Module 1 of earthquake engineering - AN INTRODUCTION TO EARTHQUAKE AND PLATE TECTONICS

1. By
Arshaghosh Ajayan

2.  The study of earthquakes dates back many centuries. Written
records of earthquakes in China dates back as far as 3000 years.
 Japanese records from the eastern Mediterranean region go back
nearly 1600 years.
 In the United States the historical record of earthquakes is much
shorter, about 350 years.
 On the seismically active west coast of the United States,
earthquake records go back only about 200 years.
 Compared with the millions of years over which earthquakes
have been occurring, humankind’s experience with earthquakes
is very brief.

3.  Earthquake is a sudden tremor or movement
of the earth’s crust, which originates naturally
at or below the surface
A list of natural and man made earthquake causes
Natural source Man made source
Tectonic earthquakes Controlled sources(explosives)
Volcanic earthquake Reservoir induces earthquakes
Rock falls/ collapse of cavity Mining induces earthquakes
microseism Cultural noise(industry, traffic etc.)

4. 4
 The most common cause of earthquakes.
 The earthquake occurs when the pressure that has built up in
tectonic plates causes the rock to break suddenly.
 This usually occurs at the boundaries of tectonic plates and
along existing faults.

5. 
The inner core is believed to be a solid
metal body. Heavier and denser
materials

The outer core is believed to be a liquid
metal layer.
 The mantle materials are in a viscous,
semi molten state

The crust is the outer layer of hard rock
At the Core, the temperature is estimated to be 2500°C,
the pressure 4 million atmospheres and density ~13.5
gm/cc; this is in contrast to ~25°C, 1 atmosphere and
1.5 gm/cc on the surface of the Earth.

6. Continental Crust Oceanic Crust
Thick thin
less dense than
oceanic crust
dense - sinks
under continental
crust
mostly old young

7. The convective flows of Mantle material cause the Crust and some portion of
the Mantle, to slide on the hot molten outer core. This sliding of Earth’s mass
takes place in pieces called Tectonic Plates

8.  The theory of plate tectonics tells us that the Earth's rigid outer shell (lithosphere)is
broken into a mosaic of oceanic and continental plates that can slide over the
plastic aesthenosphere (uppermost layer of the mantle).
 The plates are in constant motion.
 There are seven major crustal plates, subdivided into a number of smaller plates
 They are about 80 kilometers thick, moving relative to one another at rates varying
from 1 to 13 centimeters per year.
 Their pattern is neither symmetrical nor simple. Several different landform features
found on our planet - mountains, rift valleys, volcanoes, earthquakes, faulting - are
the result of geologic processes that occur where plates interact.

9.  The breaking up and formation of supercontinents appears to
have happened several times over Earth’s history
 Pangea was the supercontinent that existed 250 million years
ago during the Paleozoic and Mesozoic eras.
 During the ensuing millennia, plate tectonics slowly moved
each continent to its current position on the planet.
Each continent is still slowly moving across the face of our
world.

10.  In the early 1900's Alfred Wegener proposed
the idea of Continental Drift.
 His ideas centered around continents moving
across the face of the earth.
 The idea was not quite correct - compared to
the plate tectonics theory of today - but his
thinking was on the proper track.

17.  The earth’s crust is broken into about seven major plates, each of
which behaves for the most part as a rigid body that slides over the
partially molten mantle, in which deformation occurs plastically.
Tectonic earthquakes result from motion
between these plates. The plates are
driven by the convective motion of the
material in the Earth’s mantel, which in
turn is driven by heat generated at the
Earth’s core

18.  The surface of the earth is made up of 21
tectonic plates,
 some large and some small, that are
constantly moving.
 As the plates are forced against each other,
they deform, and eventually they crush and
fracture.
 The sudden fracture of the rock sends out a
shock wave that causes the earth's surface
to shake. This is one way earthquakes can
happen.

