Earthquake!-
- Article Narrated by Nishwanth Nichu
What
is earthquake ?
An earthquake is a weak to violent shaking of the ground
produced by the sudden movement of rock materials below the earth's surface.
The earthquakes originate in tectonic plate boundary.
Earthquakes are caused by a sudden release of stress along faults
in the earth's crust. The continuous motion of tectonic plates causes a steady
build-up of pressure in the rock strata on both sides of a fault until the
stress is sufficiently great that it is released in a sudden, jerky movement.
The resulting waves of seismic energy propagate through the ground and over its
surface, causing the shaking we perceive as earthquakes.
Causes of Earthquakes
The
Earth’s crust consists of seven large lithospheric plates and numerous smaller
plates. These plates move towards each other,
·
Convergent Boundary
·
Divergent Boundary
·
Transform Boundary
What do Convergent Boundaries Form?
Ø Convergent boundaries can form mountains, volcanos, or subduction
zones that form large trenches.
Ø When two plates collide, the crusts can push together to Form
Mountain ranges. This is how the Himalayan Mountains were formed.
Ø Convergent boundaries between oceanic and continental boundaries
feature a subduction zone. This
allows magma to escape through the lithosphere, which can form volcano’s on one
side of the boundary and mountains on the other.
Ø This occur where plates are separating from one
another. This spreading is caused by convective forces in the molten lava below the surface.
Ø When this occurs between two oceanic plates, as they
slowly spread apart, this fluid basalt lava fills the gap and quickly
solidifies as the water cools it, forming new oceanic crust.
Ø When two continental plates pull apart, a rift valley is formed. As the plates separate, faults form on either
side and the earth in between sinks. This activity often results in
earthquakes.
Ø One example of a continental divergent boundary is the
East African Rift.
What do Transform Boundaries Form?
Ø Transform boundaries occur where plates are sliding past
one another. They are also called conservative boundaries because crust is neither destroyed nor created along them.
Ø Transform boundaries are most common on the seafloor,
where they form oceanic fracture zones.
Ø When they occur on land, they produce faults. These fracture and fault lines typically connect
offsetting divergent zones.
Ø The East Anatolian and North Anatolian faults run across much of Turkey and
cause large and deadly earthquakes
What are the types of earthquake?
There are four different types of
earthquakes:
Tectonic, Volcanic,
Collapse and Explosion
Ø A tectonic earthquake is one that occurs when the earth's crust breaks due to
geological forces on rocks and adjoining plates that cause physical and
chemical changes.
EPICENTRE - The part of the earth's surface directly above the
starting point of an earthquake.
FOCUS - Point inside the
earth where the earthquake started, sometimes called the
hypocenter
WAVE FRONTS - The instantaneous boundary between the seismic waves in the earth material, and the material
that the seismic energy has not yet reached.
FAULT - A
fracture or zone of fractures between two blocks of rock (normal,
reverse, strike-slip, oblique)
FAULT SCRAP - A small step or offset on the ground surface where
one side of a fault has moved vertically with respect to the other
Ø A volcanic earthquake is any earthquake that results from tectonic forces which occur
in conjunction with volcanic activity.
Ø A collapse earthquake are small earthquakes in underground caverns and mines that are
caused by seismic waves produced from the explosion of rock on the surface.
Ø An explosion earthquake is an earthquake that is the result of the detonation of a
nuclear and/or chemical device.
Types of seismic waves
P-waves
Ø P-waves, also known as primary waves or pressure waves, travel at
the greatest velocity through the Earth. When they travel through
air, they take the form of sound waves – they travel at the speed of sound (330 ms-1) through air but may
travel at 5000 ms-1 in granite. Because of their speed, they are the
first waves to be recorded by a seismograph during an earthquake.
Ø They differ from S-waves in
that they propagate through a material by alternately compressing and expanding
the medium, where particle motion is parallel to the direction of
wave propagation – this is rather like a slinky that is partially
stretched and laid flat and its coils are compressed at one end and then
released
S-waves
Ø S-waves, also known as secondary waves, shear waves or shaking waves,
are transverse waves that travel slower than P-waves.
