EARTHQUAKE

Definition:
An earthquake is sudden movement or violent shaking of earthen rocks or volcanic action.
The sudden movement, breaking, adjustment or any change in position of earthen rocks causes
vibration on the ground that shakes the region. This shaking or vibration of earth’s crust or surface is
called Earthquake.
 Earthquakes happen along cracks in the earth’s surface, called Fault Lines.
 Can be felt over large areas.
 Usually last less than 1 minute.
 Earthquakes cannot be predicted, although scientists are working on it.
 Earthquake produces waves that shake the ground.
 It is measured on seismograph.
 These waves are called Seismic Waves.
There are 3 types of layers;
P-Wave or Push Waves or Primary Waves (can travel through liquid and solid)
S-Wave or Shake Waves or Secondary Waves (Can travel through semi-liquid Mantle but
cannot pass through the Liquid)
L-Wave or Land Waves (Travels only on the ground surface. Records only on the Earth’s
Crust)
P-Waves are primary waves, they can
travel faster than S-Waves (secondary
waves) and pass through any medium and
also reaches first on the Richter scale
whereas S-waves cannot pass through
liquids but solids.
When earthquake occurs seismic waves are
generated.
The point where earthquake is occurred
within the Earth is known as Focus.
On the 90o upward on the Earth crust from
the Focus the point known as Epicenter.
Fault lines are called to those which are occurred with the earthquake separating two blocks by
moving the crust.
Causes of Earthquake:
1. Tectonic Plates Movement
a) Transform or Lateral Movement
b) Convergent Movement
c) Divergent Movement
2. Isostatic Adjustment
3. Volcanic Eruption
4. Local or Human Activities
5. Elastic Rebound Theory
1. Tectonic Plates Movement:
 Major cause of the tectonics movement of the earth crust.
 Occurs due to the structural adjustment inside the earth.
 The earth’s lithosphere is divided into tectonic plates of various sizes.
 Major plates having area greater than 20 million km2 (African and North-American plates are
major plates).
 Minor plates are those which are smaller than 20 million km2 (Caribbean and Arabian Plates
are minor plates).
 Micro plates are all those which have an area of less than 1 million km2.
 All these plates are floating on Asthenosphere which is upper part of the Mantle.
 The movement of the tectonic plates can be classified into three main categories.
a) Transform or Lateral Movement:
o These are those in which plates rub against each other.
o Relative motion is predominantly horizontal, in either a sinistral (left lateral) or dextral (right
lateral) direction.
o Furthermore, transform faults end abruptly and are connected on both ends to other faults, ridges,
or subduction zones.
o While most transform faults are hidden in the deep oceans where they form a series of short
zigzags accommodating seafloor spreading (see graphic at right), the best-known (and most
destructive) are those on land at the margins of tectonic plates.
o Transform faults are the only type of strike-slip fault that can be classified as a plate boundary.
b) Convergent Movement:
o A convergent boundary, also known as a destructive plate boundary (because of subduction), is
an actively deforming region where two (or more) tectonic plates or fragments of the lithosphere
move toward one another and collide.
o Subduction is a geological process that takes place at convergent boundaries of tectonic
plates where one plate moves under another and is forced down into the mantle.
o As a result of pressure, friction, and plate material melting in the mantle, earthquakes and
volcanoes are common near convergent boundaries.
o When two plates move towards one another, they form either a subduction zone or a continental
collision.
o This depends on the nature of the plates involved. In a subduction zone, the subducting plate,
which is normally a plate with oceanic crust, moves beneath the other plate, which can be made
of either oceanic or continental crust.
o During collisions between two continental plates, large mountain ranges, such as the Himalayas
are formed.
 Oceanic and Continental Plates:
 When continental and oceanic plates collide the thinner and denser oceanic plate
is overridden by the thicker and less dense continental plate.
 The oceanic plate is forced down into the mantle in a process
known as "subduction".
 As the oceanic plate descends it is forced into higher
temperature environments.
 At a depth of about 100 miles (160 km) materials in the
subducting plate begin to approach their melting
temperatures and a process of partial melting begins.
 This partial melting produces magma chambers above the subducting
oceanic plate.
 If a magma chamber rises to the surface without solidifying the magma will break through in the
form of a volcanic eruption.
  Oceanic and Oceanic Plates:
 When a convergent boundary occurs between two oceanic plates one of those
plates will subduct beneath the other.
 Normally the older plate will subduct because of its
higher density.
 The subducting plate is heated as it is forced deeper
into the mantle and at a depth of about 100 miles (150
km) the plate begins to melt.
 Magma chambers are produced as a result of this
melting and the magma is lower in density than the
surrounding rock material.
 Continental and Continental Plates:
 The two thick continental plates collide and both of them have a density that is much lower than
the mantle, which prevents subduction (there may be a small amount of
subduction or the heavier lithosphere below the continental crust
might break free from the crust and subduct).
 Fragments of crust or continent margin sediments might be
caught in the collision zone between the continents forming
a highly deformed mélange of rock.
 The intense compression can also cause extensive folding
and faulting of rocks within the two colliding plates.
 This deformation can extend hundreds of miles into the
plate interior.
 The Himalaya Mountain Range is the best active example of this type of plate boundary.

