Download Lecture 10

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Lecture#10
CE-312
Engineering Geology and Seismology
Instructor:
Dr Amjad Naseer
Department of Civil Engineering
N-W.F.P University of Engineering and Technology, Peshawar
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Outlines of the Presentation
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Continental Drift Theory
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Plate Tectonic
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Elastic Rebound Theory
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Continental Drift Theory
In 1915, the observed geometric correlation and mineral similarities
between Africa and South America inspired Alfred Wegener to
hypothesize the possibility of continental drift.
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Continental Drift Theory
In essence, he proposed that the continents had once been huddled
together and then had drifted apart
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Continental Drift Theory
Evidence for continental drift
 Matching coastlines
 Matching mountains
 Matching rock types and rock ages
 Matching glacier deposits
 Matching fossils
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Evidence for continental drift
Matching
coastlines
Evidence for continental drift
Matching mountain
ranges
Evidence for continental drift
Matching rock types and ages of rocks
Evidence for continental drift
Matching glacier deposits
300 million years ago
Evidence for continental drift
Fossils of Mesosaurus (aquatic reptile) found on both sides of
Atlantic`
Continental Drift
Until 1960, the idea of continental drift was not considered to be
credible.
The main objection to Wegener's insight was the difficulty in
accepting the movement of the crust through the mantle without
either the crust or the mantle breaking apart.
This objection became a stumbling (meaning: Hesitation) block
for the continental-drift theory until the weight of the seismic
observations (concentration of earthquakes along certain narrow
zones) and the data from exploration of the sea floor (discovery of
categorical evidence about spreading of the sea floor from a central
ridge) convinced most of the professional community the continental
drift was possible.
Concentration of Earthquakes zones
sea-floor spreading
Sea-floor spreading occurs where oceanic plates are diverging
from one another. Magma rises along a rift zone and spreads out
at the surface building new sea floor.
Sea-floor spreading
The midoceanic ridge is the primary site for sea-floor spreading.
Earthquakes and volcanoes are where sea floor spreading is
occurring.
Consequences
Sea-floor spreading leads to CONTINENTAL DRIFT
Sea-floor spreading
Two Geographic Examples :
Mid Atlantic Ridge & East African Rift
Continental Drift in the making:
Incipient Sea-floor spreading
Eastern Africa and the Red
Sea
Theory of Plate tectonics
 The theory of Plate tectonics was proposed in 1960s based on the
theory of continental drift.
 This is the Unifying theory that explains the formation and
deformation of the Earth’s surface.
 According to this theory, continents are carried along on huge
slabs (plates) on the Earth’s outermost layer (Lithosphere).
 Earth’s outermost layer is divided into 12 major Tectonic Plates
(~80 km deep). These plates move relative to each other a few
centimeters per year.
Tectonic plates of Earth
Types of plate boundaries
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Divergent plate boundaries: where plates move apart
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Convergent Plate boundaries: where plates come together
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Transform plate boundaries: where plates slide past each other
Types of plate boundaries
Divergent (Tension)
(Spreading Zones)
Convergent (Compression)
(Subduction Zones)
Transform (shearing)
(Transform Plates)
Spreading zones (Divergent plate boundaries)
At spreading zones (or ridges), molten rock rises, pushing
two plates apart and adding new material at their edges
The elevated region where new material is coming forth is
called a "spreading ridge".
Most of the spreading ridges of today are to be found in the
central portion of the world's oceans.
Spreading zones usually have earthquakes at shallow depths
(within 30 kilometers of the surface).
Spreading zones (Divergent plate boundaries)
Associated with this type of seismicity is the volcanic activity
along the axis of the ridges (for example, Iceland, Azores,
Tristan da Cunha).
Midoceanic Ridge
The Midoceanic Ridge is the longest continuous mountain
system on earth and found on the ocean floor.
The Midoceanic Ridge is a site of plate divergence where
volcanic and earthquake activity occur.
The Midoceanic Ridge (light green curving feature)
Midoceanic Ridge
The East Pacific Rise at
9°N
Subduction zones (Convergent Plates)
Subduction zones are characterized by deep-ocean trenches,
shallow to deep earthquakes, and mountain ranges containing
active volcanoes.
Subduction
When two sections of the Earth's crust collide, one slab of crust
can be forced back down into the deeper regions of the Earth, as
shown in the diagram. This process is called subduction.
Subduction
The slab that is forced back into the Earth usually undergoes
melting when the edges get to a depth which is hot enough.
(A temperature hot enough to melt lithosphere is about a
thousand degrees!).
Subduction zones (Convergent Plates)
In the Philippines, ocean trenches are associated with
curved volcanic island arcs on the landward plate, for
example the Java trench
 Along the Peru - Chile trench, the Pacific plate is being
subducted under the South American plate which responds
by crumpling to form the Andes.
