| Earthquakes
The Indian subcontinent has a history of devastating earthquakes.
The major reason for the high frequency and intensity of the earthquakes
is that India is driving into Asia at a rate of appromately 47 mm/year[1].
Geographical statistics of India show that almost 54% of the land
is vulnerable to earthquakes. The latest version of seismic zoning
map of India given in the earthquake resistant design code of India
[IS 1893 (Part 1) 2002] assigns four levels of seismicity for India
in terms of zone factors. In other words, the earthquake zoning
map of India divides India into 4 seismic zones (Zone 2, 3, 4 and
5) unlike its previous version which consisted of five or six zones
for the country. According to the present zoning map, Zone 5 expects
the highest level of seismicity whereas Zone 2 is associated with
the lowest level of seismicity. The latest seismic zoning map can
be accessed from The India Meteorological Department website ([1]).
Zone 5
Zone 5 covers the areas with the highest risk zone that suffers
earthquakes of intensity MSK IX or greater. The IS code assigns
zone factor of 0.36 for Zone 5. Structural designers use this factor
for earthquake resistant design of structures in Zone 5. The zone
factor of 0.36 is indicative of effective (zero period) peak horizontal
ground accelerations of 0.36 g (36 % of gravity) that may be generated
during MCE level earthquake in this zone. It is referred to as the
Very High Damage Risk Zone. The state of Kashmir, Punjab,the western
and central Himalayas, the North-East Indian region and the Rann
of Kutch fall in this zone.
Generally, the areas having trap or basaltic rock are prone to earthquakes.
Zone 4
This zone is called the High Damage Risk Zone and covers areas liable
to MSK VIII. The IS code assigns zone factor of 0.24 for Zone 4.
The Indo-Gangetic basin and the capital of the country(Delhi, Jammu)and
bihar fall in Zone 4.
Delhi prone areas - The areas which are near to Yamuna bank are
very much prone to the earthquake. East delhi is the most earthquake
prone area. Some areas are- Shahdara, Mayur Vihar - I, II, III,
Laxmi Nagar and nearby areas, Gurgaon, rewari, noida
Zone 3
The Andaman and Nicobar Islands, parts of Kashmir, Western Himalayas
fall under this zone. This zone is classified as Moderate Damage
Risk Zone which is liable to MSK VII. The IS code assigns zone factor
of 0.16 for Zone 3.
Zone 2
This region is liable to MSK VI or less and is classified as the
Low Damage Risk Zone. The IS code assigns zone factor of 0.10 (maximum
horizontal acceleration that can be experienced by a structure in
this zone is 10 % of gravitational acceleration) for Zone 2.
What is an earthquake and what causes them to happen ?
Ans: An earthquake is caused by a sudden slip on a fault.
Stresses in the earth's outer layer push the sides of the fault
together. Stress builds up and the rocks slips suddenly, releasing
energy in waves that travel through the earth's crust and cause
the shaking that we feel during an earthquake. An EQ occurs when
plates grind and scrape against each other. In California there
are two plates the Pacific Plate and the North American Plate. The
Pacific Plate consists of most of the Pacific Ocean floor and the
California Coast line. The North American Plate comprises most the
North American Continent and parts of the Atlantic Ocean floor.
These primary boundary between these two plates is the San Andreas
Fault. The San Andreas Fault is more than 650 miles long and extends
to depths of at least 10 miles. Many other smaller faults like the
Hayward (Northern California) and the San Jacinto (Southern California)
branch from and join the San Andreas Fault Zone. The Pacific Plate
grinds northwestward past the North American Plate at a rate of
about two inches per year. Parts of the San Andreas Fault system
adapt to this movement by constant "creep" resulting in
many tiny shocks and a few moderate earth tremors. In other areas
where creep is NOT constant, strain can build up for hundreds of
years, producing great EQs when it finally releases.
Can we cause earthquakes? Is there any way to prevent earthquakes
?
Ans: Earthquakes induced by human activity have been documented
in a few locations in the United States, Japan, and Canada. The
cause was injection of fluids into deep wells for waste disposal
and secondary recovery of oil, and the use of reservoirs for water
supplies. Most of these earthquakes were minor. The largest and
most widely known resulted from fluid injection at the Rocky Mountain
Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude
5.5 followed a series of smaller earthquakes. Injection had been
discontinued at the site in the previous year once the link between
the fluid injection and the earlier series of earthquakes was established.
(Nicholson, Craig and Wesson, R.L., 1990, Earthquake Hazard Associated
with Deep Well Injection--A Report to the U.S. Environmental Protection
Agency: U.S. Geological Survey Bulletin 1951, 74 p.) Other human
activities, even nuclear detonations, have not been linked to earthquake
activity. Energy from nuclear blasts dissipates quickly along the
Earth's surface. Earthquakes are part of a global tectonic process
that generally occurs well beyond the influence or control of humans.
The focus (point of origin) of earthquakes is typically tens to
hundreds of miles underground. The scale and force necessary to
produce earthquakes are well beyond our daily lives. We cannot prevent
earthquakes; however, we can significantly mitigate their effects
by identifying hazards, building safer structures, and providing
education on earthquake safety.
What do we know about the interior of the Earth ?
Five billion years ago the Earth was formed by a massive
conglomeration of space materials. The heat energy released by this
event melted the entire planet, and it is still cooling off today.