20.  African Plate
 Antarctic Plate
 Eurasian Plate
 Indo-Australian Plate
 North American Plate
 Pacific Plate
 South American Plate
Minor Plates These smaller plates are generally shown on major plate maps, but with
the exception of the Arabian plate which do not comprise significant land area.
 Arabian Plate
 Caribbean Plate
 Cocos Plate
 Juan de Fuca Plate
 Nazca Plate
 Philippine Sea Plate
 Scotia Plate

21.  Divergent boundaries or constructive plate margin-- where new crust is
generated as the plates pull away from each other (green lines in Figure 1).
 Convergent boundaries or destructive plate margin-- where crust is destroyed
as one plate dives under another (red lines in Figure 1).
 Transform boundaries or conservative plate margin -- where crust is neither
produced nor destroyed as the plates slide horizontally past each other (grey
continuous lines in Figure 1).

24.  zones of spreading .
 In certain areas the plates move apart from each other at boundaries known as
spreading ridges or spreading rifts. Molten rock from the underlying mantle rises to
the surface where it cools and becomes part of the spreading plates.
 Ex. Mid oceanic ridges
 Seafloor spreading over the past 100 to 200 million years has caused the Atlantic
Ocean to grow from a tiny inlet of water between the continents of Europe, Africa,
and the Americas into the vast ocean that exists today.
Bridge across the Álfagjá rift valley in
southwest Iceland, that is part of the
boundary between the Eurasian and North
American continental tectonic plates.

26.  The Mid-Atlantic Ridge (MAR) is a mid-ocean ridge, a divergent tectonic plate or
constructive plate boundary located along the floor of the Atlantic Ocean, and part of
the longest mountain range in the world.
 In the North Atlantic, it separates the Eurasian and North American Plates
 The section of the ridge that includes the island of Iceland is also known as the Reykjanes
Ridge. The average spreading rate for the ridge is about 2.5 cm per year

27. Map showing the Mid-Atlantic Ridge splittingIcelandand separatingthe
North American and EurasianPlates. The map also shows Reykjavik, the
capitalof Iceland, the Thingvellir area, and the locationsof some of
Iceland's active volcanoes(red triangles),includingKrafla

29.  destructive plate boundary is an actively deforming region where
two or more tectonic plates or fragments of the lithosphere move
toward one another and collide.
 As a result of pressure, friction, and plate material melting in
the mantle, earthquakes and volcanoes are common near convergent
boundaries.
 When two plates move towards one another, they form either a
subduction zone or a continental collision.
 Subduction occurs when the relative movement of two plates is
toward each other.
 When the rate of plate convergence is high, a trench is formed at the
boundary between plates. Earthquakes are generated at the interface
between the subducting and overriding plates.

30.  There are three types of convergent boundaries:
 1. Oceanic-Continental
Oceanic plate sub ducts due higher density
2. Oceanic-Oceanic
they typically create an island arc, deep oceanic trench
 3. Continental-Continental
Himalayas

32.  The Mariana Trench or Marianas Trench is the deepest part of
the world's oceans.
 It is located in the western Pacific Ocean, to the east of the Mariana
Islands.
 The trench is about 2,550 km long but has an average width of only 69
kilometers.
 It reaches a maximum-known depth of 10.911 km at the Challenger
Deep, a small slot-shaped valley in its floor, at its southern end
 some unrepeated measurements place the deepest portion at 11.03 km.

33. The Pacific plate is subducted beneath the
Mariana Plate, creating the Mariana trench,
and (further on) the arc of the Mariana
islands, as water trapped in the plate is
released and explodes upward to form
island volcanoes.

34.  these faults neither create nor
destroy lithosphere
 most transform faults are hidden in the deep
oceans where they form a series of short
zigzags accommodating seafloor spreading
 Few occurs in land Ex. San Andreas Fault
zone California,
 here the earthquake occurs at shallow depth,
unaccompanied by volcanic activity
 Periodically large earthquakes happens

36.  When plates move towards each other
continental collisions can lead to the
formation of mountain ranges. Example:
Himalayas formed due to collision of
Australia-Indian plate with Eurasian plate.