Japan and Philippine trenches
The deepest one (~12 km) is the Marianas trench
Tectonic Activity
Subduction, Faulting & Earthquakes
Reverse Faulting at Subduction Zones
Subduction Zones: Classification
Three types of subduction
Ocean-Continent convergence (Andes, Cascades)
Ocean-Ocean convergence (Marianas, Japan)
Continent-Continent convergence (Himalayas)
Ocean- Continent convergent convergence
 Ocean-continent plates collide
 Ocean plate subducts below continent
 Forms a subduction zone
 Earthquakes and volcanoes
Ocean-ocean convergent margin
 2 oceanic plates collide
 One plate dives (subducts) beneath other
 Forms subduction zone
 Earthquakes and volcanoes
Continent-continent convergent margin
 2 continental plates collide
 Neither plate wants to subduct
 Collision zone forms high
mountains
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Earthquakes, no volcanoes
example: Himalayas
Consequences of Continent-continent collision
Subduction leads to CONTINENTAL COLLISION &
MOUNTAIN BUILDING
Another view of a continent-continent collision
Transform faults
Transform faults are found where plates slide past one
another. 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
 Earthquakes at transform faults tend to occur at shallow depths
and form fairly straight linear patterns
Transform faults
San Andreas fault
San Andreas fault photographed from the air. A linear
valley has been eroded along the main trace of the fault.
The black line at right is a fence line.
Transform faults
Nevertheless, activity does not always occur along the entire
length of the fault during any one earthquake. For instance, the
1906 San Francisco event was caused by breakage only along
the northern end of the San Andreas fault.
 Strong earthquakes can occur at transforms with focal
depths of approximately 20 km. Largest earthquakes occur
at subduction zones.
Divergent ( creative)
Ridges
Convergent (destructive)
Subduction Zones
Transform (conservative)
Transform Faults
Plates move away from
each other
Plates move towards each other
Plates slide past each
other
Morphological Expression:
Trenches
Morphological
Expression: Faults
Tectonic Activity:
Volcanism (andesitic); Arc of
volcanoes
Shallow to Deep Earthquakes
(reverse)
Tectonic Activity:
No Volcanism
Shallow Earthquakes
(strike slip)
Morphological
expression:
Mid Ocean Ridges
Tectonic Activity:
Volcanism (basaltic)
Shallow Earthquakes
(normal)
Leads to Continental Drift Leads to Collision
Leads to Mountain Building
Type of Stress:
Tensional
Types of Stress:
Compressional
Type of Stress:
Shear
What drives plate movement?
 Ultimately: heat transported from core and mantle to
surface
 Heat transported by convection
 Core is ~5,000°C and surface is ~0°C
 Where mantle rises: rifting
 Where mantle dives: subduction zones
Dynamics of the Earth's Interior
Earthquakes and Plate Boundaries
How are earthquakes connected with plate tectonics? In 1969,
Muawia Barazangi and James Dorman published the locations of all
earthquakes which occurred from 1961 to 1967. Most of the
earthquakes are confined to narrow belts and these belts define the
boundaries of the plates.
The interiors of the plates themselves are largely free of large
earthquakes, that is, they are aseismic.
Earthquakes and Plate Boundaries
There are notable exceptions to this.
An obvious one is the 1811-1812 earthquakes at New Madrid,
Missouri, and another is the 1886 earthquake at Charleston, South
Carolina.
 As yet there is no satisfactory plate tectonic explanation for these
isolated events--which are sometimes referred to as intraplate
earthquakes. Consequently, we will have to find alternative
mechanisms to explain intraplate earthquakes.
Plate Tectonics and Earthquake Prediction
How can plate tectonics help in earthquake prediction? We
have seen that earthquakes occur at the following three kinds of
plate boundary: ocean ridges where the plates are pulled apart,
margins where the plates scrape past one another, and margins
where one plate is thrust under the other.
Thus, we can predict the general regions on the Earth's
surface where we can expect large earthquakes in the future.
We know that each year about 140 earthquakes of magnitude
6 or greater will occur within this area which is 10 percent of the
Earth's surface. But on a worldwide basis we cannot say with
much accuracy when these events will occur.
Plate Tectonics and Earthquake Prediction
 The reason is that the processes in plate tectonics have
been going on for millions of years. Averaged over this
interval, plate motions amount to several millimeters per year.
But at any instant in geologic time, for example, the year
1977, we do not know exactly where we are in the worldwide
cycle of strain buildup and strain release.
Only by monitoring the stress and strain in small areas, for
instance, the San Andreas fault, in great detail can we hope
to predict when renewed activity in that part of the plate
tectonics arena is likely to take place.
Plate Tectonics and Earthquake Prediction
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