Earthquakes or Seismicity :--
Earthquake is a sudden shaking of the ground/ earth surface. It leads to vibration or oscillation of the earth surface.
Each Earthquake is associated with 3 elements :-
1) Focus(Hypocenter) :- located inside the earth interior ,below the surface from where earthquake originates. On the basis of depth the earthquakes are classified into 2 groups :
a) shallow focus earthquake above 70km depth. b) deep focus earthquake between 70-700km depth.
2) Epicentre is a point vertically above the Focus (Hypocenter) on the earth surface. It is a location of maximum damage.
3) seismic wave :- the seismic energy is released from hypocenter is P-wave and S -wave which is at epicentre produce the surface waves. See the the diagram below :
P-wave is primary seismic wave originates from focus. Characterised by it's longitudinal nature (compressional wave), passes through both solid and liquid medium, fastest among all seismic wave.
S-wave is secondary seismic wave characterised by slower than P-wave, Transverse ( distortions wave) in nature, only passes through solid medium.
Surface wave decay with depth , it consist of two components : a) Rayleigh wave -- it is a rolling wave containing perpendicular and parallel motion of particles. It moves slower than 90% of S-wave speed. b) love wave-- it is transverse wave, it is little faster than Rayleigh wave. It is most destructive wave. Love wave destroy the foundation of building structure and later, Rayleigh wave leads to collapse of building structure.
Stress causing factors leads to earthquake
Major earthquakes caused by :-
1) Tectonic Induced earthquakes at plate boundaries.
2) Volcanic Induced earthquakes at volcanic sources such as volcanic peaks in folded mountains, island arcs, hotspots, mid-oceanic ridges, rift valleys.
3) landslide induced earthquakes.
4) Dam/Reservoir Induced earthquakes.
Minor triggers for earthquakes are :-
5) sand blasting/ mining/quarrying
6) nuclear testing
7) pumping of ground waters
8) meteorite falling
Geographical distribution : seismic zone of the world. :
Magnitude of an earthquake means measurement of an amplitude of seismic wave as shown below record by seismograph/seismometer.
Seismographic recording is being calibrated using lograthmic scale with base 10. Richter scale measure the magnitude of an earthquake .it is a decimal number between 1 to 10. If magnitude of earthquake increase by 1order than amplitude increase by 10 times and energy increase by 33 times. Earthquakes is felt when magnitude is more than 5. The energy release of an earthquake , which closely correlates to it's destructive power, scales with the 3/2 power of shaking amplitude.Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 (
) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 (
) in the energy released.
Intensity of earthquake measure the damage caused by an earthquake. It depends on local factors such as geological and tectonic stability , population ,economic infrastructure, level of preparedness and disaster management , technology, etc. There are many sales varies from time to time and region to region as given below :-
The most popular scale for Intensity measurement is modified mercalli scale (MM scale). It is a Roman number from I to XII.
MM scale for intensity of earthquake
From I to XII is mainly used in USA. India used similar scale called MSK scalelike israel, Russia, CIS . The Medvedev–Sponheuer–Karnik scale, also known as the MSK or MSK-64, is a macroseismic intensity scale used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence. It is also represented by Roman numeral of 12 degree intensity.
Seismic zones of India :-
Old map consist of 5 zones while new seismic map consist of 4 zones, Bureau of Indian Standard (BIS) used building code according to seismic zones.
----------------------------------------
---- Definition, Elements, seismic waves.
---- Cause : Elastic Rebound theory of Harry Fielding Reid.
---- Stress and strain causing Factors that trigger earthquakes
--- measurement of an earthquake by
a) magnitude of an earthquake
b) intensity of an earthquake.
--- seismic map of world.
--- seismic zones if India.
--- consequences
--- consequences
--- Disaster management :-Mitigation and Adaptation measure to reduce risk of earthquake.
Earthquake is a sudden shaking of the ground/ earth surface. It leads to vibration or oscillation of the earth surface.
Each Earthquake is associated with 3 elements :-
1) Focus(Hypocenter) :- located inside the earth interior ,below the surface from where earthquake originates. On the basis of depth the earthquakes are classified into 2 groups :
a) shallow focus earthquake above 70km depth. b) deep focus earthquake between 70-700km depth.
2) Epicentre is a point vertically above the Focus (Hypocenter) on the earth surface. It is a location of maximum damage.