37.  Among the most dramatic and visible creations of plate-tectonic forces are
the lofty Himalayas, which stretch 2,900 km along the border between
India and Tibet.
 This immense mountain range began to form between 40 and 50 million
years ago.
 when two large landmasses, India and Eurasia, driven by plate movement,
collided.
 Because both these continental landmasses have about the same rock
density, one plate could not be subducted under the other.
 The pressure of the impinging plates could only be relieved by thrusting
skyward, contorting the collision zone, and forming the jagged Himalayan
peaks.

38. •About 225 million years ago, India was a large
island still situated off the Australian coast, and
a vast ocean (called Tethys Sea) separated India
from the Asian continent.
• When Pangaea broke apart about 200 million
years ago, India began to forge northward.
•About 80 million years ago, India was located
roughly 6,400 km south of the Asian continent,
moving northward at a rate of about 9 m a
century.
• When India rammed into Asia about 40 to 50
million years ago, its northward advance
slowed by about half.
•The collision and associated decrease in the
rate of plate movement are interpreted to mark
the beginning of the rapid uplift of the
Himalayas.

39.  Elastic Rebound
 Sources of stress
◦ Plate Tectonics
◦ Other stresses

40.  Tectonic plates move past each other causing
stress. Stress causes the rock to deform
◦ Plastic deformation – does not cause earthquakes
◦ Elastic deformation – rock stretches then reaches a
breaking point, releasing energy.

41.  Following the great 1906 San Francisco earthquake, Henry Feilding Reid examined
the displacement of the ground surface around the San Andreas Fault.
 From his observations he concluded that the earthquake must have been the
result of the elastic rebound of previously stored elastic strain energy in the rocks
on either side of the fault.
 The elastic rebound theory states that an earthquake is the sudden reaction of the
earth’s overly strained crust “snapping back” along the fault (Figure 2.4).
 Elastic rebound theory states that as tectonic plates move relative to each other,
elastic strain energy builds up along their edges in the rocks along fault planes.
Since fault planes are not usually very smooth, great amounts of energy can be
stored (if the rock is strong enough) as movement is restricted due to interlock
along the fault. When the shearing stresses induced in the rocks on the fault
planes exceed the shear strength of the rock, rupture occurs.
 The elastic rebound theory is an explanation for how energy is spread during
earthquakes. As plates on opposite sides of a fault are subjected to force and
shift, they accumulate energy and slowly deform until their internal strength is
exceeded. At that time, a sudden movement occurs along the fault, releasing the
accumulated energy, and the rocks snap back to their original undeformed shape.

42. Elastic Rebound
(a) Initial Time- The fence is built straight
across the fault trace.
(b) After Several Years- Tectonic movement
occurs, but the edges of the crustal blocks are
restrained by friction along the fault, and the
ground and fence bend. Elastic strain energy
builds up.
(c) Following Rupture- The edges of the blocks
along the fault try to "catch up" with the middle
as they release their strain energy during the
rupture, but don't quite make it due to fault drag.
Now the fence is offset and slightly curved

43. http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

44. 44
 Over millions of years the earth's tectonic plates have been moving continuously and
pushing against each other. These movements have forced them to deform producing
mountains and valleys in the earth's surface.
 Sometimes the rocky surface of the earth has just been bent and folded (Fig.1). Sometimes
the movements have caused the rock to deform so much that they fracture. These fractures
are called faults (Fig. 2). When the rock fractures, its sudden movement causes an
earthquake as shock waves spread away from it.
Fig. 2 How Rocks FractureFig. 1 How Rocks are bent and
folded

45. Strike-Slip
Thrust
Normal

46. Thank you