What is an Earthquake?
An earthquake is a
sudden violent shaking
of the ground due to a
build up of pressure
being suddenly released
within the Earth’s crust.
Earthquake Characteristics
Highest level of energy is
at the focus (when at the
surface the epicentre).
Energy < as waves
spread outwards.
The most
destructive,
devastating natural
disaster?
Shallow Focus: 0-70km
Intermediate Focus: 70-300km
Deep Focus: 300-700km
Foreshock occurs before
the earthquake. Not all
earthquakes feature these.
Aftershock is the biggest
earthquakes after the
main quake.
Distribution
Intraplate Earthquakes
Plate Boundaries Earthquakes
Seismic Waves
P-Waves (Primary): Fastest
waves that shake the Earth by
the wave spreading out then
contracting to create
vibrations as they are
longitudinal waves.
They move through core,
mantle and crust.
Seismic Waves
S-Waves (Seconary): Slower
waves that move sideways
motion shaking the Earth at
right angles as they are
transverse waves, hence move
in a S shape. They move
through mantle and crust.
They’re more destructive.
Seismic Waves
Surface Waves: Waves that
travel nearer to the surface
more slowly.
Consist of love waves which
moves side to side, and Raleigh
waves which move up and
down. They are even slower
however are more destructive.
The Causes
Tectonic Activity
Volcanic Activity
Friction of plates
when they
collide, found at
different plate
Boundaries.
Pressure of built
up on volcanos
when the vent is
blocked by lava.
Old fault lines
deep in the
ground
reactivated by
seismic activity.
Reactivated Faults
Immense
pressure caused
by human
activities on and
into the crust.
Human Activity
Tectonic Activity
The most common cause of earthquakes is tectonic activity. The process is
called elastic-rebound theory. Friction builds up in the lithosphere causing the
rocks to undergo an increasing amount of stress and stored strain energy
around the fault. Once released the rock surrounding it breaks and seismic
waves are released.
Mid Ocean Ridges
Conservative Boundary
Destructive Boundary
Continental Plate Boundaries
Collision / Constructive
Mid Ocean Ridge: As the two oceanic
plates diverge the new crust created in
seafloor spreading occurs at different
rates. This creates transform slip faults.
The friction at these faults create
shallow focus earthquakes as activity is
low and the lithosphere is weak.
Conservative: The two continental
plates slide past one another creating a
great build up of friction causing large
strains to build up. This energy is
released in the terms of strong shallow
focus earthquakes.
Destructive: Subduction occurs of the
oceanic plate under the continental
plate. The friction between these two
plates occurs at the Benioff zone. The
pressure is released in terms of shallow
and deep focus earthquakes.
Continental: At collision plate boundary
high levels of seismicity occur due to
the pressure of the rock buckling to
create fold mountains, resulting in large
earthquakes. At constructive plate
boundary pressure builds up at the fault
lines caused by the brittle crust
fracturing as the crust updomes.
Examples!
Kobe Earthquake
17th January 1995 (5:46)
Mw: 6.8 Nojima Fault
“Most expensive natural disaster in
modern history”
Northridge Earthquake
17th January 1994 (4:31)
Mw: 6.7 Blind Thrust Fault
“Considered a federal disaster”
Haiti Earthquake
12th January 2012 (16:53)
Mw: 7.0 Enriquillo-Plaintain Fault
“One of the deadliest earthquakes
of all time”
Reactivated Fault Lines
Intraplate earthquakes can also occur in the middle of a plate hence not by a
plate boundary…
This is caused by stress released at distant fault lines deep in the ground.
These are fault lines once active in the past but are now no longer
considered. Movement and stress can occur at these faults in the same way it
does at a plate boundary however these earthquakes usually have a pretty
small magnitude and are cause little destruction.
Market Rasen Earthquake – 27th February 2008
5.2 magnitude earthquake (richter
scale) lasted 10 seconds. It was
caused by a sudden rupture at a
strike-slip fault 18.6km below
ground. With 9 aftershocks!
Human Activities
95% of earthquakes are not located at plate boundaries which brings the
question… What causes the other 5%?
Human activities including dams, mining, and burying waste.
• A dam holds an extremely huge amount of water. Due to
this huge pressure there is a huge increase in stress load
on the crust, which increases the level of seismicity.
For example The Hoover Dam.
• When mining a huge amount of material is being
removed. This largely changes the mass and at times can
cause a deformation. This process changes the stresses of
an area and can increase seismicity.
For example, Newcastle in New South Wales, 5.6 earthquake.
Other example includes the hydraulic fracking in Blackpool
causing a 2.3 magnitude earthquake.
Volcanic Activity
These earthquakes have different characteristics to tectonically caused earthquakes.
They are large earthquakes, more rare but contained to a 10-20 mile radius.
This is mostly associated with acidic lava volcanoes which cools and sets quickly
when it meets the air. If this occurs the vent can get trapped. Due to this, huge
pressure builds up until the volcano explodes on a large scale, causing a huge
magnitude earthquake to occur.
The lava of Mt Pelee was of an acidic viscous nature hence solidified at the vent
before it had a chance to flow down the volcano. This caused immense pressure
to build up and once released resulted in a large earthquake.
Measuring Earthquakes
Seismic Records
Seismic records are formed by
using a seismograph. This
technology allows
seismologists to record data
from earthquake vibrations.
This data is used in the
following equations for
calculating the magnitude
scale of the earthquake.
They also allow seismologists
to map the interior of the
Earth via the type of waves
and properties of that wave.
The Richter Scale
Quantifies the amount of energy an earthquake produce at the epicentre.
Uses the ML equation based on the waves recorded by a seismograph.
It is based on recording the surface waves (love and Rayleigh).
It is logarithmic on a scale of 10^1, 10^2, 10^3…
Hence the energy of an earthquake is 10 times more at each real number.
The Mercalli Scale
The scale measures the intensity of the earthquake.
This is not done by using calculations and equations but by using your eyes.
It is the scale of the effects such as damage to structural buildings and the environment.
Measured from a scale of I to XII.
There is a general pattern
that as distance increases
from the epicentre that the
Mercalli scale number
decreases, hence destruction
decreases as you move
further away. However there
are anomalies…
This scale is measured from
what humans percieve rather
than quantitative data.
The Moment Magnitude Scale
Measures the amount of energy an earthquake produces at the epicentre.
Uses the Mw equation based on the waves recorded by a seismograph.
It is based on recording the actual shake that took place at the epicentre.
It is logarithmic on a scale of 10^1.5, 10^3, 10^4.5 etc..
It increases by approximately 32 times each real number.
It is similar to the Richter Scale however the equation has been adapted.
This scale is becoming more largely recognised around the world.