Earthquake plate tectonic- Geomorphology Chapter

An earthquake is a trembling or
shaking of the ground caused by
the sudden release of energy
stored in the rocks beneath the
earth’s surface.
  Most earthquakes occur along pre-
existing faults, but a new fault can be
created during an earthquake.
        Two terms are used to describe
the point of origin for earthquakes:
1.  FOCUS
This is the actual location where fault
movement begins.  Almost every
earthquake has its focus located below
the earth's surface.

2.  EPICENTER
  This is the point on the land surface
directly above the focus and is the
location normally reported in the news
or shown on maps.

• An Earthquake is a sudden tremor or movement of the earth's
crust, which originates naturally at or below the surface. The
word natural is important here, since it excludes shock waves
caused by French nuclear tests, man made explosions and
landslides caused by building work.
• There are two main causes of earthquakes.
• Firstly, they can be linked to explosive volcanic eruptions; they
are in fact very common in areas of volcanic activity where
they either proceed or accompany eruptions.
• Secondly, they can be triggered by Tectonic activity associated
with plate margins and faults. The majority of earthquakes
world wide are of this type.
• (A volcano is an opening, or rupture, in a planet's surface or
crust, which allows hot magma, volcanic ash and gases to
escape from below the surface)

• Most earthquakes are causally related to compressional or
tensional stresses built up at the margins of the huge moving
lithospheric plates that make up the earth's surface.
• The immediate cause of most shallow earthquakes is the
sudden release of stress along a fault, or fracture in the earth's
crust, resulting in movement of the opposing blocks of rock
past one another.
• These movements cause vibrations to pass through and around
the earth in wave form, just as ripples are generated when a
pebble is dropped into water. Volcanic eruptions, rockfalls,
landslides, and explosions can also cause a quake, but most of
these are of only local extent.
•

Stresses
Stress is the FORCE acting on a body of rock.
Strain is the response of a rock to stress. It
generally involves a change in shape or
volume of the rock.
Types of stress:
•Compressional (squeezing)
•Tensional (stretching)
•Shear (side to side shearing)
Types of strain:
•Elastic deformation - changes in shape of rock
are reversible. Deform it, remove the stress, and it
returns to its original shape (like a rubber band or
a piece of elastic)
•Plastic deformation - changes in shape of rock
are permanent and not reversible (like folding).
Rock Behavior
•Brittle - the rock breaks
•Ductile - the rock flows or bends (folds are
produced)
Once the elastic limit is surpassed, rocks will
deform plastically if the rock is ductile or they will
fracture (rupture) if the rock is brittle.

Reid's Elastic Rebound Theory
From an examination of the displacement of the ground surface
which accompanied the 1906 earthquake, Henry Fielding Reid,
Professor of Geology at Johns Hopkins University, concluded that
the earthquake must have involved an "elastic rebound" of
previously stored elastic stress.
If a stretched rubber band is broken or cut, elastic energy stored in
the rubber band during the stretching will suddenly be released.
Similarly, the crust of the earth can gradually store elastic stress
that is released suddenly during an earthquake.
This gradual accumulation and release of stress and strain is now
referred to as the "elastic rebound theory" of earthquakes. Most
earthquakes are the result of the sudden elastic rebound of
previously stored energy.

• Earthquakes are three dimensional events, the waves move
outwards from the focus, but can travel in both the horizontal
and vertical plains. This produces three different types of waves
which have their own distinct characteristics and can only move
through certain layers within the Earth. Lets take a look at these
three forms of shock waves.
• Types of shockwaves
• P-Waves
Primary Waves (P-Waves) are identical in character to sound
waves. They are high frequency, short-wavelength, longitudinal
waves which can pass through both solids and liquids. The
ground is forced to move forwards and backwards as it is
compressed and decompressed. This produces relatively small
displacements of the ground.

• L-waves move particles in
a circular path.
S-waves move particles at 90° to
the wave's direction
Particles are compressed and expanded in
the wave's direction.