3) seismic wave :- the seismic energy is released from hypocenter is P-wave and S -wave which is at epicentre produce the surface waves. See the the diagram below :
P-wave is primary seismic wave originates from focus. Characterised by it's longitudinal nature (compressional wave), passes through both solid and liquid medium, fastest among all seismic wave.
S-wave is secondary seismic wave characterised by slower than P-wave, Transverse ( distortions wave) in nature, only passes through solid medium.
Surface wave decay with depth , it consist of two components : a) Rayleigh wave -- it is a rolling wave containing perpendicular and parallel motion of particles. It moves slower than 90% of S-wave speed. b) love wave-- it is transverse wave, it is little faster than Rayleigh wave. It is most destructive wave. Love wave destroy the foundation of building structure and later, Rayleigh wave leads to collapse of building structure.
How earthquake occurs?
Mechanism of an earthquake is explained by elastic rebound theory given by Harry fielding Reid in 1906. According to elastic rebound concept, when applied stress on rock exceeds the elasticity/strength of rocks then strain developed, resulting into sudden sliping/sliding of rock from where seismic energy is released in different direction. Location from seismic energy is released called as faults/fracture/gap/crack in the block.
There is two type of slide/slip against hanging wall block and footwall block
a) Dip-slide/slip slopes for convergenceplate boundary as Thrust fault or divergence plate boundaries as Normal fault.
b) strike-slip fault develops at transform/parllel plate boundaries.
Stress causing factors leads to earthquake
Major earthquakes caused by :-
1) Tectonic Induced earthquakes at plate boundaries.
2) Volcanic Induced earthquakes at volcanic sources such as volcanic peaks in folded mountains, island arcs, hotspots, mid-oceanic ridges, rift valleys.
3) landslide induced earthquakes.
4) Dam/Reservoir Induced earthquakes.
Minor triggers for earthquakes are :-
5) sand blasting/ mining/quarrying
6) nuclear testing
7) pumping of ground waters
8) meteorite falling
Geographical distribution : seismic zone of the world. :
1) Circum-Pacific region/belt
2) Mid-continental belt or Alpide belt
3) carribean belt
4) mid-afternoon oceanic ridges of Atlantic, Indian and Arctic ocean and South east pacific rise
5) east African rift valley
6)volcanic hotspot
7) San-Andreaus Fault.
Measurement of an Earthquakes by two ways :-
1) Magnitude(M)
2) Intensity (I)
) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 () in the energy released.
Intensity of earthquake measure the damage caused by an earthquake. It depends on local factors such as geological and tectonic stability , population ,economic infrastructure, level of preparedness and disaster management , technology, etc. There are many sales varies from time to time and region to region as given below :-
The most popular scale for Intensity measurement is modified mercalli scale (MM scale). It is a Roman number from I to XII.
MM scale for intensity of earthquake
From I to XII is mainly used in USA. India used similar scale called MSK scalelike israel, Russia, CIS . The Medvedev–Sponheuer–Karnik scale, also known as the MSK or MSK-64, is a macroseismic intensity scale used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence. It is also represented by Roman numeral of 12 degree intensity.
Seismic zones of India :-
Old map consist of 5 zones while new seismic map consist of 4 zones, Bureau of Indian Standard (BIS) used building code according to seismic zones.
Zone 5 :- very high risk zone, here magnitude of an earthquake is more than 8, and MSK scale is > IX . It includes Himalayan states jammu and Kashmir to arunachal Pradesh,north eastern states and Andaman and Nicobar Islands due to collision of Indian plate in north with Eurasian plate and in east with Sunda/Burmese plate. It also include Lunch region of Gujarat. So, zone 5 lies under tectonically active region. The earthquake intensity for this zone is MSK-IX or greater. The zone factor for this zone is 0.36, which is indicative of effective (zero period) level earthquake in this zone.
Zone 4 :- High risk zone, here M =7-8, I =MSK value between VIII to IX. It include areas surrounding zone 5 in Himalayas, north east, kutch region as well as Northern plains and coastal plains of India. The zone factor for this zone is 0.24.
Zone 3 :- moderate risk zone, here M =6-7, MSK value = VII to VIII,
The zone factor for this zone is 0.16. It includes the river valleys of peninsular India.
Zone 2:- low risk zone, here M =5-6, I = MSK value = Vi to VII, The zone factor for this zone is 0.10. It includes the other parts of Peninsular India .since it is a stable landmass away from fault lines.
Consequences of an earthquakes
1) disaster by destroyed
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