Map of epicenter of May 12, 2008 earthquake in Sichuan Province in China (Photo credit: Wikipedia)
The Sichuan province is located in the country of China and it experiences many earthquakes with the most recent being in the year 2008 as at 2012. There are different reasons why the region experiences these earthquakes such as in 2008 it was caused by the fracture of the Longmenshan fault due to the movement of the Indo-Australian Plate and Eurasian Plate. For more information about other earthquakes that are experienced in the area you can visit sites such as wikipedia.
The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which float on the fluid-like (visco-elastic solid) asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions. This map shows 15 of the largest plates. Note that the Indo-Australian Plate may be breaking apart into the Indian and Australian plates, which are shown separately on this map. (Photo credit: Wikipedia)
The Sichuan Earthquake was caused by a thrust on a Northeast – Southwest (Longmenshan) fault line, which moved the earth in two sections. Geologists believe since the quake was shallow (about 3km beneath the surface), it resulted to cause a greater effect at the epicentre, which was highly populated. It is the 21st deadliest earthquake of all time.
Taken from the Port Hills overlookingChristchurch when the quake hit.
As New Zealanders remember the earthquake of 22nd February a year ago, a range of media have covered many aspects of the event. Here I attempt to summarise some of my own thoughts and facts…
*** Quake factsheet at my re-launched Learn From Nature blog | follow me at twitter
Just as Christchurch was beginning to recover from the huge impact of 4 September 2010 earthquake, a massive aftershock delivered an even more deadly and destructive blow to the city. News reports of the Anglican Cathedral without its characteristic tower, flashed across the internet and newspapers. In January, NAEE co-chair Henricus Peters visited his family who lives there to see the city for himself.
Out of sight, out of mind. Such folly, as we all know now, when it comes to nature. Many Cantabrians probably thought a major earthquake would not happen in their lifetime, despite occasional warnings from scientists, council planners, engineers, and Civil Defence workers that there was still a good chance it would.
The threat was, it was thought, might be from the Alpine Fault, which runs through the western spine of the South Island. Instead, it was hidden or ‘blind’ faults, under the Canterbury Plains. What happened on Saturday 4 September 2010 at 4.35am and continued on Tuesday 22 February 2011 at 12.51pm, proved to be damaging …. Canterbury’s fertile plains are the result of millions of years of mountain building, glaciation and river action. These deposits masked the greywacke bedrock with its tell-tale splinters and cracks resulting from the pressure of the colliding Australian and Pacific tectonic plates [1].
Vast amounts of energy were released in the first few hours of 22 February, changing the shape of Christchurch. The Port Hills are 40 cm taller in places, and Port of Lyttelton is now several centimetres closer to the city.
Everyone has been affected by this natural disaster, turned human disaster for all those who have lost loved ones and property. Schools are sharing premises, since of their locations has been devastated and is now ‘red zoned’ – cannot be occupied.
182 people died as a result of 22 February. This was because a shaken city was now rocked and people were inside these already-affected structures.
21 – the number of earthquakes exceeding magnitude 5 since 4 September
247 – number of earthquakes exceeding magnitude 4 since 4 September
6016 – number of earthquakes detected in Canterbury since 4 September
563 million – number of hits on GeoNet in the six days after 4 September
Christchurch has been presented with a rare opportunity. We have the chance to build a better city. Christ’s College [2], where my brother teaches, has lost a large number of buildings. They are now designing far better, more sustainable premises, which will benefit future generations of students.
At intervals inside the $6 billion span’s box girders are anchor blocks, called deadmen.
They are meant to be used perhaps in the next century, when the concrete girders start to sag.
By running cables from deadman to deadman and tightening them, workers will be able to restorethe girders to their original alignment.
The deadmen are one sign that the new eastern span of the Bay Bridge is meant to last at least 150years after its expected opening in 2013. (The existing eastern bridge will then be torn down.)
At some point, the new span may have to survive a major earthquake, like the one that destroyedmuch of San Francisco in 1906 or the one that partly severed the Bay Bridge in 1989.
Keeping the bridge intact in an earthquake is the engineers’ chief goal. So they are designingflexible structures in which any potential damage would be limited to specific elements.
“The flexibility of the system is such that it basically rides the earthquake,” said its lead designer,Marwan Nader, a vice president at the engineering firm T. Y. Lin International.
Another potential approach, making the bridge structures large enough, and rigid enough, to resistmovement, was rejected.
The new design includes a 160-meter-tall suspension bridge tower made up of four steel shafts thatshould sway in a major quake, up to about 1.5 meters at the top. But the brunt of the force wouldbe absorbed by connecting plates between the shafts, called shear links.