• S-Waves
Secondary Waves (S-Waves) travel more slowly than P-Waves
and arrive at any given point after the P-Waves. Like P-Waves
they are high frequency, short-wavelength waves, but instead
of being longitudinal they are transverse. They move in all
directions away from their source, at speeds which depend
upon the density of the rocks through which they are moving.
They cannot move through liquids. On the surface of the
Earth, S-Waves are responsible for the sideways displacement
of walls and fences, leaving them 'S' shaped.

• L-Waves
Surface Waves (L-Waves) are low frequency transverse
vibrations with a long wavelength. They are created close to the
epicentre and can only travel through the outer part of the
crust. They are responsible for the majority of the building
damage caused by earthquakes. This is because L Waves have a
motion similar to that of waves in the sea. The ground is made
to move in a circular motion, causing it to rise and fall as visible
waves move across the ground. Together with secondary
effects such as landslides, fires and tsunami these waves
account for the loss of approximately 10,000 lives and over
$100 million per year

Depth of the earthquakes
The max. depth is 670 km. They are classified into
three groups.
1. Shallow                                  0-70 km      85%
2. intermediate focus            70-350 km           12%
3. Deep focus                     350-700 km          3%
The shallow focus quakes are more common, because
rocks are more brittle on the surface.

• EARTHQUAKE STRENGTH
• The strength or size of the earthquake is measured in two ways
• 1. INTENSITY SCALE
• This is a measure of the damage caused by the earthquake on humans and
buildings.
• Intensities are pressed in roman numerals I-XII on the modified MERCALLI
SCALE, where higher numbers indicate more damage.
• VI. everybody feels it, they are scared run 0utdoors (damaged chimneys)
• VII. Damage slight. Partial collapse in weak badly designed buildings
• VIII. Damage slight in well designed building, worse in poorly designed ones.
so on.
• XII. damage is total.
• There is a problem with using this scale, because it is only measures the
amount of damage caused in human structures, and there is big difference in
building designs. Underlying geology is also important. Houses built on solid
rocks are much better for surviving than unconsolidated sediment.

• MAGNITUDE SCALE
• The second method is to measure the Magnitude of an earthquake (RICHTER SCALE). In
this method people calculate the energy released during the quake.
Richter Scale Comments
2.0 Smallest earthquake detectable by people
5.0 Energy released by the Hiroshima atomic bomb.
6.0-6.9 About 120 shallow earthquakes of this magnitude occur each year
6.7 Northridge, California earthquake 1994.
7.0 Major earthquake.
7.4 Turkey earthquake August 17, 1999. More than 12,000 people killed.
7.6 Deadliest earthquake this century. Tangshan, China, 1976. About
250,000 people died.
8,3 San Francisco earthquake of 1906.
8.6 Most powerful earthquake recorded in the last 100 years. Southern
Chile 1960. Claimed 5,700 lives.

• EFFECT OF EARTHQUAKES
• 1.GROUND MOTION
• This is the trembling and shaking of the land that can cause the buildings to vibrate.
In large quakes the motion is visible.
• Because proper building construction can reduce the damage in a great deal,
building codes has to be very strict in special areas. Their location also has to be
controlled. (hard rock!)
• 2.FIRE
• Particularly serious problem after an earthquake, because of the frequently broken
gas and water pipes, and fallen electrical wires. (San Francisco 1906)
• 3.LANDSLIDES
• They can be triggered by shaking the ground (1959 Madison Canyon) 1970 Peru
huge mud flows in the Andes!!
• 4. PERMANENT DISPLACEMENT OF THE SURFACE
• 5.AFTERSHOCKS
• Small earthquakes following the big ones. Although they are smaller they still can
cause damage in the weakened constructions.

• The level of damage done to a structure depends on the
amplitude and the duration of shaking. The amplitudes are
largest close to large earthquakes and the duration generally
increases with the size of the earthquake (larger quakes shake
longer because they rupture larger areas). Regional geology
can affect the level and duration of shaking but more
important are local site conditions. Although the process can
be complicated for strong shaking, generally shaking in soft
sediments is larger and longer than when compared with the
shaking experienced at a "hard rock" site.

