Earthquakes

Earthquakes

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What is an earthquake?
• Mobile crust
• Stress
• Pressure release
• Focus (hypocenter)
• Epicentre
• Depth of focus…surface damage


• Focus: The point at which the stress is
released
• Epicentre: The point directly above that on
the earth’s surface
• Seismic waves: shockwaves released
from the earthquake


0-43 miles (0-70 km) below ground: shallow earthquakes
43-186 miles (70-300 km) below ground: intermediate earthquakes
deeper than 186 miles (300 km) below ground: deep earthquakes


• An earthquake is a vibration of the earth
caused by the rapid release of energy
• Every year 3000 of moderate intensity
earthquakes occur – most cause little
damage
• Tectonic forces slowly deform the
crustal rocks on both sides of the fault,
much like a stick being bent.
• As the forces holding are overcome the
rock slips at its weakest point (focus)
allowing the rock to snap back into
place.

• Earthquakes are a result of the rapid
release of elastic energy stored ion the
rock.
• Aftershocks occur after the events and are
usually weaker they can cause significant
destruction e.g. Armenia 1988.


Seismic activity
• Seismic Waves
A seismic wave is an elastic wave generated
by an impulse such as an earthquake or an
explosion. Seismic waves may travel either
along or near the earth's surface (Rayleigh
and Love waves) or through the earth's
interior (P and S waves).
• A body wave is a seismic wave that moves
through the interior of the earth, as opposed
to surface waves that travel near the earth's
surface. P and S waves are body waves.
Each type of wave shakes the ground in
different ways.


Primary (P) waves are fastest and can move
through solids and liquids.

Secondary (S) waves move with a sideways
motion and can not move through liquids.
They make the ground move horizontally,
hence they cause much damage.

When P and S waves reach the surface,
some of them are transformed into surface
waves. These can cause the ground to move
sideways (Love waves) and up and down
(Rayleigh waves)


Earthquake waves


Seismic Waves
When a sudden break or shift occurs in the earth's crust, the energy radiates out as seismic waves, just as the energy
from a disturbance in a body of water radiates out in wave form. In every earthquake, there are several different types of
seismic waves.

Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth. Surface
waves -- sometimes called long waves, or simply L waves -- are responsible for most of the damage associated with
earthquakes, because they cause the most intense vibrations. Surface waves stem from body waves that reach the
surface.
There are two main types of body waves.

•Primary waves, also called P waves or compressional waves, travel about 1 to 5 miles per second (1.6 to 8 kps),
depending on the material they're moving through. This speed is greater than the speed of other waves, so P waves arrive
first at any surface location. They can travel through solid, liquid and gas, and so will pass completely through the body of
the earth. As they travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and then
back together -- in line with the direction the wave is traveling. These waves typically arrive at the surface as an abrupt
thud.
•Secondary waves, also called S waves or shear waves, lag a little behind the P waves. As these waves move, they
displace rock particles outward, pushing them perpendicular to the path of the waves. This results in the first period of
rolling associated with earthquakes. Unlike P waves, S waves don't move straight through the earth. They only travel
through solid material, and so are stopped at the liquid layer in the earth's core.

Both sorts of body waves do travel around the earth, however, and can be detected on the opposite side of the planet from
the point where the earthquake began. At any given moment, there are a number of very faint seismic waves moving all
around the planet.
Surface waves are something like the waves in a body of water -- they move the surface of the earth up and down. This
generally causes the worst damage because the wave motion rocks the foundations of manmade structures. L waves are
the slowest moving of all waves, so the most intense shaking usually comes at the end of an earthquake.
In the next section, we'll see how scientists can calculate the origin of an earthquake by detecting these different waves.


Modified Mercalli Intensity Scale

• I. People do not feel any Earth movement.
• II. A few people might notice movement if they are at rest and/or on
the upper floors of tall buildings.
• III. Many people indoors feel movement. Hanging objects swing
back and forth. People outdoors might not realize that an
earthquake is occurring.
• IV. Most people indoors feel movement. Hanging objects swing.
Dishes, windows, and doors rattle. The earthquake feels like a
heavy truck hitting the walls. A few people outdoors may feel
movement. Parked cars rock.
• V. Almost everyone feels movement. Sleeping people are
awakened. Doors swing open or close. Dishes are broken. Pictures
on the wall move. Small objects move or are turned over. Trees
might shake. Liquids might spill out of open containers.
• VI. Everyone feels movement. People have trouble walking.
Objects fall from shelves. Pictures fall off walls. Furniture moves.
Plaster in walls might crack. Trees and bushes shake. Damage is
slight in poorly built buildings. No structural damage.
•


• VII. People have difficulty standing. Drivers feel their cars shaking.
Some furniture breaks. Loose bricks fall from buildings. Damage is
slight to moderate in well-built buildings; considerable in poorly built
buildings.
• VIII. Drivers have trouble steering. Houses that are not bolted down
might shift on their foundations. Tall structures such as towers and
chimneys might twist and fall. Well-built buildings suffer slight damage.
Poorly built structures suffer severe damage. Tree branches break.
Hillsides might crack if the ground is wet. Water levels in wells might
change.
• IX. Well-built buildings suffer considerable damage. Houses that are
not bolted down move off their foundations. Some underground pipes
are broken. The ground cracks. Reservoirs suffer serious damage.
• X. Most buildings and their foundations are destroyed. Some bridges
are destroyed. Dams are seriously damaged. Large landslides occur.
Water is thrown on the banks of canals, rivers, lakes. The ground
cracks in large areas. Railroad tracks are bent slightly.
• XI. Most buildings collapse. Some bridges are destroyed. Large cracks
appear in the ground. Underground pipelines are destroyed. Railroad
tracks are badly bent.
• XII. Almost everything is destroyed. Objects are thrown into the air. The
ground moves in waves or ripples. Large amounts of rock may move.


