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Contents
一、The Knowledge of Earthquake
(一)Earthquake fault types
(二)Size and frequency of occurrence
(三)Induced seismicity
(四)Effects/impacts of earthquakes
二、Latest Earthquakes in the World
(一)Wenchuan earthquake
(二)Haiti devastated by massive earthquake
三、Earthquake survival tips
一、The Knowledge of Earthquake
From:http://en.wikipedia.org/wiki/Earthquake
An earthquake (also known as a quake, tremor, or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event — whether a natural phenomenon or an event caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
(一) Earthquake fault types
There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.
Earthquakes away from plate boundaries
Where plate boundaries occur within continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms. All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes
Shallow-focus and deep-focus earthquakes
The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers). These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.
Earthquakes and volcanic activity
Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helenseruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltimeters (a device which measures the ground slope) and used as sensors to predict imminent or upcoming eruptions. Size and frequency of occurrence
Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.[13] Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years. This is an example of the Gutenberg-Richter law.
The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.
The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable. In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.
With the rapid growth of mega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.
(二) Induced seismicity
Main article: Induced seismicity
While most earthquakes are caused by movement of the Earth's tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: constructing large dams and buildings, drilling and injecting liquid into wells, and by coal mining and oil drilling. Perhaps the best known example is the 2008 Sichuan earthquake in China's Sichuan Province in May; this tremor resulted in 69,227 fatalities and is the 19th deadliest earthquake of all time. The Zipingpu Dam is believed to have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault. The greatest earthquake in Australia's history was also induced by humanity, through coal mining. The city of Newcastle was built over a large sector of coal mining areas. The earthquake was spawned from a fault which reactivated due to the millions of tonnes of rock removed in the mining process.
(三) Effects/impacts of earthquakes
Shaking and ground rupture
Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.
Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.
Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.
Landslides and avalanches
Main article: Landslide
Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.
Fires
Fires of the 1906 San Francisco earthquake
Earthquakes can cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake itself.
Soil liquefaction
Main article: Soil liquefaction
Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.
Tsunami
The tsunami of the 2004 Indian Ocean earthquake
Main article: Tsunami
Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers, and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour, depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.
Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.
Floods
Main article: Flood
A flood is an overflow of any amount of water that reaches land. Floods occur usually when the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which then collapse and cause floods.
The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.
Tidal forces
Research work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.
Human impacts
Damaged infrastructure, one week after the 2007 Peru earthquake
Earthquakes may lead to disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings. Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage, as in the "Year Without a Summer" (1816).
二、Latest Earthquakes in the World
(一) Wenchuan earthquake
From : www.rr365.com
Expert: Wenchuan earthquake as huge as Tangshan earthquake in 1976
BEIJING, May 13 -- The strength of this earthquake is being compared to the destructive force of the Tangshan earthquake in 1976 which killed tens of thousands. Seismologists are urging calm and telling the public not to believe rumors about more earthquakes.
Jiang Haikun says the energy from the Wenchuan earthquake was as huge as the Tangshan earthquake in 1976, which killed 240,000 people.
Jiang Haikun, China Seismological Network Center, said, "The strength of the earthquake is big and it has released a huge amount of energy so that many parts of the country felt the quake."
He emphasized that Beijing did not experience a separate earthquake.
Jiang Haiku said, "The quake we felt in Beijing today was not a separate earthquake. It was just the aftershocks from the Wenchuan earthquake in the afternoon."
Jiang Haikun also says his center's research shows China and its neighbors are currently in a relatively-active period for earthquakes.
Jiang Haiku said, "For example, a 7.3 magnitude earthquake occurred in Japan recently. On the Chinese mainland, a 6.9 magnitude quake took place in Tibet in January, while March saw a 7.3 magnitude quake in Yutian in Xinjiang and now we have a 7.8 magnitude quake in Wenchuan. So at the moment there is a trend of earthquakes in China's mainland and its neighboring countries."
He also urged residents not to trust rumors they heard. He stressed no destructive earthquake near Beijing is likely to occur in the near future.
From :http://news.iciba.com/patch/listen/listen_arc.php?aid=22496
Seismologist explains Wenchuan earthquake
The strength of this earthquake is being compared to the destructive force of the Tangshan earthquake in 1976 which killed tens of thousands. Seismologists are urging calm and telling the public not to believe rumors about more earthquakes.
Jiang Haikun says the energy from the Wenchuan earthquake was as huge as the Tangshan earthquake in 1976, which killed 240,000 people.
Jiang Haikun, China Seismological Network Center, said, "The strength of the earthquake is big and it has released a huge amount of energy so that many parts of the country felt the quake."
He emphasized that Beijing did not experience a separate earthquake.
Jiang Haiku said, "The quake we felt in Beijing today was not a separate earthquake. It was just the aftershocks from the Wenchuan earthquake in the afternoon."
Jiang Haikun also says his center's research shows China and its neighbors are currently in a relatively-active period for earthquakes.