• 6. TSUNAMIS
• The term tsunami comes from the Japanese 津波 , composed of the two
kanji 津 (tsu) meaning "harbor" and 波 (nami), meaning "wave". The
waves generated in the oceans triggered by high magnitude earthquakes in
the ocean-floor s (exceeding 7.5 on Richter scale).
• Seaquake; initiate huge wave.... (seismic sea waves) Hurricane can have a
wavelength of 400 m, however a tsunami may have a wavelength of 100
miles and may be moving 450 miles/hour. The wave height near the shore
can be 15-30 m.
• Because of the long wavelength of the tsunamis it will not withdraw fast,
but the water will rise for 5-10 minutes, causing flood.

• 7. LIQUEFACTION
• In areas where unconsolidated sediment (Unconsolidated sediments are
sediments ranging from clay to sand to gravel, with connected pore spaces
that allow Groundwater to be stored and transported) is water filled, during
the quake the sediment turns into liquid that is not capable to support
buildings. Buildings will collapse an sewage tanks and other underground
constructions will float up to the surface. Liquefaction is a phenomenon in
which the strength and stiffness of a soil is reduced by earthquake shaking
or other rapid loading. Liquefaction and related phenomena have been
responsible for tremendous amounts of damage in historical earthquakes
around the world.
Liquefaction occurs in saturated soils, that is, soils in which the space
between individual particles is completely filled with water. This water
exerts a pressure on the soil particles that influences how tightly the
particles themselves are pressed together. Prior to an earthquake, the water
pressure is relatively low. However, earthquake shaking can cause the water
pressure to increase to the point where the soil particles can readily move
with respect to each other.

• Ring of Fire (Circum-Pacific Belt: The circum-pacific belt
includes the eastern and western coastal areas of the Pacific
ocean ( or the western coastal margins of North and South
Americas and the eastern coastal margins of Asia).

• Mid-Continental Belt: This belt includes Alpine mountain
chains and the Mediterranean Sea and the fault zone of
eastern Africa.

• Earthquakes and Plate Tectonics
• The world's earthquakes are not randomly distributed over the
Earth's surface. They tend to be concentrated in narrow zones. Why
is this? And why are volcanoes and mountain ranges also found in
these zones, too?
• Plate tectonics tells us that the Earth's rigid outer shell (lithosphere)
is broken into a mosaic of oceanic and continental plates which can
slide over the plastic aesthenosphere, which is the uppermost layer
of the mantle. The plates are in constant motion. Where they
interact, along their margins, important geological processes take
place, such as the formation of mountain belts, earthquakes, and
volcanoes.
• How are earthquakes connected with plate tectonics?

• Major tectonic events associated with these plate boundaries
are ruptures and faults along the constructive plate
boundaries, faulting and folding along the destructive plate
boundaries and transform faults along the conservative plate
boundaries. All sorts of disequilibrium are caused due to
different types of plate motions and consequently
earthquakes of varying magnitudes are caused.

• Plate tectonics confirms that there are four types of seismic zones.
• Shallow and moderate earthquakes (25-35km , few 60km) are
caused along constructive plate boundaries because the faults and
rate of rupture of the crust and consequent movement of plates
away from the mid-oceanic ridges is rather slow and the rate of
upwelling of lavas due to fissure flow is also slow. Theses types
follow the line of mid-ocean ridges. Activity is low, and it occurs at
very shallow depths. The point is that the lithosphere is very thin
and weak at these boundaries, so the strain cannot build up
enough to cause large earthquakes. Associated with this type of
seismicity is the volcanic activity along the axis of the ridges (for
example, Iceland, Azores, Tristan da Cunha).

• The second type of earthquake associated with plate
tectonics is the shallow-focus event unaccompanied by
volcanic activity. The San Andreas fault is a good example of
this, so is the Anatolian fault in Northern Turkey. In these
faults, two mature plates are scraping by one another. The
friction between the plates can be so great that very large
strains can build up before they are periodically relieved by
large earthquakes.

