Category Science

Modern building technologies mean that homes, offices and other buildings can be designed to withstand the effect of an earthquake. Tall buildings are built with a strong central column from which the structure “hangs”. Conical or triangular designs are able to absorb shocks more easily, while the use of new materials allows buildings to be constructed in earthquake zones at a relatively low cost.

After the massive earthquake near Japan one wonders if it’s possible to build an earthquake-proof building. The answer is yes and no. There are of course, engineering techniques that can be used to create a very sound structure that will endure a modest or even strong quake. However, during a very strong earthquake, even the best engineered building may suffer severe damage. Engineers design buildings to withstand as much sideways motion as possible in order to minimize damage to the structure and give the occupants time to get out safely.

Buildings are basically designed to support a vertical load in order to support the walls, roof and all the stuff inside to keep them standing. Earthquakes present a lateral, or sideways, load to the building structure that is a bit more complicated to account for. One way to make a simple structure more resistant to these lateral forces is to tie the walls, floor, roof, and foundations into a rigid box that holds together when shaken by a quake.

The most dangerous building construction, from an earthquake point of view, is unreinforced brick or concrete block.  Generally, this type of construction has walls that are made of bricks stacked on top of each other and held together with mortar.  The roof is laid across the top.  The weight of the roof is carried straight down through the wall to the foundation.  When this type of construction is subject to a lateral force from an earthquake the walls tip over or crumble and the roof falls in like a house of cards.

Construction techniques can have a huge impact on the death toll from earthquakes. An 8.8-magnitude earthquake in Chile in 2010 killed more than 700 people. On January 12, 2010, a less powerful earthquake, measuring 7.0, killed more than 200,000 in Haiti.

The difference in those death tolls comes from building construction and technology. In Haiti, the buildings were constructed quickly and cheaply. Chile, a richer and more industrialized nation, adheres to more stringent building codes.

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Perhaps the world’s best known fault line is the San Andreas Fault. Situated in California, USA, it is an area of the world where earthquakes and tremors occur frequently. The citizens of San Francisco know that a very powerful quake (often referred to as “The Big One”) could occur at any time.

Viewed from space, the San Andreas Fault looks like a long, narrow valley that marks where the North America plate meets the Pacific plate. This narrow break between the two plates is called a fault. But viewed up close, there are actually many fractures and faults that mark the zone where the two plates slide past one each other. Sometimes the boundary is a zone of several smaller faults, one or more of which may break during an earthquake. Sometimes it is a single fault. 

On the ground, one can find the San Andreas Fault by looking for landforms it created. For example, sharp cliffs called scarps form when the two sides of the fault slide past each other during earthquakes. “The dominant motion along the fault is primarily horizontal, but some areas also have vertical motion,” noted Shimon Wdowinski, a geophysicist at the University of Miami’s Rosentiel School of Marine & Atmospheric Sciences who has studied the San Andreas Fault. And stream channels with sharp jogs — the channels are offset across the fault line — can be visited in the central California’s Carrizo Plain National Monument.

On the west side of the fault sits most of California’s population, riding the Pacific Plate northwest while the rest of North America inches south. The Pacific Plate is moving to the northwest at 3 inches (8 centimeters) each year, and the North American Plate is heading south at about 1 inch (2.3 cm) per year.

The San Andreas Fault was born about 30 million years ago in California, when the Pacific Plate and the North America plate first met. Before then, another oceanic plate, the Farallon plate, was disappearing beneath North America at a subduction zone, another type of plate boundary. The new configuration meant the two plates slid past one another instead of crashing into each other, a boundary called a strike-slip fault.

Researchers have measured identical rocks offset by 150 miles (241 kilometers) across either side of the fault. For example, the volcanic rocks in Pinnacles National Park south of Monterey match volcanic rocks in Los Angeles County (called the Neenach volcanics). Geologists think the total amount of displacement along the fault is at least 350 miles (563 km) since it formed.

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The shock wave of a powerful earthquake can easily destroy buildings and other structures, but there are some side-effects of the quake itself. Underground gas pipes may rupture, leading to serious fires and explosions. The health of survivors is but at risk by damaged sewerage systems allowing disease to spread. In mountainous areas, landslides or avalanches can be triggered, and an undersea earthquake can generate a huge wave called a tsunami.

An earthquake is a sudden shaking movement of the surface of the earth. It is known as a quake, tremblor or tremor. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

So far, there have been sixty-two earthquakes in India. The first recorded earthquake in India was on 6th June 1505 it occurred in Saldang, Karnali zone. And the most recent one happened in India as on 31st January 2018 and occurred in Kashmir, Pakistan, Afghanistan, and Tajikistan.

