Earthquake Proof Buildings

Technology designed to keep buildings from collapsing works essentially in two ways: By making buildings stronger, or by making them more flexible, so they sway and slide above the shaking ground rather than crumbling.The latter technology employs an idea called "base isolation."For about 30 years, engineers have constructed skyscrapers that float on systems of ball bearings, springs and padded cylinders. They don't sit directly on the ground, so they're protected from some earthquake shocks. In the event of a major earthquake, they sway up to a few feet. The buildings are surrounded by "moats," or buffer zones, so they don't swing into other structures."You actually take the foundation of building and you put it either on almost like springs or on a mechanism so it is allowed to move a little bit with the earthquake," said Armstrong of the building code council.Well-designed buildings with base-isolation systems ensure that no lives will be lost, no matter the strength of an earthquake, said Michael Constantinou, a professor of civil engineering at the University at Buffalo, State University of New York.More difficult than perfecting the technology, he said, is figuring out how large of an earthquake will hit a certain area.

Tectonics Plates

This theory is now called plate tectonics. The plates "float"on the asthenosphere, a layer of soft, molten rock, and are pushed about by convection currents rising up from the mantle below. The plates move just an inch or two each year. But that's enough to create enormous stress when two plates run into each other.

Timeline of Recent Earthquake ( TOP 10 )

Chile Earthquake May Have Shortened Days on Earth

The massive 8.8 earthquake that struck Chile may have changed the entire Earth's rotation and shortened the length of days on our planet, a NASA scientist said Monday.

The quake, the seventh strongest earthquake in recorded history, hit Chile Saturday and should have shortened the length of an Earth dayby 1.26 milliseconds, according to research scientist Richard Gross atNASA's Jet Propulsion Laboratory in Pasadena, Calif.

"Perhaps more impressive is how much the quake shifted Earth's axis," NASA officials said in a Monday update.

The computer model used by Gross and his colleagues to determine the effects of the Chile earthquake effect also found that it should have moved Earth's figure axis by about 3 inches (8 cm or 27 milliarcseconds).

The Earth's figure axis is not the same as its north-south axis, which it spins around once every day at a speed of about 1,000 mph (1,604 kph).

The figure axis is the axis around which the Earth's mass is balanced. It is offset from the Earth's north-south axis by about 33 feet (10 meters).

This is a file from the Wikimedia Commons.

Driving forces of plate motion

Friction

Basal drag

Large scale convection currents in the upper mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.

Slab suction

Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting wherein basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Gravitation

Gravitational sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.

Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push". This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example:

1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear topographical feature that can offset or at least affect the influence of topographical ocean ridges.

2. Mantle plumes impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor.

Slab-pull 

Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches.  There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The overall driving force for plate motion and its energy source remain subjects of ongoing research.

Driving forces of plate motion

Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. The current view, although it is still a matter of some debate, is that excess density of the oceanic lithosphere sinking in subduction zones is the most powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone. Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists.

Two- and three-dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally varying density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density is mantle convection from buoyancy forces. How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: friction and gravity.

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Q : What Are Some Significant Earthquakes In The Past

New Madrid Quakes

December 16, 1811 through February 1812, early settlers of the Central Mississippi Valley must have thought the world was coming to an end when three of the most powerful earthquakes ever to happen in the Continental USA hit the New Madrid Missouri, and Marked Tree, Arkansas area.

There are accounts of the Mississippi and Arkansas rivers heaving upward, rolling backwards for a few seconds then rerouting their banks before settling down. A series of finger lakes disappeared and new ones were created. Reelfoot Lake in TN is said to have been created by the quakes.

Damage reports were as far away as Washington, DC, Philadelphia, PA and Charleston, SC. Each of the three quakes would have measured 8.0 plus on the Richter Scale, if a seismograph would have been in place.

San Francisco April 18, 1906 Quakes

At 5:12 am Pacific time, San Francisco was devastated with a a series of violent shocks starting with a 7.7 on the Richter scale. It took the lives of 700 to 800 people (some 3,000 total following a consortium of aftershocks) and left 225,000 homeless, with 28,000 buildings destroyed. The largest problem in San Francisco was fire, gas mains and lights ruptured creating a fire which burned for many days.

It ranks as one of the most significant earthquakes of all time. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli Intensities (MMI's) of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace

Anchorage, Alaska 1964 Quake

It was March 27, around 5:30 p.m., on a Good Friday when cars filled chasms were sidewalks once stood after the 9.2 shock hit Anchorage, A Tsunami ripped into Prince William Sound killing 115 people and 15 more inland. A 30 block Area of dwellings and commercial buildings were damaged or destroyed in the downtown area. The tsunami devastated many towns along the Gulf of Alaska, and left damage at Alberni and Port Alberni, Canada, along the West Coast of the United States (15 killed), and waves hit in Hawaii. The Uplifted sea floor at Cape Cleare, Montague Island, Prince William Sound, in the area of greatest recorded tectonic uplift on land (33 feet).

San Francisco 1989 (Loma Prieta Quake)

At 5:04 P.M., Tuesday, October 17, 1989, as over 62,000 fans filled Candlestick Park for the third game of the World Series and the San Francisco Bay Area commute moved into its heaviest flow, a Richter magnitude 7.1 earthquake struck. It was an emergency planner's worst-case scenario. The 20-second earthquake was centered about 60 miles south of San Francisco, and was felt as far away as San Diego and western Nevada. Scientists had predicted an earthquake would hit on this section of the San Andreas Fault and considered it one of the Bay Area's most dangerous stretches of the fault.

Northridge California January 17, 1994 Quake

The 6.7 Magnitude Northridge Earthquake struck early in the early morning of January 17, 1994 at 4:31 AM. The Epicenter was in Northridge, California, a suburb of Los Angeles in the San Fernando Valley. This same general area had been hit by a 6.6 Magnitude quake in 1971. It was one of the most expensive natural disasters in US history, with total damage estimated at $15 billion. The death toll of 57 and 1500 serious injuries is considered low for a quake of this size in a heavily populated area. This was one of the best prepared areas in the world. Most people were safe in their beds at the time of the quake, reducing the casualties. Earthquake awareness in California was high following the 1971 San Fernando quake in this same area and the 1989 Loma Prieta quake in northern California. Building codes required earthquake resistant designs that prevented outright collapse in almost all cases.

What Causes Earthquakes?

The surface of the Earth is in continuous slow motion. This is plate tectonics--the motion of immense rigid plates at the surface of the Earth in response to flow of rock within the Earth. The plates cover the entire surface of the globe. Since they are all moving they rub against each other in some places, sink beneath each other in others, or spread apart from each other. At such places the motion isn't smooth--the plates are stuck together at the edges but the rest of each plate is continuing to move, so the rocks along the edges are distorted. As the motion continues, the strain builds up to the point where the rock cannot withstand any more bending. With a lurch, the rock breaks and the two sides move. An earthquake is the shaking that radiates out from the breaking rock.

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Group Name : Cactus Juice

Members Included : Joshua Chin (13) , Jovan Lim (14) , Khoo Jer Rick (15) , Kok Wei Jing (16)

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