Ø In this case, particle motion
is perpendicular to the direction of wave propagation. Again, imagine
a slinky partially stretched, except this time, lift a section and
then release it, a transverse wave will travel along the length of
the slinky.
Earth waves
Ø Seismic waves are waves that
travel through or over Earth. They are usually generated by movements of the
Earth's tectonic plates (earthquakes) but may also be caused by explosions,
volcanoes and landslides. They can tell us much about the Earth's structure.
Ø S-waves cannot travel through
air or water but are more destructive than P-waves
because of their larger amplitudes
Surface waves
Ø Surface waves are similar in
nature to water waves and travel just under the Earth’s surface.
Ø They are typically generated
when the source of the earthquake is close to the Earth’s surface.
Ø Although surface waves travel
more slowly than S-waves, they can be much larger in amplitude and
can be the most destructive type of seismic wave.
Ø There are two basic kinds of
surface waves:
·
Rayleigh waves,
also called ground roll, travel as ripples similar to those on the surface
of water. People have claimed to have observed Rayleigh waves during an
earthquake in open spaces, such as parking lots where the cars move up and down
with the waves.
·
Love waves
cause horizontal shearing of the ground. They usually travel slightly faster
than Rayleigh wave.
What are the 3 ways of
measuring earthquakes?
WAVE
AMPLITUDE
Ø In P or compressional waves, the vibration of the rock is in the
direction of propagation. P waves travel fastest and are the first to arrive
from the earthquake. In S or shear waves, rock oscillates perpendicular to the
direction of wave propagation. In rock, S waves generally travel about 60% the
speed of P waves, and the S wave always arrives after the P wave. For example,
sound waves are P waves at a high enough frequency to hear with your ear. An
example of an S wave is wiggling or shaking a rope which is tied down at one or
both ends.
Ø Both P
and S waves travel outward from an earthquake focus inside the earth. The waves
are often seen as separate arrivals recorded on seismographs at large distances
from the earthquake. The direct P wave arrives first because its path is
through the higher speed, dense rocks deeper in the earth. The PP (one bounce)
and PPP (two bounces) waves travel more slowly than the direct P because they
pass through shallower, lower velocity rocks. The different S waves arrive
after the P waves.
Ø The
slowest (and latest to arrive on seismograms) are surface waves, such as the L
wave. L waves are named for the Cambridge mathematician A.E.H. Love who first
described them. The surface waves are generally the largest recorded from an
earthquake. Body waves in the earth's interior lose their amplitude rapidly as
they get farther from the earthquake because they spread out inside the volume
of the earth. Surface waves, however, spread out more slowly and only on the
earth's surface. The energy from surface waves is confined to a smaller volume
at the surface and the wave amplitude to carry that energy is therefore larger
than body waves.
FAULT SIZE
Ø The magnitude of slip is simply how far the two
sides of the fault moved relative to one another; it's a distance usually a few centimeters for
small earthquakes and meters for large events. The direction of slip is measured
on the fault surface, and like the strike and dip, it is specified as an angle.
Ø Fault is
a fracture or crack where two rock blocks slide past one to another. If this
movement may occur rapidly, it can be causes earthquike or slowly, in the form
of creep.
Ø Types
of faults include strike-slip faults, normal faults, reverse
faults, thrust faults, and oblique-slip faults.
Ø It
can be small and large complex interconnection fault systems and can replace
one type of fault in one location with another type in another. Many faults are
associated with folds.
Ø Faults
are separated, bifurcated, converge, or move away from distances, sometimes
creating complex fracture systems.
AMOUNT OF SLIP
Ø The rate of motion obtained
when the amount of offset is divided by a time interval. The common units of
measure are millimeters per year or meters per thousand years (mm/yr or m/k.y.; equivalent units).
Ø The average slip rate at a
point along a fault is commonly determined from geodetic measurements,
displacement of manmade features, or from offset geologic features whose age
can be estimated or measured.
Ø Offset is measured parallel
to the predominant slip direction or estimated from the vertical or horizontal
separation of geologic features.
Ø In special cases, interval slip rates
