Revelation of 5'12 Quake, Sichuan, China Part I Background of Earthquake

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Revelation of 5.12 Quake, Sichuan, ChinaPart I
Background of Earthquake

• Supercourse China
• http//www.SuperCourse.cn/
• 2008-6-6

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West Sichuan Earthquake, 12th May 2008

• Magnitude 7.9 Richter scale
• Local earthquake time 14.48 Beijing-time
• Location 30.986N, 103.364E
• Depth 19 km (11.8 miles)

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Outline

• 1.1 Basic knowledge about earthquake
• 1.2 Natural factors related to the frequency and
  magnitude of earthquake
• 1.3 Why 5.12 Quake happened?
• 1.4 Secondary disasters of earthquake

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1.1 Basic knowledge about earthquake
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What is the Earthquake?
The shaking of earth caused by waves moving on
and below the earth's surface and causing
surface faulting, tremors vibration,
liquefaction, landslides, aftershocks and/or
tsunamis.
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How Earthquake Happens?

• It caused by a sudden slip on a FAULT.
• Stresses in the earth's
• outer layer push sides of
• fault together.
• Stress builds up rocks
• slips suddenly, releasing
• energy in waves that travel
• through the earth's CRUST
• cause the shaking that we
• Feel during an earthquake.

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Earthquake Strength Measures I) Magnitude II)
Intensity

• I) Magnitude
• Definition A measure of actual physical energy
  release at its source as estimated from
  instrumental observations.
• Scale Richter Scale
• By Charles Richter, 1936
• Open-ended scale
• The oldest most widely used

Noji 1997
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Earthquake Strength Measures I) Magnitude II)
Intensity

• II) Intensity
• Definition a measure of the felt or perceived
  effects of an earthquake rather than the strength
  of the earthquake itself.
• Scale Modified Mercalli (MM) scale
• 12-point scale, ranges from barely perceptible
  earthquakes at MM I to near total destruction
  at MM XII

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Magnitude versus Intensity

• Magnitude refers to the force of the earthquake
  as
• a whole, while intensity refers to the
  effects of an
• earthquake at a particular site.
• An earthquake can have just one magnitude, while
• intensity is usually strongest close to the
  epicenter
• is weaker the farther a site is from the
  epicenter.
• The intensity of an earthquake is more germane
  to
• its public health consequences than its
  magnitude.

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1.2 Natural factors that influence earthquakes
frequency and magnitude
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Earthquake Strength
Magnitude and intensity are two measures of the
strength of an earthquake and are frequently
confused by laypeople (22). The magnitude of an
earthquake is a measure of actual physical energy
release at its source as estimated from
instrumental observations. A number of magnitude
scales are in use. The oldest and most widely
used is the Richter magnitude scale, developed by
Charles Richter in 1936. Although the scale is
open-ended, the strongest earthquake recorded to
date has been of Richter magnitude 8.9.
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Topographic Factors
Topographic factors substantially affect the
impact of earthquakes. Violent ground shaking
in areas constructed on alluvial soils or
landfill, both of which tend to liquify and
exacerbate seismic oscillations, can produce
significant damage and injuries at a given
location far from the actual earthquake epicenter
(23). Both the impact of the 1985 earthquake on
Mexico City, where an estimated 10,000 people
died, and that of the 1989 Loma Prieta earthquake
are good examples of how local soil conditions
can play important roles in producing building
damage of greater severity than what may occur in
areas closer to the earthquake's epicenter.
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Volcanic Activity
Earthquakes often occur in association with
active volcanoes, sometimes triggered by magmatic
flow and sometimes releasing pressure that allows
magmatic intrusion. The so-called harmonic
tremors associated with actual magmatic flow are
generally not damaging however, relatively
severe earthquakes can immediately precede or
accompany actual volcanic eruptions and can
contribute to devastating mudslides.
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1.3 Why did 5.12 Quake happen?
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By the end of the Paleozoic era, about 250
million years ago, most of the continents had
collided to form the super-continent Pangea.
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Pangea lasted about 50 million years, then it
began to break apart. India broke away from
Antarctica about 120 million years ago and
drifted northward across the old Tethys Ocean.
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India first encountered the southern edge of Asia
about 50 million years ago, initiating a
continental collision.
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The India-Asia continental collision has
continued ever since, with India ramming ever
more deeply into southeast Asia.
To view or download a computer movie showing the
breakup of Pangea, visit http//emvc.geol.ucsb.ed
u/downloads.php
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The lithosphere of India was old, cold and strong
while the lithosphere beneath the rim of Asia was
young, warm and weak.
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Thus, as it collided with Asia, India acted as a
rigid indenter. It crumpled and piled up the
weak Asian crust in front of it as it entered.
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As the Tibetan crust became thick and high, it
heated up and became unstable.
To view or download a computer movie showing the
India-Asia continental collision, visit
http//emvc.geol.ucsb.edu/downloads.php
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The unstable Tibetan crust began to flow
sideways, mostly toward the east, in a process
called extrusion tectonics or tectonic
escape.
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The Sichuan basin lies on top of the Sichuan
block, an old, rigid block that is embedded
within the Asian lithosphere.
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As the weak Tibetan crust flowed eastward, it
encountered this strong block. Some of the flow
was diverted southward, moving through a slot
between the Sichuan block and the indenting
Indian block.
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The Tibetan edge above the Sichuan basin is laced
with large, curved strike-slip faults that guide
the crustal flow around the corner. This flow
is actually accomplished in jerks when
earthquakes rupture these faults. Many of
Chinas largest, most destructive earthquakes
occur here.
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The flow is also pressing eastward against the
Sichuan block, forming a steep mountain front and
running over the block with folds and thrust
faults. During the May 12 earthquake, one of
these thrust faults ruptured and moved the
mountains as much as 8 meters up and over the
basin.
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Tectonics of Sichuan Earthquake

