Natural Hazards
Terra firma. The firm
Earth we say. The term
itself shows that we
perceive the ground
under our feet to be
solid and stable.
However, this
perception is violently
shaken when an
Earthquake strikes.
Earthquake Essentials
56
An Earthquake is exactly what its name
suggests it is! It is a quaking or shaking
of the ground.
The cause of Earthquakes was stated
correctly in 1760 by British engineer
John Michell, one of the First Fathers of
Seismology, who wrote that Earthquakes
are caused by “…shifting masses of rock
miles below the surface.”
Most Earthquakes occur at depths of
less than 80 km from the Earth’s surface.
The deepest Earthquakes occur at plate
boundaries where the Earth’s crust is
being subducted into the Earth’s mantle.
These occur as deep as 750 km below
the surface.
Powerful Earthquakes can shake large
sections of the ground quite violently.
However, minor Earthquakes are sensed
as mere vibrations such as that caused
when a heavy truck rumbles past.
Earthquakes never kill directly. A person
will not die just because the ground
under one’s feet is shaking.
However, people can be: crushed by
collapsing building, electrocuted by
snapped power lines, hurt by falling
objects, drowned by the flood caused
by a broken dam; buried under a
landslide, and burnt by fires caused by
overturned stoves/broken gas/electric
lines. Apparently, more damage was
done by the resulting fire after the 1906
San Francisco (USA) Earthquake than by
the quake itself.
The hypocentre of an Earthquake is the
location beneath the Earth’s surface
where the rupture of the fault begins.
The epicentre of an Earthquake is the
location directly above the hypocentre
on the surface of the Earth.
A large Earthquake has tremors of
different intensities. The most severe
shock is called Main shock. Shocks
preceding it are called Foreshock and
those that follow are called Aftershock
Estimating Earthquakes
I.
II.
III.
SCIENCE REPORTER, February 2010
Seismograph is a device
used
to
measure
E a r t h q u a k e s .
Seismographs record a
zigzag trace that shows the
varying amplitude of ground
oscillations beneath it.
The modern seismograph was invented
in Japan around 1880 by John Milne.
The Mercalli scale named after
Giuseppe Mercalli who invented it in
1902 is used to estimate intensity of
Earthquakes. The following Levels are
taken into account:
No discernible movement.
People, notably those on the upper
floors of tall buildings notice.
People indoors but not those outdoors
sense it. Hanging objects swing.
IV. Crockery windows, and doors rattle. Some
people outdoors may feel movement.
V. Everybody senses it. Sleeping people
are awakened. Objects may fall down
and liquids spill out of open containers.
VI. Pictures fall off walls. Furniture moves.
Plaster in walls might crack. Slight
damage to poorly constructed houses.
VII. People have difficulty standing. Moderate
damage to well-built buildings.
VIII.Drivers have trouble steering. Water
levels in wells might change.
IX. Well-built buildings suffer considerable
damage. The ground cracks. Reservoirs
suffer serious damage.
X. Most buildings and their foundations
are destroyed. Some bridges are
destroyed. Dams are seriously
damaged. Large landslides occur.
Ground cracks in large areas. Railway
tracks are bent slightly.
Spring
A Modern
Seismograph
Pen
Weight
Rotating Drum
Frame
Natural Hazards
Damaged Rail Track (left) and Road (above)
Elements of Earthquakes
XI. Railway tracks are severely damaged.
XII. Almost everything is destroyed. The
ground moves in waves or ripples.
The Richter scale, a mathematical
formula named after Charles F. Richter
who invented it in 1934, is used to
estimate the magnitude of Earthquakes
but not used to express damage.
The magnitude is derived from the
logarithm of the amplitude of waves
recorded on a seismogram at a certain
period.
Since it is a logarithmic scale, an
Earthquake rating 4.0 on the Richter
scale is 10 times larger than one that
measures 3.0.
An Earthquake below
2.0 is called a
microquake.
Moderate
Earth- quakes score less
than 6.0 or so; those
rating more than 6.0 can
cause
significant
Charles F. Richter
damage.
Moment Magnitude Scale was
introduced by Tom Hanks and Hiroo
Kanamori (1979) to compare the energy
released by Earthquakes.
An anciant
Seismograph
Seismic waves propagate within the
Earth or along its surface when an
Earthquake happens.
Those waves that pass through the body
of the earth are called Body Waves;
those restricted to near the Earth’s
surface are called Surface Waves.
Body waves are of two types: Primary
(P) waves and Secondary (S) waves.
Surface waves are either Love waves
or Raleigh waves. These do not travel
through liquids.
The P waves are the fastest and also
the first ones we feel, followed by S
waves, Love waves and Raleigh waves.
The P waves cause the back and forth
motion.
S waves shake the rocks up and down
and also from side to side. Raleigh
waves give an elliptical motion to rock
particles while Love waves displace
particles perpendicularly to the
direction of propagation. Love waves
have no vertical component.
Earthquakes and India
Earthquake Experiences
It is estimated that there are 500,000
detectable Earthquakes each year.
100,000 of those can be felt, but only
100 of them cause damage.
Although Earthquakes can happen
anywhere, most strike a within 40,000
km long, horseshoe shaped zone called
circum-Pacific seismic belt or Ring of
Fire. The Alpide belt extending along
the southern margin of Eurasia accounts
for 17% of Earthquakes worldwide and
the third belt follows the submerged
mid-Atlantic Ridge.
The earliest recorded evidence of an
Earthquake is as old as 1831 BC
(Shandong province of China).
The most devastating Earthquake in
recorded history is likely to have been
the one that hit China’s Shaanxi
Province in 1556 killing 830,000
people. It is likely to have been rated 9
on the Mercalli scale.
The biggest Earthquake of the last
century originated off the coast of Chile
on 22 May 1960—it had a magnitude
of 9.5. Seismographs recorded seismic
waves that travelled all around the Earth
and shook it for many days—a
phenomenon called the free oscillation
of the Earth.
Earthquakes also take place under the
oceans; some trigger tsunamis.
The ice sheets of Antarctica experience
icequakes that are similar to
Earthquakes, but occur within the ice
sheet itself instead of the land
underneath the ice.
India Meteorological Department (IMD)
is the nodal agency of Government of
India for monitoring seismic activity in
and around the country.
India’s first seismological observatory
was set up at Kolkata in 1898.
Bureau of Indian Standards has
grouped the country into seismic zones.
Of these, zone V is rated as the most
seismically active region.
Codes and guidelines for Earthquakeproof buildings were developed by the
Bureau of Indian Standards first in
1962.
The Indian Earthquake of the highest
magnitude till date rated 8.7 on the
Richter scale. It was recorded in the
Shillong plateau on 12 June 1897.
The 1819 Runn of Kutch Earthquake is
considered one of the largest intra-plate
Earthquakes anywhere.
The Latur Earthquake on 30 September
1993 was the most devastating Stable
Continental Range Earthquake in the
world.
A major Earthquake registering 6.9 (US
Geological Survey put it as 7.9) on the
Richter scale struck Bhuj, Gujarat on
26 January 2001.
Dr Sukanya Datta
Scientist, NISCAIR posted to Director General's
Technical Cell, CSIR HQ
Email: [email protected]
SCIENCE REPORTER, February 2010
57