An earthquake
that follows a large magnitude earthquake called, "main shock" and originates
in or around therupture zone of the main shock. Generally,
major earthquakes are followed by a number of aftershocks, which show a
decreasing trend in magnitude and frequency with time.
Arrival is the
appearance of a wave, representing seismic energy, on a seismic record. The
time at which a particular wave / phase arrives at a station or detector is
called arrival time.
The innermost
layer of the Earth below the mantle, with an approximate radius of 3500 km.
Seismic wave studies led to a subdivision of the core into an "outer core”,
which in relation to seismic waves acts as a fluid (no resistance to shear,
i.e. no shear strength) and an "inner core” which acts as a solid. The radius
of the Earth is about 6371 kilometers. The core-mantle boundary also represents
a sharp thin discontinuity in physical properties such as a precipitous fall of
the compressional wave velocity from 13.7 to 8.1 km/s and cessation of shear
waves.
It is the rigid
outermost layer of the Earth. Beneath the oceans, the crust varies in
thickness, between 5-8 km.Thickness of
the crust beneath continents is much more variable but averages about 25-40 km;
under large mountain ranges, such as the Alps or the Sierra Nevada, however,
the base of the crust can be as deep as 60-70 km. Like the shell of an egg, the
Earth's crust is brittle and can break.
Grievous
impact of a sudden (sometimes prolonged, as in the case of drought) adverse
event leaving considerable damage and destruction in its wake as well as other
debilitating consequences (epidemics, erosion of life support systems, etc.).
Severity of a disaster, whilst
subject to the magnitude (amount of energy unleashed) of the event, therefore
largely depends on the resilience of the affected community (assimilated
knowledge and understanding to anticipate future hazards and preparedness to
cope with them).
It encompasses
all acts necessary for eliminating, minimizing and reducing the possible
adverse impacts of a potentially disastrous event which may be anticipated or
impending or has already happened.
It calls for a planned, sustained
and orchestrated applications of knowledge, technology (including information
and communication systems), community education and management principles, to
reduce vulnerability by enhancing hazard consciousness and by progressive
assimilation of hazard resistant land use plans, building and slope grading
codes and advance warning systems in the life and works of communities exposed
to natural hazards.
Earthquakes
are the manifestations of sudden release of strain energy accumulated in the
rocks over extensive periods of time in the upper part of the Earth.
Earthquakes are classified as, Slight (M<5.0), Moderate (5.0<M<6.9)
and Great (M>7.0) depending upon the magnitude on Richter‟s scale. An
earthquake having a magnitude, M<2.0 is termed as microearthquake.
It is the
point on the surface of the Earth, vertically above the place of origin
(Hypocenter or Focus) of an earthquake. This point is expressed by its
geographical coordinates in terms of latitude and longitude.
A subjective
measure of the effects of an earthquake at a particular place on humans,
structures and (or) the land itself. The intensity at a point depends not only
upon the strength of the earthquake (magnitude) but also on the distance from
the earthquake to the point and the local geology at that point. Intensity
grades are commonly given in Roman numerals (in the case of the Modified
Mercalli Intensity Scale, from I for "not perceptible” to XII for "total
destruction”).
An abrupt
movement of soil and / or rock masses downhill in response to gravity.
Landslides may be triggered by an earthquake or other natural causes. Undersea
landslides can also lead to tsunamis.
The word cyclone has been derived from Greek word ‘cyclos’ which
means ‘coiling of a snake’. The word cyclone was coined by Heary Piddington who
worked as a Rapporteur in Kolkata during British rule. The terms
"hurricane" and "typhoon" are region specific names for a
strong "tropical cyclone". Tropical cyclones are called "Hurricanes”
over the Atlantic Ocean and "Typhoons” over the Pacific Ocean.
The
frequencies of cyclonic storms crossing different coastal states of India
during 1891-2006 are shown in the figure below. The frequency of severe
cyclonic storms is maximum for Andhra Pradesh while that of cyclone is maximum
for Orissa. Considering west coast only, Gujarat is most vulnerable.
The
phenomenon Tsunami is a series of traveling ocean waves of extremelylong length generated primarily by
earthquakes occurring below or near the ocean
floor;
•
A tsunami is a series of waves
with a long wavelength and period (time between
crests). Time between crests of the wave can vary from a few minutes to over an hour.
•
Tsunamis are often incorrectly
called tidal waves; they have no relation to the
daily ocean tides.
•
Tsunami (soo-NAH-mee) is a
Japanese word meaning harbour wave.
•
Tsunamis are generated by any
large, impulsive displacement of the sea bed
level.
•
Earthquakes generate tsunamis by
vertical movement of the sea floor. If the sea
floor movement is horizontal, a tsunami is not generated. Earthquakes of M > 6.5 are critical for tsunami generation.
•
Tsunamis are also triggered by
landslides into or under the water surface, and
can be generated by volcanic activity and meteorite impacts.
•
Tsunami velocity is dependent on
the depth of water through which it travels (Velocity
equals the square root of water depth h times the gravitational acceleration g, that is V =√g h).
•
Tsunamis travel approximately at
a velocity of 700 kmph in 4000 m depth of sea
water. In 10 m of water depth the velocity drops to about 36 kmph.
For example, the tsunami from
Sumatra coastal earthquake travelled to Tamil
Nadu coast in about two hours.
Even on shore tsunamis speed is 35 –
40 km/h, hence much faster than a person
can run.