Denser materials like iron (Fe) sank into the core of the Earth,
while lighter silicates (Si), other oxygen (O) compounds, and water
rose near the surface. The earth is divided into four main layers:
the inner core, outer core, mantle, and crust. The core is composed
mostly of iron (Fe) and is so hot that the outer core is molten,
with about 10% sulfur (S). The inner core is under such extreme
pressure that it remains solid. Most of the Earth's mass is in the
mantle, which is composed of iron (Fe), magnesium (Mg), aluminum
(Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000
degrees C, the mantle is solid but can deform slowly in a plastic
manner. The crust is much thinner than any of the other layers,
and is composed of the least dense calcium (Ca) and sodium (Na)
aluminum-silicate minerals. Being relatively cold, the crust is
rocky and brittle, so it can fracture in earthquakes. (Univ. of
Nevada).
What are plate tectonics?
Plate tectonics is the continual slow movement of the tectonic
plates, the outermost part of the earth. This motion is what causes
earthquakes and volcanoes and has created most of the spectacular
scenery around the world.
What is a fault and what are the different types?
A fault is a fracture or zone of fractures between two
blocks of rock. Faults allow the blocks to move relative to each
other. This movement may occur rapidly, in the form of an earthquake
- or may occur slowly, in the form of creep. Faults may range in
length from a few millimeters to thousands of kilometers. Most faults
produce repeated displacements over geologic time. During an earthquake,
the rock on one side of the fault suddenly slips with respect to
the other. The fault surface can be horizontal or vertical or some
arbitrary angle in between.
Earth scientists use the angle of the fault with respect to the
surface (known as the dip) and the direction of slip along the fault
to classify faults. Faults which move along the direction of the
dip plane are dip-slip faults and described as either normal or
reverse, depending on their motion. Faults that move horizontally
are known as strike-slip faults and are classified as either right-lateral
or left-lateral. Faults, which show both dip-slip and strike-slip
motion are known as oblique-slip faults.
The following definitions are adapted from The Earth by Press and
Siever.
Normal fault- a dip-slip fault in which the block above the fault
has moved downward relative to the block below. This type of faulting
occurs in response to extension and is often observed in the Western
United States Basin and Range Province and along oceanic ridge systems.
Thrust fault- a dip-slip fault in which the upper block, above
the fault plane, moves up and over the lower block. This type of
faulting is common in areas of compression, such as regions where
one plate is being sub ducted under another as in Japan. When the
dip angle is shallow, a reverse fault is often described as a thrust
fault.
Strike-slip fault - a fault on which the two blocks slide past
one another. The San Andreas Fault is an example of a right lateral
fault.
A left-lateral strike-slip fault is one on which the displacement
of the far block is to the left when viewed from either side.
A right-lateral strike-slip fault is one on which the displacement
of the far block is to the right when viewed from either side.
At what depth do earthquakes occur ?
Earthquakes occur in the crust or upper mantle, which ranges
from the earth's surface to about 800 kilometers deep (about 500
miles).
What is "surface rupture" in an earthquake ?
Surface rupture occurs when movement on a fault deep within
the earth breaks through to the surface. NOT ALL earthquakes result
in surface rupture.
What is the relationship between faults and earthquakes?
What happens to a fault when an earthquake occurs ?
Earthquakes occur on faults - strike-slip earthquakes occur
on strike-slip faults, normal earthquakes occur on normal faults,
and thrust earthquakes occur on thrust or reverse faults. When an
earthquake occurs on one of these faults, the rock on one side of
the fault slips with respect to the other. The fault surface can
be vertical, horizontal, or at some angle to the surface of the
earth. The slip direction can also be at any angle.
How do we know a fault exists ?
if the EQ left surface evidence, such as surface ruptures or fault
scarps (cliffs made by EQs).
if a large EQ has broken the fault since we began instrumental recordings
in 1932.
if the faults produces small EQs that we can record with the denser
seismographic network established in the 1970s.
Where can I go to see the/a fault ?
The closest fault depends on where you live. Some earthquakes
produce spectacular fault scarps, and others are completely buried
beneath the surface. Sometimes you may not even know that you are
looking at a fault scarp.
What does an earthquake feel like ?
Generally, during an earthquake you first will feel a swaying
or small jerking motion, then a slight pause, followed by a more
intense rolling or jerking motion. The duration of the shaking you
feel depends on the earthquake's magnitude, your distance from the
epicenter, and the geology of the ground under your feet. Shaking
at a site with soft sediments, for example, can last 3 times as
long as shaking at a stable bedrock site such as one composed of
granite. If the site is in a building, then the height of the building
and type of material it is constructed from are also factors. For
minor earthquakes, ground shaking usually lasts only a few seconds.
Strong shaking from a major earthquake usually lasts less than one
minute. For example, shaking in the 1989 magnitude 7.1 Loma Prieta
(San Francisco) earthquake lasted 15 seconds; for the 1906 magnitude
8.3 San Francisco earthquake it lasted about 40 seconds. Shaking
for the 1964 magnitude 9.2 Alaska earthquakes, however, lasted three
minutes.
Foreshocks, aftershocks - what is the difference ?
"Foreshock" and "aftershock" are relative
terms. Foreshocks are earthquakes, which precede larger earthquakes
in the same location. Aftershocks are smaller earthquakes, which
occur in the same general area during the days to years following
a larger event or "mainshock", defined as within 1-2 fault
lengths away and during the period of time before the background
seismicity level has resumed. As a general rule, aftershocks represent
minor readjustments along the portion of a fault that slipped at
the time of the main shock. The frequency of these aftershocks decreases
with time. Historically, deep earthquakes (>30km) are much less
likely to be followed by aftershocks than shallow earthquakes. (Univ.
of Washington).
Two earthquakes occurred on the same day. Are they related
?
Often, people wonder if an earthquake in Alaska may have
triggered an earthquake in California; or if an earthquake in Chile
is related to an earthquake that occurred a week later in Mexico.
Over these distances, the answer is no. Even the Earth's rocky crust
is not rigid enough to transfer stress fields efficiently over thousands
of miles.
Source:http://earthquake.usgs.gov/hazards/
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