Revelation of 5.12 Quake, Sichuan, China

Part I Background of Earthquake

Supercourse Chinahttp://www.SuperCourse.cn/

2008-6-6

• 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)

West Sichuan Earthquake, 12th May 2008

Outline

1.1 Basic knowledge about earthquake1.2 Natural factors related to the frequency and

magnitude of earthquake1.3 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.

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

Earthquake Strength Measures

I) Magnitude & II) 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.

Magnitude versus Intensity

1.2 Natural factors that influence earthquake’s frequency and magnitude

Earthquake StrengthMagnitude 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 FactorsTopographic 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 ActivityEarthquakes 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 super-continent 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 RisksOne 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 factorsNonstructural 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/