Download UGRC 144_Session 3

UGRC 144
Science and Technology in Our
Lives/Geohazards
Session 3 – Understanding Earthquakes and
Earthquake Hazards
Lecturer: Dr. Patrick Asamoah Sakyi
Department of Earth Science, UG
Contact Information: [email protected]
College of Education
School of Continuing and Distance Education
2014/2015 – 2016/2017
Session Overview
• Earthquakes are natural ground motions caused as
the Earth releases energy.
• The science of earthquakes is seismology, "study of
shaking" in scientific Greek. Earthquake energy
comes from the stresses of plate tectonics.
• As plates move, the rocks on their edges deform and
take up strain until the weakest point, a fault,
ruptures and releases the strain.
• The occurrence of earthquake also results in various
hazards that may bring about disasters.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 2
Session Outline
The key topics to be covered in the session are as follows:
• Topic One – What is an earthquake
• Topic Two – Mechanism for earthquake
• Topic Three – Seismic Waves
• Topic Four - Earthquake instruments and seismic
records
• Topic Five – Earthquake Measurement & Size
• Topic Six - Earthquake Hazards
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 3
Reading List
• Chapter 4 of Environmental Geology 4th Edition – by
Carla W Montgomery (1995)
• Unit 1, Section 3 of UGRC 140 II Geohazards –
Institute of Continuing and Distance Education
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 4
Topic One
WHAT IS AN EARTHQUAKE
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 5
What is an Earthquake
• An earthquake is the vibration of Earth
produced by the rapid release of energy
• This energy radiates in all directions from its
Source, the FOCUS, in the form of SEISMIC
WAVES.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 6
What is an Earthquake
• Where do earthquakes occur?
• Globally, earthquakes occur in well-defined zones
marked by tectonic plate boundaries and faults .
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 7
What is an Earthquake
• Where do earthquakes occur?
– Earthquake epicenters are not randomly
distributed over the earth.
– Areas that have no history of earthquakes are
not likely to experience earthquakes in future.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 8
What is an Earthquake
• Focus and Epicenter
• Focus is the point within
the Earth where the
earthquake starts. It is
also called the
Hypocenter
• Epicenter is the location
on the surface directly
above the focus.
• Faults
• Faults are fractures in the
Earth where movement
has occurred.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 9
Topic Two
MECHANISM FOR EARTHQUAKE
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 10
Mechanism for Earthquake
• Elastic Rebound Hypothesis
– The mechanism for earthquake generation
is termed Elastic Rebound
– Most earthquakes are produced by the rapid
release of elastic energy stored in a rock
that has been subjected to great forces.
– When the strength of the rock is exceeded,
it suddenly breaks, causing vibrations of the
earth, leading to an earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 11
Mechanism for Earthquake
• Elastic Rebound
Hypothesis
– It has been noticed
that earthquakes occur
along large Faults and
plate boundaries.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 12
Mechanism for Earthquake
• Elastic Rebound
Hypothesis
– Tectonic Forces over tens
or hundreds of years
slowly deform the
crustal rocks on both
sides of the fault.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 13
Mechanism for Earthquake
• Elastic Rebound
Hypothesis
– Under these conditions,
the rocks bend and store
elastic energy much like
a wooden stick would if
bent.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 14
Mechanism for Earthquake
• Elastic Rebound
Hypothesis
– Eventually, the Frictional
resistance holding the
rocks together is
overcome.
– The vibrations we know
as an earthquake occur
as the rock elastically
snaps back to its original
shape.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 15
Mechanism for Earthquake
• Elastic Rebound
Hypothesis
– Summary of
causes and
mechanism for
earth quake
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 16
Mechanism for Earthquake
• Aftershocks and Foreshocks
– An aftershock is a small earthquake that follows
the main earthquake.
– Foreshock is a small earthquake that often
precedes a major earthquake.
• The study of seismic waves related to
earthquakes is called Seismology
• Scientists who study seismic waves produced
by earthquakes are called Seismologists
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 17
Topic Three
SEISMIC WAVES
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 18
What is Seismic Wave
• Seismic Wave is the energy released when a fault
ruptures, causing a vibration of earth in an
earthquake
• This energy radiates in all directions from its source
called the focus.
• Seismic waves spreads from the focus in three
different forms:
– Compression waves, exactly like sound waves (P waves)
– Shear waves, like waves in a shaken jump-rope (S waves)
– Surface waves resembling water waves.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 19
What is Seismic Wave
• Surface waves are seismic waves that travel
along Earth’s outer layer.
• Surface waves are slower still and cause the
majority of damage associated with
earthquake disasters.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 20
What is Seismic Wave
• Body Waves, identified as P and S waves are
seismic waves that travel in the interior of the
earth
• P waves
– Are push-pull waves that push (compress) and pull
(expand) in the direction that the waves travel
– Travel through solids, liquids, and gases
– Have the greatest velocity of all earthquake waves
– P waves always arrive first and do little or no
damage.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 21
What is Seismic Wave
• S waves
– They are seismic waves that travel along Earth’s outer
layer
– The shake particles at right angles to the direction
that they travel,
– Travel only through solids,
– Slower velocity than P waves
– They may cause damage
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 22
Topic Four
EARTHQUAKE INSTRUMENTS AND
SEISMIC RECORDS
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 23
Earthquake Instrumentation and
Seismic Records
• Seismographs are
instruments that
measure and record
seismic waves in the
earth during an
earthquake.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 24
Earthquake Instrumentation and
Seismic Records
• Seismograms are traces
of amplified,
electronically recorded
ground motion made by
seismographs
• A seismogram shows all three types of seismic waves—
surface waves, P waves, and S waves
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 25
Topic Five
EARTHQUAKE MEASUREMENT AND
SIZE
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 26
Earthquake Measurement and Size
• Historically, scientists have used two different types of
measurements to describe the size of an earthquake,
namely: intensity and magnitude.