Why do some earthquakes cause more than others?

Depth of focus - The more shallow the focus – deep earthquakes may occur up to
700km below ground level, but those less than 40km deep cause the most
damage as the seismic waves as the seismic waves haven’t dissipated by the
time they reach areas of human habitation.
Magnitude – Earthquakes are measured using the Richter Scale (see below). The
stronger the earthquake, the more likely it is to do damage. The Modified Mercalli
Scale may also be used.
Duration – Earthquakes may last a fraction of a second, or may go on for several
minutes if you include aftershocks.
Distance from epicentre – the further away, the less severe the seismic waves (in
the same way as depth of focus).
Underlying geology – softer rocks, muds and clays may cause more damage
including liquefaction (see related hazards)
Other factors – time of day may influence damage to people (night = more deaths
as people are asleep and less likely to get out of falling buildings etc.). Quality of
buildings, and other human factors, are of course equally important as any
physical factors.


Where do they occur?
• Destructive plate boundaries
• Conservative margins…faults
– Normal
– Reverse
– Thrust
– Strike-slip


Normal faults within the subducting
slab can slip causing shallow
earthquakes
‘thrusts’ at the plate crush
past each other cause
shallow earthquakes.

Slab pull and mineral
collapse at intermediate
and deep levels can
cause slippage.


Pacific plate moves northwards at a rate
of 6cm relative to the North American
plate.


OTHER CAUSES…
• Underground subsidence
• Lubrication of old faults by water / oil
• Dam / reservoir construction (Lake Kariba
in Zambia)
– Extra weight
– Seepage of water into groundwater zone
• Windscreen effect of pressure at a margin
spreading to the mid-plate regions


At least 9,500 people were killed, 30,000 injured, more than 100,000 people were left
homeless, and severe damage was caused in parts of Mexico City and in several states of
central Mexico.

According to some sources, the death toll from this earthquake may be as high as 35,000.

It is estimated that the quake seriously affected an area of approximately 825,000 square
kilometers, caused between 3 and 4 billion U.S. dollars of damage, and was felt by almost 20
million people.

Four hundred twelve buildings collapsed and another 3,124 were seriously damaged in
Mexico City.

The maximum Modified Mercalli intensity was IX at Mexico City, Ciudad Guzman and the
Pacific Coast towns of Lazaro Cardenas, Ixtapa and La Union. Felt reports were received
from Mazatlan, Sinaloa to Tuxtla Gutierrez, Chiapas, and as far away as Guatemala City,

Landslides caused damage at Atenquique, Jalisco and near Jala, Colima. Rockslides were
reported along the highways in the Ixtapa area and sandblows and ground cracks were
observed at Lazaro Cardenas.

A tsunami was generated which caused some damage at Lazaro Cardenas, Zihuatenejo and
Manzanillo. Estimated wave heights were 3 meters at Zihuatenejo and 2.8 meters at Lazaro
Cardenas.

A large percentage of the buildings which were damaged in Mexico City were between 8 and
18 stories high, indicating possible resonance effects with dominant two-second period
horizontal ground accelerations which were recorded in the area.


The Mw7.4 earthquake that struck western Turkey on August 17, 1999 (also known
as the Kocaeli, Turkey, earthquake) occurred on one of the world's longest and
best studied strike-slip (horizontal motion) faults: the east-west trending North
Anatolian fault. This fault is very similar to the San Andreas fault in California.


The Kobe earthquake struck at the
edge of the densely populated
Osaka-Kyoto corridor in southwest
Japan.


Effects of an earthquake
Primary Secondary

The ground shaking…  Soil liquefaction
 Landslides / avalanches
Depends on…  Tsunamis
 Magnitude  Effects on people and
 Depth the environment
 Distance from focus


Effects on people and the environment
Short-term Long-term
•People killed or injured •Homelessness – lack of shelter

•Buildings collapse •Communication / infrastructure
damage
•Bridges / roads / railways collapse
•Disruption to key services (little
•Water / gas / electricity pipes clean water…disease)
rupture
•Long-term emotional impact
•Fires break out
•Orphans
•Landslides in mountainous areas
•Cost of rebuilding
•Tsunami impact coastal regions
•Loss of income (economic impact)
•Worry / trauma / anxiety
•Refugees / out-migration


COUNTRY GDP PER CAPITA (PPP)
PAKISTAN $2,600
TURKEY $8,900
JAPAN $33,100


Why do people live in hazard
zones?
• Perceptions of the hazard

Based on…

• Prediction
• Prevention
• Preparation


SEISMIC GAP THEORY


Earthquakes

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