• The third type of earthquake is related to the collision of oceanic
and continental plates. One plate is thrust or subducted under
the other plate so that a deep ocean trench is produced. In the
Philippines, ocean trenches are associated with curved volcanic
island arcs on the landward plate, for example the Java trench.
This type of earthquake can be shallow, intermediate, or deep,
according to its location on the downgoing lithospheric slab.
Such inclined planes of earthquakes are know as Benioff zones.

• Studies of fault motions from seismic waves generated in this
zone indicate that the type of faulting varies with depth. Near
the walls of the trench, normal faulting is typical, resulting
from tensional stresses generated by the initial bending of the
plate. In the zone of the shallow earthquakes, thrust faulting
dominates as the descending lithosphere slides beneath the
upper plate. At intermediate depths, extension or
compression can occur, depending on the specific
characteristics of the subduction zone. Extension and normal
faulting result when a descending slab, which is denser than
the surrounding mantle, sinks under its own weight.
Compression results if the mantle resists the downward
motion of the descending slab of lithosphere, indicating that
the mantle material at the depth resists the movement of the
descending plate.

0–100
100–200
200–300
300–400
400–500
>600
Focal depthNormal
faulting from
tension and
bending
of plate
Thrust
faulting
from plates
sliding past
each other
Extension
or
compression
faulting from
sinking plate

• The fourth type of seismic zone occurs along the boundaries of
continental plates. Typical of this is the broad swath of seismicity
from Burma to the Mediterranean, crossing the Himalayas, Iran,
Turkey, to Gibraltar. Within this zone, shallow earthquakes are
associated with high mountain ranges where intense
compression is taking place. Intermediate- and deep-focus
earthquakes also occur and are known in the Himalayas and in
the Caucasus.

• In summary, plate tectonics is a blunt, but,
nevertheless, strong tool in earthquake
prediction. It tells us where 90 percent of the
Earth's major earthquakes are likely to occur.
It cannot tell us much about exactly when
they will occur. For that, we must study in
detail the plate boundaries themselves.
Perhaps the most important role of plate
tectonics is that it is a guide to the use of finer
techniques for earthquake prediction.

• The generation of magma at Divergent plate margins
• Why and how is magma generated along spreading centers
rather than in some other places? The answer lies in the special
characteristics of temperature and pressure in the
asthenosphere and in their relationship to the melting of
minerals in the mantle. The generation of magma along
divergent plate margins is not the result of high temperature
alone; it is also related to the effects of pressure and
temperature at which melting occurs.
• The balance of temperature, pressure and composition in
asthenosphere (between 100 and 200 km below the surface)
allows some melting to occur. In the overlying lithosphere, the
temperature is too low for melting to occur. In the underlying
mantle, the confining pressure is so great that the rocks are kept
well under their melting point. The balance that permits some
minerals to melt is reached in the asthenosphere.

• Each mineral in a rock has its own melting point.
• One factor particularly enhances the generation of magma
along spreading centers. As the magma moves upward, the
pressure is reduced and the decrease in pressure lowers the
temperature at which melting occurs. Magma is thus
generated along spreading centers, in contrast to other
zones, largely because of pressure.

• The generation of magma at Convergent plate margins

• The first is a Convergent boundary, where one plate is literally
going over another plate. Through a subduction zone (where one
plate goes under another) this causes movement and earthquakes.
Thrust faults are then created because the upper plate is being
pushed into a fold or broken by thrust faults. Rock layers also
scrape off the ocean floor and are then stacked into piles against
the upper plate creating island and mountain chains.

• Another boundary is the Divergent boundary: this is where the
plates separate and move apart. They often form a rift zone.
Most are located on the oceanic floor where new seafloor is
created at the separating edges. One example is the Mid-
Atlantic Ridge.

• Another plate boundary is called the Strike-Slip or Transform
Fault where the plates are sliding against one another without
spreading apart or going over or under each other. They are
also found on the ocean floor. The San Andreas fault in
California .

Earthquake plate tectonic- Geomorphology Chapter

Earthquake plate tectonic- Geomorphology Chapter

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