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

Revelation of 5.12 Quake,
Sichuan, China
Part I Background of Earthquake
Supercourse China
http://www.SuperCourse.cn/
2008-6-6
West Sichuan Earthquake, 12th May 2008
• Magnitude: 7.9 Richter scale
• Local earthquake time: 14.48 Beijing-time
• Location: 30.986°N, 103.364°E
• Depth: 19 km (11.8 miles)
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
1.1 Basic knowledge about earthquake
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.
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.
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
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
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.
1.2 Natural factors that influence
earthquake’s frequency and magnitude
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.
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.
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.
1.3 Why did 5.12 Quake happen?
By the end of
the Paleozoic
era, about 250
million years
ago, most of the
continents had
collided to form
the supercontinent
Pangea.
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.
India first encountered the southern edge of Asia about 50
million years ago, initiating a continental collision.
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.edu/downloads.php
The lithosphere of India was old, cold and strong while the
lithosphere beneath the rim of Asia was young, warm and
weak.
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.
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
The unstable Tibetan crust began to flow sideways, mostly
toward the east, in a process called “extrusion tectonics” or
“tectonic escape”.
The Sichuan basin lies on top of the Sichuan block, an old, rigid
block that is embedded within the Asian lithosphere.
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.
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 China’s largest,
most destructive earthquakes occur here.
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.
Tectonics of Sichuan Earthquake
• Motion on a northeast striking reverse fault or thrust
fault on the northwestern margin of the Sichuan
Basin
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.
1.4 Secondary disasters of earthquake
FACTORS INFLUENCING
EARTHQUAKE MORBIDITY AND
MORTALITY
• Natural Factors
•
Landslides
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.
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.
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 11:41
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.
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).
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.
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.
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.
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).
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).
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/