EARTHQUAKES
What is an Earthquake ?
Earthquake is a violent movement of the Earth as a result of
tectonic upheaval caused by the sudden release of
accumulated stress, perhaps along a fault or fault zone. This
caused by sudden movements of the Earth’s crust
Classification of Earthquakes by depth of Focus
The earthquake focus can be deep, intermediate or shallow. A
deep focus happens in the mantle , whereas a shallow focus
in the crust.
EARTHQUAKES
An earthquake with a shallow focus causes
more damage at the surface.
Classification
Shallow – less than 70 kilometres
Intermediate – 70 to 300 kilometres
Deep – more than 300 kilometres
The place underground where the rock snaps is
called the focus . The place at the surface
directly above the focus is called the epicentre.
Rings of seismic waves spread from the
epicentre like ripples in a pond.
STRUCTURE OF AN EARTHQUAKE
NATURE OF EARTHQUAKES
A fault is a fracture accompanied by a movement along one side
or both sides. The total movement is called a shift. Displacement
can be vertical (throw) or horizontal (heave).
Faulting movement is usual during an earthquake.. Earthquakes
occur from the deformation of outer brittle portions of “tectonic
plates”, the earth’s outermost layer of crust and upper mantle.
The degree of folding depends on the relative force on the rock
from each direction.
Rock on both sides of an active fault is slowly bent over (fold)
many years as tectonic forces are applied. Energy accumulates
in the bent rock, just as it does in a bent crossbow. When a
critical point is reached, the strain is relieved by slippage on the
fault and a large quantity of energy is instantaneously released
in the form of seismic waves.
NATURE OF EARTHQUAKES
CAUSES OF EARTHQUAKES
Earthquakes occur at fault zones where
tectonic plates collide or slide against each
other. Immense stress can build up between
plates over years. When this stress is released
quickly, it sends massive vibrations called
seismic waves to the surface.
STAGES OF AN EARTHQUAKE
There are three main stages to an earthquake :
Foreshocks – relate to the initial shattering of obstructions or
bonds along the failure plane.
Principal Shock – is the most severe shock . It may last from
just a few seconds to a couple of minutes.
Aftershocks – recur as the shockwaves travel around the Earth .
They generally decrease in frequency and intensity over time ,
but may occur over a period of several days to several months.
They have great potential to damage, as structures have already
been weakened by the principal shock.
RECORDING & MEASURING OF EARTHQUAKES
Scientist record seismic waves from
earthquakes on seismometers.
Seismometers are so sensitive that they can
pick up earthquake vibrations from the other
side of the world.
Earthquake vibrations are shown as a
seismograph.
RECORDING & MEASURING OF EARTHQUAKES
The strength of an earthquake is described
using two different scales. Scientist assign a
magnitude rating to earthquakes based on
the strength and duration of their seismic
waves.
An earthquake measuring 3 to 5 is
considered light
5 to 7 is moderate to strong
7 to 8 is major
8 or more is great
RECORDING & MEASURING OF EARTHQUAKES
The strength of an earthquake is described
using two different scales.
1) Richter Scale
2) Mercalli Scale
The Richter scale measures the total amount
of energy released. The Richter scale is
logarithmic. This means a level 5
earthquake is 10 times more powerful than
a level 4.
RECORDING & MEASURING OF EARTHQUAKES
The Mercalli scales measures the amount of
damage caused by the earthquake. The
scale ranges from 1 to 12.
Occurrences
Some 80% of all the planet’s earthquakes
occur along the Pacific Ocean known as the
‘Ring of Fire’.
IMPACTS OF EARTHQUAKES
Total or partial destruction of building structures.
Interruption of water supply
Breakage of sewage disposal systems
Loss of public utilities such as electricity or gas
Floods from collapsed dams
Release of hazardous material
Fires
Spread of chronic illnesses.
EFFECTS OF EARTHQUAKES
The effects of earthquakes are grouped into
two categories . They are primary effects and
secondary effects.
Primary effects are hazards which happen
immediately when an earthquake strikes.
Secondary effects are problems faced in the
hours and days after an earthquake.
EFFECTS OF EARTHQUAKES
PRIMARY EFFECTS
1. Building Collapse – buildings which have
not been built to withstand earthquakes
may collapse, trapping and killing the
people inside. In some cases the land
becomes liquefied and buildings actually
sink
2. Falling Objects – objects such as signs, may
fall from buildings. Glass falling from broken
windows is a major hazard
EFFECTS OF EARTHQUAKES
SECONDARY EFFECTS:
1. Fire – gas pipes ripped open in an earthquake may
catch fire.
2. Tsunami – huge waves, up to 30metres high travelling
at 500km hour, can occur after an earthquake which
happens out at sea
3. Disease- cholera and typhoid may spread through the
population of an area if the water supply has been cut
off
4. Communications – buckled roads and railways and
fallen telephones cables, make travelling and
communicating difficult
5. Economy- factories may have to close and people lose
their jobs. The country will spend millions of pounds on
repairs