• Intensity classifies the degree of shaking and magnitude
is a measure of the amount of energy released
• Intensity is gauged from inspection of the damage and
other effects of an earthquake. It is greatest close to the
epicentre, diminishing with distance Intensity
• The 12-point Mercalli Scale is used to measure intensity.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 27
Earthquake Measurement and Size
• Earthquake magnitude is determined from the amplitude of
body waves recorded by sensitive seismographs. Earthquake
magnitude measures how big an earthquake is, that is, how
much energy is released in seismic waves
• The Richter Scale is used to measure earthquake magnitude
• Based on the amplitude of the largest seismic
wave
• Each unit of Richter magnitude equates to
roughly a 32-fold energy increase
• Does not estimate adequately the size of very large
earthquakes
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 28
Earthquake Measurement and Size
• Momentum Magnitude
– Derived from the amount of displacement that
occurs along the fault zone
– Momentum magnitude is the most widely used
measurement for earthquakes because it is the
only magnitude scale that estimates the energy
released by earthquakes.
– Measures very large earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 29
Earthquake Measurement and Size
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 30
Topic Six
EARTHQUAKE HAZARDS
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 31
Earthquake Hazards
• Possible hazards from earthquakes can be
classified as follows:
 Ground Motion
 Fire
 Landslides
 Changes in Ground Level
 Tsunami
 Flooding
 Liquefaction
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 32
Earthquake Hazards
• Ground Motion - Shaking of the ground caused by the passage of seismic
waves, especially surface waves, near the epicenter of the earthquake
are responsible for the most damage during an earthquake and is thus a
primary effect of an earthquake.
• The intensity of ground shaking depends on
• Local geologic conditions in the area. In general, loose
unconsolidated sediment is subject to more intense shaking than
solid bedrock.
 Size of the Earthquake. In general, the larger the earthquake, the
more intense is the shaking and the duration of the shaking.
 Distance from the Epicenter. Shaking is most severe near the
epicenter and drops off away from the epicenter. The distance factor
depends on the type of material underlying the area.
 Damage to structures from shaking depends on the type of
construction.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 33
Earthquake Hazards
• The structural damage include
buildings and other structures
such as railways, roads, dams,
telephone and power
transmission poles and lines
• Factors that determine
structural damage;
 Intensity and duration of the
vibrations
 Nature of the material upon which
the structure is built
 The design of the structure
 Concrete and masonry structures
are brittle and thus more
susceptible to damage
 wood and steel structures are
more flexible and thus less
susceptible to damage.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 34
Earthquake Hazards
• Fire - Fire is a secondary
effect of earthquakes.
• Because power lines may be
knocked down and because
natural gas lines may rupture
due to an earthquake, fires are
often started closely following
an earthquake.
• The problem is compounded if
water lines are also broken
during the earthquake since
there will not be a supply of
water to extinguish the fires
once they have started.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 35
Earthquake Hazards
• Faulting and Ground Rupture - Ground
rupture generally occurs only along the fault
zone that moves during the earthquake, and
are thus a primary effect.
 Thus structures that are built across fault
zones may collapse
 whereas structures built adjacent to, but
not crossing the fault may survive
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 36
Earthquake Hazards
• Landslides - In mountainous regions subjected
to earthquakes ground shaking may trigger the
following, all of which are secondary effects;
 Landslide
 Rock and debris falls
 Rock and debris slides
 Slumps
 Debris avalanches
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 37
Earthquake Hazards
• Tsunami - Tsunami a
secondary effect that are giant
ocean waves that can rapidly
travel across oceans.
• Earthquakes that occur beneath
sea level and along coastal
areas can generate tsunami,
which can cause damage
thousands of kilometers away
on the other side of the ocean.
• A tsunami, triggered by an
earthquake occurs where a slab
of the ocean floor is displaced
vertically along a fault.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 38
Earthquake Hazards
• A tsunami also can occur when the vibration of
a quake sets an underwater landslide into
motion
• Tsunami is the Japanese word for “harbour
wave.”
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 39
Earthquake Hazards
• Changes in Ground Level - A secondary or tertiary
effect that is caused by faulting.
• Earthquakes may cause both uplift and subsidence
of the land surface.
• This causes changes in ground level
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 40
Earthquake Hazards
• Flooding - Flooding is a
secondary effect that may
occur due to rupture of
human made dams and
levees, due to tsunami,
and as a result of ground
subsidence after an
earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 41
Earthquake Hazards
• Liquefaction (soil) describes a phenomenon where
by a water-saturated or partially saturated soil or
unconsolidated sediment substantially loses
strength and stiffness in response to an applied
stress, usually earthquake shaking or other sudden
change in stress condition, causing it to behave like
a liquid.
• In areas underlain by such material, the ground
shaking causes the grains to lose grain to grain
contact, and thus the material tends to flow.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 42
Summary
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 43
END
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 44