Earthquakes
or seismicity : cause, distribution, measurement, consequences,seismic zones of
India, mitigation and adaptations
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 th
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)
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 (=({10^{1.0}})^{(3/2)})
in the energy released; a difference in magnitude of 2.0 is equivalent to a
factor of 1000 (=({10^{2.0}})^{(3/2)}) 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.