• Motion on a northeast striking reverse fault or
  thrust fault on the northwestern margin of the
  Sichuan Basin

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The continental collision continues today and
into the future, unabated. The obvious
conclusion is that large earthquakes in this
region are natural and inevitable, so that
continual earthquake preparedness is of the
utmost importance.
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1.4 Secondary disasters of earthquake
FACTORS INFLUENCING EARTHQUAKE MORBIDITY AND
MORTALITY

• Natural Factors
• Landslides

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Tsunamis ("Seismic Sea Waves")
Submarine earthquakes can generate damaging
tsunamis (also known as seismic sea waves), which
can travel thousands of miles undiminished before
bringing destruction to low-lying coastal areas
and around bays and harbors. A tsunami can be
created directly by underwater ground motion
during earthquakes or by landslides, including
underwater landslides. Tsunamis can travel
thousands of miles at 300-600 mph with very
little loss of energy.
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Aftershocks
Most earthquakes are followed by many
aftershocks, some of which may be as strong as
the main shock itself. Many fatalities and
serious injuries occurred from a strong
aftershock that followed 2 days after the
September 19, 1985, Mexico City earthquake that
killed an estimated 10,000 people (45). In some
cases landslides may be triggered by an
aftershock, after having been primed by the main
shock. Some major debris flows start slowly with
a minor trickle and then are triggered in waves.
In these cases there may be sufficient warning
that allows a community that is aware of this
hazard to evacuate in time.
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Time of Day
Time of day is an important determinant of a
population's risk for death or injury, primarily
because it affects people's likelihood of being
caught in a collapsing building. For example,
the 1988 Armenia earthquake occurred at 1141 AM,
and thus many people were trapped in schools,
office buildings, or factories. If the
earthquake had occurred at another time of day,
very different patterns of injury and places of
injury would have occurred.
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Human-Generated Factors
Fires and dam bursts following an earthquake are
examples of major human-caused complications that
aggravate the destructive effects of the
earthquake itself. In industrialized countries,
an earthquake may also be the cause of a major
technological disaster by damaging or destroying
nuclear power stations, research centers,
hydrocarbon storage areas, and complexes making
chemical and toxic products. In some cases, such
"follow-on" disasters can lead to many more
deaths than those caused directly by the
earthquake (60).
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Fire Risks
One of the most severe follow-on or secondary
disasters that can follow earthquakes is fire
(62). Severe shaking may cause overturning of
stoves, heating appliances, lights, and other
items that can ignite materials into flame.
Historically, earthquakes in Japan that trigger
urban fires cause 10 times as many deaths as
those that do not (62). The Tokyo earthquake of
1923, which killed more than 140,000 people, is a
classic example of the potential that fires have
to produce enormous numbers of casualties
following earthquakes.
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Dams
Dams may also fail, threatening communities
downstream. A standard procedure after any
sizeable earthquake should be an immediate damage
inspection of all dams in the vicinity and a
rapid reduction of water levels in reservoirs
behind any dam suspected of having incurred
structural damage.
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Structural Factors (cont.)
Glass (1976) was one of the first to apply
epidemiology to the study of building collapse
(67). He identified the type of housing
construction as a major risk factor for injuries.
Those living in the newer style adobe houses
were at highest risk for injury or death, while
those living in the traditional mud and stick
construction houses were at the least risk.
Figure 8-6 shows the breakdown of earthquake
fatalities by cause for each half of this
century. By far the greatest proportion of
victims have died in the collapse of unreinforced
masonry (URM) buildings (e.g., adobe, rubble
stone, or rammed earth) or unreinforced
fired-brick and concrete-block masonry buildings
that can collapse even at low intensities of
ground shaking and will collapse very rapidly at
high intensities.
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Structural Factors (cont.)
Time and again, wood-frame buildings such as
suburban houses in California have been
pronounced among the safest structures one could
be in during an earthquake. Indeed, these
buildings are constructed of light wood
elements--wood studs for walls, wood beams and
joists for floors, and wood beams and rafters for
roofs (75). Even if they did collapse, their
potential to cause injury is much less than that
of unresistant old stone buildings, like those
often used for businesses, offices, or schools.
The relative safety of wood-frame buildings was
shown quantitatively following the 1990
Philippine earthquake. People inside buildings
constructed of concrete or mixed materials were
three times more likely to sustain injuries (odds
ratio OR 3.4 95 confidence interval
CI,1.1-13.5)than were those inside wooden
buildings (76).
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Nonstructural factors
Nonstructural elements and building contents have
been known to fail and cause significant damage
in past earthquakes. Facade cladding, partition
walls, roof parapets, external architectural
ornaments, unreinforced masonry chimneys, ceiling
tiles, elevator shafts, roof water tanks,
suspended ceilings and light fixtures, raised
computer floors, and building contents such as
heavy fixtures in hospitals are among the
numerous nonstructural elements that can fall in
an earthquake, sometimes causing injury or death
(78).
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Supercourse China has already made more
just-in-time PPT about the Sichuan Earthquake,
which concerned with self-rescue and mutual-help
in the earthquake, public health problems, as
well as the first and secondary rescue( including
psychological reconstruction ) after the disaster
ect. Please visit http//www.supercourse.cn/