Download SGES 1302 Lecture18

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
Lecture 18: Earthquakes 1
Earthquakes
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Earthquakes are natural vibrations of the ground, some of which are
caused by movement along faults in Earth’s crust.
Most earthquakes are the result of movement of Earth’s crust
produced by plate tectonics.
As a whole, tectonic plates tend to move gradually.
Along the boundaries between two plates, rocks in the crust often
resist movement.
Over time, stress builds up. Stress is the total force acting on crustal
rocks per unit of area.
When stress overcomes the strength of the rocks involved,
movement occurs along fractures in the rocks, releasing the energy
built up as a result of stress.
The vibrations caused by this sudden movement are felt as an
earthquake.
The characteristics of earthquakes are determined by the orientation
and magnitude of stress applied to rocks, and by the strength of the
rocks involved.
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Distribution of Earthquakes
Frequency of
Occurrence of
Earthquakes
http://earthquake.usgs.gov/earthqua
kes/eqarchives/year/eqstats.php
Magnitude
Annual Ave
>8
1
7 - 7.9
15
6 - 6.9
134
5 - 5.9
1319
4 - 4.9
13,000
3 - 3.9
130,000
2 - 2.9
1,300,000
Concentration of earthquakes worldwide near plate boundaries, with a small but significant
number of intra-plate quakes.
Almost 80 percent of all earthquakes occur on the Circum-Pacific Belt and about 15
percent on the Mediterranean-Asian Belt across southern Europe and Asia.
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Deadliest Eartquakes
(wikipedia)
No
Name
Date
Location
Fatalities
Magnitude
1
Shaanxi
Jan 23, 1556
Shaanxi, China
820,000–830,000
8.0 (est.)
2
Tangshan
Jul 28, 1976
Tangshan, China
242,419–779,000
7.5–7.8
3
Antioch
May 21, 525
Antioch, Turkey
250,000
8.0 (est.)
4
Gansu
Dec 16, 1920
Ningxia–Gansu, China
235,500
7.8
5
Indian Ocean
Dec 26, 2004
Indian Ocean, Sumatra
230,210+
9.1–9.3
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Aleppo
Oct 11, 1138
Aleppo, Syria
230,000
Unknown
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Haiti
Jan 12, 2010
Haiti
222,570
7.0
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Damghan
Dec 22, 856
Damghan, Iran
200,000
7.9 (est.)
9
Ardabil
Mar 22, 893
Ardabil, Iran
150,000
Unknown
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Great Kantō
Sep 1, 1923
Kantō region, Japan
142,000
7.9
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Messina
Dec 28, 1908
Messina, Italy
123,000
7.1
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Ashgabat
Oct 6, 1948
Ashgabat, Turkmenistan
110,000
7.3
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Genroku
Dec 31, 1703
Edo, Japan
108,800
Unknown
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Lisbon
Nov 1, 1755
Lisbon, Portugal
10,000–100,000
8.5–9.0 (est.)
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2011 Tōhoku earthquake and tsunami
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Also known as the Great East Japan Earthquake, it was a magnitude 9.0 undersea
megathrust earthquake off the coast of Japan that occurred on Friday, 11 March 2011
The epicenter approximately 70 km east of the Oshika Peninsula of Tōhoku at depth
of ~32 km
It was the most powerful known earthquake to have hit Japan, and one of the five
most powerful earthquakes in the world overall since modern record-keeping began
in 1900.
The earthquake triggered extremely destructive tsunami waves of up to 40.5 m. In
some cases traveling up to 10 km inland.
In addition to loss of life and destruction of infrastructure, the tsunami caused a
number of nuclear accidents, primarily the level 7 meltdowns at three reactors in the
Fukushima I Nuclear Power Plant complex, and the associated evacuation zones
affecting hundreds of thousands of residents.
The overall cost could exceed US$300 billion, making it the most expensive natural
disaster on record.
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2004 Indian Ocean earthquake
and tsunami
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It was an undersea megathrust earthquake that
occurred on Sunday, December 26, 2004, with an
epicentre off the west coast of Sumatra
The earthquake was caused by subduction and triggered a series of
devastating tsunamis along the coasts of most landmasses
bordering the Indian Ocean
The tsunami killed over 230,000 people in fourteen countries, and
inundating coastal communities with waves up to 30 meters
It was one of the deadliest natural disasters in recorded history.
Indonesia was the hardest hit, followed by Sri Lanka, India, and
Thailand.
With a magnitude of between 9.1 and 9.3, it is the third largest
earthquake ever recorded on a seismograph.
This earthquake had the longest duration of faulting ever observed,
between 8.3 and 10 minutes.
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2008 Sichuan earthquake
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It was a deadly earthquake that measured at 8.0 occurred on
Monday, May 12, 2008 in Sichuan province of China, killing an
estimated 68,000 people.
It is also known as the Wenchuan earthquake, after the location of
the earthquake's epicenter, Wenchuan County in Sichuan province.
The epicenter was 80 kilometres (50 mi) west-northwest of
Chengdu, the capital of Sichuan, with a focal depth of 19 km.
The earthquake was also felt in nearby countries and as far away as
both Beijing and Shanghai;1,500 km and 1,700 km away – where
office buildings swayed with the tremor.
