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Geotechnical Earthquake
Engineering
by
Dr. Deepankar Choudhury
Professor
Department of Civil Engineering
IIT Bombay, Powai, Mumbai 400 076, India.
Email: [email protected]
URL: http://www.civil.iitb.ac.in/~dc/
Lecture - 7
Module – 3
Engineering Seismology
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Plate Tectonics
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Layers of the Earth




Crust:
 Continental crust (25-40 km)
 Oceanic crust (~6 km)
Mantle
 Upper mantle (650 km)
 Lower mantle (2235 km)
Core
 Outer core: liquid (2270 km)
 Inner core: solid (1216 km)
Values in brackets represent the approximate thickness of each layer
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Layers of the Earth
The earth is divided into four main layers: Inner core, outer core,
mantle and crust.
The core is composed mostly of iron (Fe) and is so hot that the outer
core is molten, with about 10% sulphur (S). The inner core is under
such extreme pressure that it remains solid.
Most of the Earth's mass is in the mantle, which is composed of iron
(Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O)
silicate compounds. At over 1000 degrees C, the mantle is solid but
can deform slowly in a plastic manner.
The crust is much thinner than any of the other layers, and is
composed of the least dense calcium (Ca) and sodium (Na)
aluminum-silicate minerals. Being relatively cold, the crust is rocky
and brittle, so it can fracture in earthquakes.
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Continental drift
Theory
that continents and plates move on the
surface of the Earth proposed by Alfred Wegener
in 1915.
Alfred Wegener
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Maps by Wegener (1915),
showing continental drift
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Theory of continental drift
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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
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Evidence for continental drift
Matching
mountain
ranges
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Evidence for continental drift
Matching
rock types
and ages
of rocks
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Evidence for continental drift
Matching glacier
deposits 300
million years
ago
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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.
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Earth’s magnetic field
Basic Data used in
formulating plate tectonics:
Magnetic stripes on the seafloor. Magnetic field of
Earth reverses on semiregular basis. Minerals act
like compass needles and
point towards magnetic
north. “Hot” rocks record the
direction of the magnetic
field as they cool.
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Tectonic plates of Earth
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Tectonic plates of Earth
Subduction zone
Strike-slip (transform)
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Uncertain plate boundary
Ridge axis faults
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Major Earthquakes
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Types of plate boundaries
Divergent plate boundaries: where plates
move apart
 Convergent Plate boundaries: where plates
come together
 Transform plate boundaries: where plates
slide past each other

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Types of plate boundaries
Divergent (Tension)
Convergent (Compression)
Transform (shearing)
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Types of plate boundaries
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Divergent Plate Boundaries
• Plates move away from
each other (tension)
• New lithosphere is
formed
• normal faults
• Causes volcanism
• not very explosive
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Divergent Plate Boundaries
Upwarping
Continental Crust
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Divergent Plate Boundaries
Rift Valley
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Divergent Plate Boundaries
Linear Sea
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Convergent Plate Boundary
•
•
•
•
•
Plates move toward
each other
(compression)
lithosphere is
consumed
reverse/thrust faults
and folds
Mountain building
explosive volcanism
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Ocean- Continent convergent margin
• Ocean-continent
plates collide
• Ocean plate subducts
below continent
• Forms a subduction
zone
• Earthquakes and
volcanoes
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Ocean-ocean convergent margin
• 2 oceanic plates collide
• One plate dives (subducts)
beneath other
• Forms subduction zone
• Earthquakes and volcanoes
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Continent-continent convergent margin
• 2 continental plates
collide
• Neither plate wants
to subduct
• Collision zone
forms high
mountains
• Earthquakes, no
volcanoes
example: Himalayas
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Transform plate margin
• Two plates slide past
each other
• strike slip faults.
• Lithosphere is neither
consumed nor
destroyed.
• Earthquakes, no
volcanoes
• Responsible for most of
the earthquakes
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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
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Types of Faults
and
Seismic Waves
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Elastic Rebound Theory
• This theory was discovered by making
measurements at a number of points across a fault.
• Prior to an earthquake it was noted that the rocks
adjacent to the fault were bending. These bends
disappeared after an earthquake suggesting that
the energy stored in bending the rocks was
suddenly released during the earthquake.
Elastic Rebound Theory
Sequence of elastic rebound: Stresses
Sequence of elastic rebound: Bending
Sequence of elastic rebound: Rupture
Sequence of elastic rebound: Rebound
Sudden Slip by Elastic Rebound
• Stresses (force/area) are applied to a fault.
• Strain (deformation) accumulates in the
vicinity of friction-locked faults.
• Strain accumulation reaches a threshold and
fault slips suddenly
• Rupture (slip) continues over some portion
of the fault. Slip is the distance of
displacement along a fault.
Fault
A fracture (crack) in the earth, where the two sides move past each
other and the relative motion is parallel to the fracture.
90˚ dip = vertical fault plane, 0˚ strike = north parallel fault plane