar to sound waves and travel in solid, liquid, and gaseous media. They have short wavelengths and high-frequency.
b) Secondary or ‘S’ waves-
These are transverse waves, also known as shear waves, travel only in solid media. In comparison to primary waves, they are slow in motion. They travel at varying velocities through the solid parts, proportional to the density of the materials. They are also having a short wavelength and high frequency.
c) ‘L’ waves-
These are transverse vibrations and are confined to the outer skin of the crust. They are also known as surface waves or Rayleigh (R) waves. They have low velocity, low frequency, and long-wavelength. These are responsible for most of the destructive force of the earthquake.
Measurement Scale of Earthquake Intensity
Various scales have been proposed to estimate the intensity of earthquakes from the amount of damage caused. These scales are:
1. Rosi-Forrel Scale: In the rosi forrel scale, the intensity has classified into severe, catastrophic, and disastrous.
2. Mercalli- Scale: The Mercalli intensity scale has derived twelve numbers with the increase of intensity. In this case- number 1 detected only by seismographs.
3. Richter Scale of the Earthquake magnitudes: In the Richter scale, the scale number ranges from ‘0’ to ‘9’. Here it is particularly important to notice that a magnitude- ‘8’ earthquake is 10 times larger than a magnitude- 7 earthquakes, 100 times larger than a magnitude- 6 earthquakes, and 1000 times larger than a magnitude-5 earthquake.
NOTE:-The instrument used for recording seismic shocks is known as ‘Seismograph’, and the records of seismic shocks prepared and presented by seismographs are known as “seismograms”.
Recording of Earthquake
The instrument used for recording seismic shocks is known as ‘Seismograph’, and the records of seismic shocks prepared and presented by seismographs are known as “seismograms”.
Fig: Earthquake magnitude levels vector illustration diagram, Richter scale seismic activity diagram with shaking intensity, from moving furniture to crashing buildings.
Distribution Of Earthquake
• The destructive earthquakes are concentrated in a ring surrounding the pacific Ocean. This ring coincides with the circum Pacific Ring Of Fire.
• The second chain is termed an East-Indian, which extends over Indonesia, Andaman-Nicobar, Islands, and Burma.
• The third belt extends over Himalayas, Kun-Lun, tien Saban, and Altai Range up to Lake Baikal.
• Another belt extends from the Pamir knot to Afghanistan, Iran, Turkey Greece, Rumania, Atlas Mountains, Gibraltar, and the Azores Islands.
• A belt also extends from the Gulf of Aden, between Seychelles and Maldive islands, turns to the west-south of Africa, and goes up to the Falkland Islands.
Source of map: – Introducing physical geography by Alan Strahler.
PREVENTIVE AND MITIGATION MEASURES OF EARTHQUAKE
- Pre-earthquake Preventive Measures
- Re-framing building codes, guidelines, manuals, and byelaws and their strict implementation. Tougher legislation for highly seismic areas.
- Incorporating earthquake-resistant features in all buildings in high-risk areas.
- Making all public utilities like water supply systems, communication networks, electricity lines, etc. earthquake-proof. Creating alternative arrangements to reduce damages to infrastructure facilities.
- Constructing earthquake-resistant community buildings and buildings (used to gather large groups during or after an earthquake) like schools, dharamshalas, hospitals, prayer halls, etc., especially in seismic zones of moderate to higher intensities.
- Supporting R&D in various aspects of disaster mitigation, preparedness and prevention, and post-disaster management.
- Evolving educational curricula in architecture and engineering institutions and technical training in polytechnics and schools to include disaster-related topics.
- Post-Earthquake Preventive Measures
- Maintenance of law and order, prevention of trespassing, looting, etc.
- Evacuation of people.
- Recovery of dead bodies and their disposal.
- Medical care for the injured.
- Supply of food and drinking water.
- Temporary shelters like tents, metal sheds, etc.
- Repairing lines of communication and information.
- Restoring transport routes.
- A quick assessment of destruction and demarcation of destroyed areas, according to the grade of damage.
- Cordoning off severely damaged structures that are liable to collapse during aftershocks. (http://admis.hp.nic.in/himachal/home/HomeGuards/pdfs/MEASURES%20FOR%20EARTHQUAKE%20RISK%20REDUCTION.pdf)
What to Do and Don’t During an earthquake
- Before an earthquake
- Know well seismic zonation of our area, get your house evaluated for retrofitting (if any) and ensure expert civil engineer’s help in making your house earthquake resistant.
- Go to any safe meeting places that are near to reach. Practice Drop, Cover, and Hold on in each safe place at least once a month.
- Make an emergency kit and place it in a safe place in the house which should contain all necessary items for your protection and comfort, sufficient for at least three days.
- To date prediction of earthquakes is not possible. Don’t listen to or spread rumors.
- During an earthquake
- First of all, don’t panic; stay chill and keep others calm, think and take necessary further action.
- Try to Protect yourself first then help others, drop to the floor, take cover under a sturdy desk or table and hold on to it so that it doesn’t move away from you. Wait there until the shaking stops.
- Make sure to Stay away from glass windows, heavy furniture, and anything that could fall, such as lighting fixtures or other similar items.
- If you are on the upper floor of the building, don’t jump from windows or balconies. Do not try and run out of a building, you may be hit by falling debris. Stay inside till the shaking stops and check if it is safe to go outside.
- If you are outdoors, find a clear spot away from buildings, trees, electrical lines, and narrow streets. Drop to the ground and stay there until the shaking stop.
- If you are in a vehicle, pull over to a clear location, stop and stay there with your seatbelt fastened until the shaking stops. Avoid bridges, flyovers, or ramps that might have been damaged by the earthquake.
- If in coastal areas, move to higher ground and check tsunami warning.
- If you are in a hilly area be alert and move away from slopes in case of landslides and falling rocks.
- After an earthquake
- Check-up: radio, TV, online updates, a social network for emergency information, and safety guidance.
- Watch out for fallen power lines or broken gas lines and stay out of damaged areas.
- Don’t enter partially damaged buildings. Strong aftershocks can cause further damage to the buildings and weak structures may collapse.
- Don’t use your two-wheeler/car to drive around the area of damage. Rescue and relief operations need the road for mobility.
- Anticipate aftershocks, if shaking lasts longer than usual.
- Leave a message stating where you are going if you must evacuate your residence.
- Evaluate damages and repair any deep cracks in the ceiling, beam, column, and foundation with the advice of an expert. (http://nidm.gov.in/PDF/IEC/EQ-14.pdf)