2. Isostatic Adjustment:
 Isostatic adjustment refers to the transient
(102−104 years) or long term (> 105 years)
nonelastic response of the earth's lithosphere to
loading and unloading due to erosion,
deposition, water loading, desiccation, ice
accumulation, and deglaciation.
 Isostatic uplift is the process by which land
rises out of the sea due to tectonic activity. It
occurs when a great weight is removed from
the land, e.g., the melting of an ice cap.
 Eustatic changes are the dropping of sea levels
when eater is locked away as ice, and its rising
as it melts.
 It mainly happen due to erosion.
 There are mainly four agents of erosion.
 a) Air b) Ice c) Water d) Gravity

3. Volcanic Eruption:
 Earthquakes are sometimes caused by volcanic
activity but they are usually local and seldom
cause any extensive damage.
4. Local or Human Activities:
 Earthquakes of small intensity are also a result of human activities which contains;
 Land sliding – May cause small intensity earthquakes.
 Explosions – For mining and cutting of mountains to make tunnels or roads and underground
atomic explosions may cause earthquakes due there shock waves.
 Storage in dams reservoirs (2008 Sichuan, China earthquake)- increasing load and destabilizing
fault-
 Groundwater extraction (2011 Lorca earthquake) - Taking water out of ground, which causes
the water table to drop, can also destabilize an existing fault.
 Fracking and injection wells (2011 Oklahoma earthquake) - Extraction of oil or gas, afterward,
when waste fracking fluid is injected back underground into deep wells. The fluid can seep out
and lubricate faults, causing them to slip more easily.
 Skyscrapers – Having thousands of tons weight can destabilize the fault may cause small
earthquakes.

5. Elastic Rebound Theory:

 The elastic rebound theory is an explanation for how
energy is spread during earthquakes.
 As rocks on opposite sides of a fault are subjected to
force and shift, they accumulate energy and slowly
deform until their internal strength is exceeded.

Earthquakes Occurred in History

1. 26 October 2015 Pakistan (Gilgit-Baltistan and Khyber Pakhtunkhwa) –Hindu Kush –
Magnitude 7.5 -363 Deaths & 1200 Injuries.
2. 25 December 2015 Pakistan (Gilgit-Baltistan and Khyber Pakhtunkhwa) –Hindu Kush –
Magnitude 6.3 -40 Deaths & 88 Injuries.
3. 12 September 2016 South Korea (Gyeongju) - Magnitude 5.4 -8 Injuries

Seismograph
 Seismometers are instruments that measure motion of the ground, including those of seismic waves
generated by earthquakes, volcanic eruptions, and other seismic sources.
 Records of seismic waves allow seismologists to map the interior of the Earth, and locate and
measure the size of these different sources.
 It is measured on Rector Scale.