The bridge’s concrete piers are designed to sway as well, limiting damage to areas with extra steelreinforcing. And at joints along the entire span there are 18-meter sliding steel tubes, called hingepipe beams, with sacrificial sections of weaker steel that should help spare the rest of the structureas it moves in a quake.
“At the seismic displacement that we anticipate, there will be damage,” Mr. Nader said. “But thedamage is repairable and the bridge can be serviceable.”
It was the Loma Prieta quake of 1989 that made this 3.5-kilometer replacement span necessary.The 6.9-magnitude quake caused part of the existing span to collapse, killing a motorist and closingthe bridge for a month. That quake caused movement far greater than the 1930s-vintage bridgehad been designed to handle. Most experts believe a stronger quake could cause a total collapse ofthe span.
Unlike more conventional suspension bridges, in which parallel cables are slung over towers andanchored at both ends in rock or concrete, the 624-meter suspension bridge has only a singletower and a single cable that is anchored to the road deck itself, looping from end to end and back. (With a conventional design it would have been extremely difficult to create an anchorage on thebridge’s eastern end, in the middle of the bay.)
The new bridge is the longest self-anchored suspension bridge in the world, with one side of thespan longer than the other.
In a self-anchored design, the road deck has to be built first. “You have a kind of chicken-and-eggsituation,” Mr. Nader said. “You need the deck to carry the compression so that the cable anchorsinto it, but the deck can’t carry itself until the cable is there to carry it. So you have to build atemporary system,” which needs to be seismically secure as well.
The single tower created design problems. It’s “just like a pole,” he said. “If you have a pole and thepole starts shaking, all the damage will occur at the bottom.”
The solution was to split the tower into four shafts and tie them together with the shear links.
The links are of a special grade of steel that deforms more easily. Their placement at points alongthe length of the tower affects how the shafts will move in a quake.
Mr. Nader said the shear links about two-thirds of the way up would be most damaged in a majorearthquake. But the tower would still be structurally sound, he said, and the links would not have tobe replaced immediately.
It’s like a fender bender, he said. “Your car is perfectly drivable, and it’s designed that way, with abumper that can take the shock.
“So you basically stop, just to make sure,” he went on. “You see everything’s O.K., and you cancome in anytime you want to repair your bumper.”
Scientists have rejected fears that a series of highly destructive large-scale earthquakes in the past few years, in countries bordering the Pacific Ocean, signal an increased global risk of these deadly events. Several vast earthquakes have taken place since 2004 — in Chile, Indonesia and Japan — leading some academics to express concern that they may be linked.
But a new study suggests that the pattern of earthquakes, although improbable, is likely to be random and that the risk of large earthquakes is no higher today than it was historically.
The conclusion of the study, published online in Proceedings of the National Academy of Sciences last month (19 December), challenges speculation that the above-average rate of earthquakes of magnitude 8 and above on the Richter scale in recent years reflects a change in the underlying rate of activity.
The magnitude 9 earthquake that shook Japan on March 11 dragged parts of the country 15 feet eastward and moved some seafloor transponders up to 230 feet, the largest earthquake-induced surface displacement ever recorded. More than 20,000 people died, most as a result of the tsunami that hit the coastline a half-hour after the quake. Although Japan has the world’s most advanced earthquake-monitoring system, few researchers had expected a quake of such magnitude. Discover asked Earth scientists and disaster-preparedness experts about the top lessons from the Great East Japan Earthquake. Here is what they said:
– Take the very long view. Models of earthquake risk in Japan were based on a 400-year historical record, but paleoseismic records suggest quakes of this size occur in the country’s Tohoku region every thousand years or so. “If your thinking is based on the last few hundred years, and you haven’t captured a representative time frame for that system, you’re going to be surprised,” says Mark Simons, a geophysicist at Caltech who studied the dynamics of the quake.
– Watch the seafloor.Japan’s earthquake-monitoring network includes 1,200 GPS sensors that track the deformation of the Earth’s crust to help researchers measure and locate the buildup of seismic strain, but these devices do not work underwater. This is a serious limitation: Like 90 percent of all earthquakes greater than magnitude 8, Japan’s temblor happened at sea. Studying movement of the ocean floor currently requires installing acoustic transponders on the bottom and sending a research vessel out to ping them, which can cost half a million dollars per data point. “Land-based measurements have been the cheap answer,” says Georgia Tech geophysicist Andrew Newman, “but a buoy with a GPS and satellite communications could take the place of a ship and help us get more frequent seafloor measurements more affordably.”
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