An earthquake is measured on Richter’s scale. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph. The strength, or magnitude, of an earthquake, is measured using the Richter scale. Quakes measuring around 7 or 8 on the Richter scale can be devastating.

Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake. In southern Italy in 1909 more than 100,000 people perished in an earthquake that struck the region. Almost half of the people living in the region of Messina were killed due to the easily collapsible structures that dominated the villages of the region. A larger earthquake that struck San Francisco three years earlier had killed fewer people (about 700) because building construction practices were different type (predominantly wood). Survival rates in the San Francisco earthquake was about 98%, that in the Messina earthquake was between 33% and 45%) (Zebrowski, 1997). Building practices can make all the difference in earthquakes, even a moderate rupture beneath a city with structures unprepared for shaking can produce tens of thousands of casualties.

Although probably the most important, direct shaking effects are not the only hazard associated with earthquakes, other effects such as landslides, liquefaction, and tsunamis have also played important part in destruction produced by earthquakes.

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Deep beneath the Earth’s surface, the Earthquake place where the earthquake actually occurs is called the focus. This is where the greatest amount of rock movement is to he found. The ground directly above the focus is known as the epicentre. This is where the most damage occurs.

An earthquake’s hypocenter is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture. This occurs directly beneath the epicenter, at a distance known as the focal or hypocentral depth.

The focal depth can be calculated from measurements based on seismic wave phenomena. As with all wave phenomena in physics, there is uncertainty in such measurements that grows with the wavelength so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock.

Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring. The expanding wave front from the earthquake’s rupture propagates at a speed of several kilometers per second; this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through. With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from the initial estimated hypocenter. These equations are solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times, and a new estimated hypocenter is computed. The system iterates until the location is pinpointed within the margin of error for the velocity computations.

A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake with a hypocenter depth exceeding 300 km. They occur almost exclusively at convergent boundaries in association with subducted oceanic lithosphere. They occur along a dipping tabular zone beneath the subduction zone known as the Wadati–Benioff zone.

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The size, or the magnitude, of an earthquake is recorded using an instrument called a seismometer. Using very heavy weights that remain still while the room it is in is shaking, the machine records the amount of movement on a rotating drum of paper. This type of record is measured on the Richter scale. The physical and visible effects of a quake are measured using the Vertical Modified Mercalli scale (see below).

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was (figure 5). A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about theintensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

The Modified Mercalli scale:

1 Only detected by instruments. Doors begin to swing.

2 Some people inside high buildings may feel a tremor.

3 Rapid vibrations possibly felt indoors.

4 Stationary cars rock; windows shake; people indoors feel something.

5 Effects felt outdoors; small objects fall over; some buildings shake.

6 Trees begin to shake; crockery broken; everyone in the area feels it.

7 People alarmed; chimneys begin to crack; windows break.

8 Cars crash; buildings and trees damaged.

9 Many people panic; cracks in the ground; buildings fall down.

I0 Buildings destroyed; underground services disrupted; rivers affected.

II Bridges collapse; landslides happen; railways affected.

12 Widespread devastation; landscape changed.

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Earthquakes can happen anywhere, but they occur most frequently above the boundaries of the Earth’s tectonic plates. The most powerful earthquakes occur where the plates are moving deep below the surface. These boundaries are known as transform faults or fault lines.

Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth:

The world’s greatest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of our planet’s largest earthquakes occur. It has earned the nickname “Ring of Fire”. Why do so many earthquakes originate in this region? The belt exists along boundaries of tectonic plates, where plates of mostly oceanic crust are sinking (or subducting) beneath another plate. Earthquakes in these subduction zones are caused by slip between plates and rupture within plates. Earthquakes in the curcum-Pacific seismic belt include the M9.5 Great Chilean Earthquake [Valdivia Earthquake] (1960) and the M9.2 Great Alaska Earthquake (1964).

The Alpide earthquake belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world’s largest earthquakes, including some of the most destructive, such as the 2005 M7.6 shock in Pakistan that killed over 80,000 and the 2004 M9.1 Indonesia earthquake, which generated a tsunami that killed over 230,000 people. 

The third prominent belt follows the submerged mid-Atlantic Ridge. The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary). Most of the mid-Atlantic Ridge is deep underwater and far from human development, but Iceland, which sits directly over the mid-Atlantic Ridge, has experienced earthquakes as large as at least M6.9.

The remaining shocks are scattered in various areas of the world. Earthquakes in these prominent seismic zones are taken for granted, but damaging shocks can occur outside these zones. Examples in the United States include New Madrid, Missouri (1811-1812) and Charleston, South Carolina (1886). However, many years usually elapse between such shocks.

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