The earthquake left about 4.8 million people homeless, though the
number could be as high as 11 million.
Strong aftershocks, some exceeding magnitude 6, continued to hit
the area even months after the main quake, causing new casualties
and damage.
The central government announced that it will spend 1 trillion yuan
(about US$146.5 billion) to rebuild areas ravaged by the earthquake.
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Types of seismic waves
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The vibrations of the ground during an earthquake
are called seismic waves.
Every earthquake generates three types of seismic
waves:
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primary waves,
secondary waves, and
surface waves.
Primary waves, also referred to as P-waves,
squeeze and push rocks in the direction along
which the waves are traveling.
The compressional movement of P-waves is similar
to the movement along a loosely coiled wire.
Secondary waves, called S-waves, are named with
respect to their arrival times. They are slower than
P-waves, so they are the second set of waves to be
felt.
S-waves have a motion that causes rocks to move
at right angles in relation to the direction of the
waves.
The movement of S-waves is similar to the
movement of a jump rope that is jerked up and
down at one end.
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Types of seismic waves
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Surface waves are the slowest
type of waves, which travel only
along Earth’s surface.
Surface waves can cause the
ground to move sideways and up
and down like ocean waves
These waves usually cause the
most destruction because they
cause the most movement of the
ground, and take the longest
time to pass.
Seismograms provide a
record of the seismic waves
that pass a certain point.
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Generation of seismic waves
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The first body waves generated by an
earthquake spread out from the point of
failure of crustal rocks.
The point where the waves originate is the
focus of the earthquake.
The focus is usually several kilometers below
Earth’s surface.
The point on Earth’s surface directly above
the focus is the epicenter .
Surface waves originate from the epicenter
and spread out.
After a major earthquake, rocks around the
focus continue to shake as they readjust to
their new positions, producing numerous,
often smaller earthquakes known as
aftershocks.
The speed of the seismic waves and the
distance travelled are affected by the
properties of the Earth material.
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Measuring Earthquakes
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Magnitude and intensity of earthquakes can be determined by
various ways.
The Richter scale, devised by Charles Richter in 1935, is a
numerical rating system that measures the energy of the largest
seismic waves, called the magnitude, that are produced during an
earthquake.
The numbers in the Richter scale are determined by the height,
called the amplitude.
The amplitude is the largest seismic wave traced on a seismogram
of a Wood-Anderson seismograph placed 100 km away from the
epicenter.
Each successive number represents an increase in amplitude of a
factor of 10 (logarithmic scale).
A magnitude 1.0 earthquake would swing the arm of a WoodAnderson seismograph 1/1000 mm; magnitude 2.0 1/100 mm and
so on.
Each increase in magnitude corresponds to about a 33-fold increase
in seismic energy.
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Measuring Earthquakes
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Moment magnitude scale – most commonly used today.
The moment magnitude scale is a rating scale that measures the
energy released by an earthquake, taking into account the size of
the fault rupture,the amount of movement along the fault, and the
rocks’ stength.
It is more accurate than the Richter scale, especially at higher
magnitudes because it is calculated directly using information from
the source, while Richter scale is calculated from the amplitude
resulted from an earthquake.
Modified Mercalli scale describes the intensity earthquakes with
respect to the amount of damage they cause – descriptive scale
based on observation.
It rates the types of damage and other effects of an earthquake as
noted by observers during and after its occurrence.
It cannot be used to determine:
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The epicenter accurately,
The actual magnitude of the earthquake,
The intensity in places where no people live.
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Modified Mercalli Intensity scale
I. Instrumental
Generally not felt by people unless in favorable conditions.
II. Weak
Felt only by a few people at best, suspended objects may swing.
III. Slight
Felt quite noticeably by people indoors. Vibration similar to the passing of a truck.
IV. Moderate
Felt indoors by many people, outdoors by few people during the day. Dishes, windows, doors
disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing
cars rock noticeably.
V. Rather Strong Felt outside by most. Dishes and windows may break. Vibrations like large train passing
close to house.
VI. Strong
Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware
broken; books fall off shelve. Damage slight.
VII. Very Strong
Difficult to stand; furniture broken; damage slight to moderate in well-built ordinary structures;
considerable damage in poorly built structures; some chimneys broken. Noticed by people
driving motor cars.
VIII. Destructive
Considerable in ordinary substantial buildings with partial collapse. Damage great in poorly
built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture
moved.
IX. Violent
General panic; damage considerable in specially designed structures. Damage great in
substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Intense
Some well built wooden structures destroyed; most masonry and frame structures destroyed
with foundation. Rails bent.
XI. Extreme
Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Cataclysmic Total destruction – Everything is destroyed. Objects thrown into the air.
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Locating an Earthquake
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The epicenter’s location, as well as the time of
occurrence, can be determined using
seismograms and travel-time curves.
P-waves reach a seismograph station before the
S-waves and the gap in their arrival times will be
greater when the distance traveled is longer.
Seismologists determine the distance to an
earthquake’s epicenter by measuring the
separation on a seismogram and plot the
separation time on the travel-time graph.
A circle is plotted around the seismic station with
a radius equal to the distance to the epicenter.
When data from 3 or more seismic stations are
available, the intersection marks the epicenter.
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