Jiang Haiku said, "For example, a 7.3 magnitude earthquake occurred in Japan recently. On the Chinese mainland, a 6.9 magnitude quake took place in Tibet in January, while March saw a 7.3 magnitude quake in Yutian in Xinjiang and now we have a 7.8 magnitude quake in Wenchuan. So at the moment there is a trend of earthquakes in China's mainland and its neighboring countries."
He also urged residents not to trust rumors they heard. He stressed no destructive earthquake near Beijing is likely to occur in the near future.
(二)Haiti devastated by massive earthquake
From:http://news.bbc.co.uk/2/hi/8455629.stm
A massive 7.0-magnitude earthquake has struck the Caribbean nation of Haiti.
The extent of the devastation is still unclear but there are fears thousands of people may have died.
Haiti's worst quake in two centuries hit south of the capital Port-au-Prince on Tuesday, wrecking the presidential palace, UN HQ and other buildings.
A "large number" of UN personnel were reported missing by the organisation. Many people have spent the night outside amid fears of more aftershocks.
The Red Cross says up to three million people have been affected.
Describing the earthquake as a "catastrophe", Haiti's envoy to the US said the cost of the damage could run into billions.
A number of nations, including the US, UK and Venezuela, are gearing up to send aid.
The quake, which struck about 15km (10 miles) south-west of Port-au-Prince, was quickly followed by two strong aftershocks of 5.9 and 5.5 magnitude.
The tremor hit at 1653 (2153 GMT) on Tuesday, the US Geological Survey said. Phone lines to the country failed shortly afterwards.
There is still no official word on casualties and the extent of the devastation is only now becoming clearer with dawn breaking.
China has already indicated in reports in state media that eight of its peacekeepers are buried and feared dead, with another 10 unaccounted for.
The AFP news agency quoted the Jordanian army as saying three of its peacekeepers had been killed and 21 wounded.
The Brazilian army said four of its peacekeepers were killed and a large number were missing.
A French official told AFP about 200 people were missing in the collapsed Hotel Montana, which is popular with tourists.
There have also been some reports of looting overnight.
Rachmani Domersant, an operations manager with the Food for the Poor charity, told Reuters that overnight the capital was in total darkness.
"You have thousands of people sitting in the streets with nowhere to go. There are people running, crying, screaming.
"People are trying to dig victims out with flashlights. I think hundreds of casualties would be a serious understatement."
Earlier, bodies white with dust could be seen piled on the back of a pick-up truck as vehicles tried to ferry the injured to hospital.
Haiti is the poorest country in the western hemisphere and has suffered a number of recent disasters, including four hurricanes and storms in 2008 that killed hundreds.
'Thoughts and prayers'
In a statement issued in New York, the UN said that its local HQ in Haiti had "sustained serious damage along with other UN installations" and "a large number" of personnel were missing.
UN peacekeeping chief Alain Le Roy said it was unclear how many people were in the building.
The head of the UN mission in Haiti, Hedi Annabi, was reported to have been inside and is unaccounted for.
French Foreign Minister Bernard Kouchner said he was believed to be dead.
The UN's stabilisation mission plays a vital role in ensuring security in Haiti.
Raymond Joseph, Haiti's ambassador to the US, said the presidential palace, the tax office, the ministry of commerce and the foreign ministry had all been damaged, but the airport was intact.
He and Haiti's ambassador to Mexico, Robert Manuel, both said that President Rene Preval and his wife had survived the quake.
The World Bank said its local offices were destroyed but most of the staff were accounted for, Reuters reported.
US President Barack Obama said his "thoughts and prayers" were with the people of Haiti and that he expected "an aggressive, coordinated [aid] effort by the US government".
Venezuela says it will send a 50-strong "humanitarian assistance team".
The Red Cross is dispatching a relief team from Geneva and the UN's World Food Programme is flying in two planes with emergency food aid.
The Inter-American Development Bank said it was immediately approving a $200,000 grant for emergency aid.
The UK said it was mobilising help and was "ready to provide whatever humanitarian assistance may be required".
Canada, Australia, France and a number of Latin American nations have also said they are mobilising their aid response.
Pope Benedict XVI has called for a generous response to the "tragic situation" in Haiti.
'Shouting and screaming'
In the minutes after the quake, Henry Bahn, a visiting official from the US Department of Agriculture, said he had seen houses which had tumbled into a ravine.
"Everybody is just totally, totally freaked out and shaken," said Mr Bahn, who described the sky as "just grey with dust".
He said he had been walking to his hotel room when the ground began to shake.
"I just held on and bounced across the wall," he said. "I just heard a tremendous amount of noise and shouting and screaming in the distance."
Reports on the Twitter message site, which cannot yet be verified by the BBC, expressed the chaos in the wake of the quake.
Tweets from troylivesay spoke of the worst damage being in the Carrefour district, where "many two and three storey buildings did not make it".
In the immediate aftermath of the quake, a tsunami watch was put out for Haiti, Cuba and the Bahamas, but this was later lifted.
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