Category The World Around us

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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Earthquakes are one of the most destructive forces on Earth. They happen quite frequently, though most of them are relatively minor. Powerful quakes, depending on where they happen, cause severe damage, toppling buildings and sometimes killing many thousands of people. They happen when tension created by the movement of the Earth’s tectonic plates is released, causing the rocks to shift and break suddenly. The incredible amount of force required to break the rocks is what makes earthquakes so devastating.

If your heart beats rapidly during an earthquake, it still doesn’t compete with high-frequency waves generated by the quake. These waves shake the ground faster than your ticker’s thrumming and cause the most damage to smaller structures, such as house­­s.

Researchers now have a new explanation for the source of these poorly understood high-frequency seismic waves. The longer a fault heals between earthquakes, the faster the waves once the fault finally breaks again, according to a new study detailed in the Oct. 31 issue of the journal Nature.

“We can think of a fault as just as crack or a cut in the ground. When they heal, it may not be all that different than how a cut in your skin heals. There are physical and chemical changes that occur right on the surface,” said Gregory McLaskey, lead study author and a postdoctoral researcher at the U.S. Geological Survey in Menlo Park, Calif.

Though the next quake may not be bigger in terms of magnitude, it could be much more intense, with more rapid shaking, he said.

“It doesn’t just affect the strength of it, it affects the way the ground will shake when it ruptures. The more the fault has healed, the more rapid vibrations and jolts will be produced when the earthquake does come,” McLaskey told OurAmazingPlanet.

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The whole of the Earth’s crust is subject to continental drift, including the ocean floor. Most of the tectonic plates are both continental (part of the land) and oceanic (part of the ocean floor). Evidence of movement on the sea bed is found in different magnetic alignments in the rock and volcanic activity on the ocean floor.

Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources. The structure of the oceans, starting with the continents, begins usually with a continental shelf, continues to the continental slope – which is a steep descent into the ocean, until reaching the abyssal plain – a topographic plain, the beginning of the seabed, and its main area. The border between the continental slope and the abyssal plain usually has a more gradual descent, and is called the continental rise, which is caused by sediment cascading down the continental slope.

The mid-ocean ridge, as its name implies, is a mountainous rise through the middle of all the oceans, between the continents. Typically a rift runs along the edge of this ridge. Along tectonic plate edges there are typically oceanic trenches – deep valleys, created by the mantle circulation movement from the mid-ocean mountain ridge to the oceanic trench.

Hotspot volcanic island ridges are created by volcanic activity, erupting periodically, as the tectonic plates pass over a hotspot. In areas with volcanic activity and in the oceanic trenches there are hydrothermal vents – releasing high pressure and extremely hot water and chemicals into the typically freezing water around it.

Deep ocean water is divided into layers or zones, each with typical features of salinity, pressure, temperature and marine life, according to their depth. Lying along the top of the abyssal plain is the abyssal zone, whose lower boundary lies at about 6,000 m (20,000 ft). The hadal zone – which includes the oceanic trenches, lies between 6,000–11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.

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Fossilized remains found in different parts of the world are good evidence that the continents were once joined together. Remains of the same animal have been found in both South America and Africa, which means it must have lived at a time when the continents were part of the same land mass. Plant fossils of the same type and age have been found all over the world, and geologists have identified parts of the same mountain range in different continents.

Alfred Wegener collected diverse pieces of evidence to support his theory, including geological “fit” and fossil evidence. It is important to know that the following specific fossil evidence was not brought up by Wegener to support his theory. Wegener himself did not collect the fossils but he called attention to the idea of using these scientific doc   uments stating there were fossils of species present in separate continents in order to support his claim.

Geological “fit” evidence is the matching of large-scale geological features on different continents. It has been noted that the coastlines of South America and West Africa seem to match up, however more particularly the terrains of separate continents conform as well. Examples include: the Appalachian Mountains of eastern North America linked with the Scottish Highlands, the familiar rock strata of the Karroo system of South Africa matched correctly with the Santa Catarina system in Brazil, and the Brazil and Ghana mountain ranges agreeing over the Atlantic Ocean.

Another important piece of evidence in the Continental Drift theory is the fossil relevance. There are various examples of fossils found on separate continents and in no other regions. This indicates that these continents had to be once joined together because the extensive oceans between these land masses act as a type of barrier for fossil transfer. Four fossil examples include: the Mesosaurus, Cynognathus, Lystrosaurus, and Glossopteris.

The Mesosaurus is known to have been a type of reptile, similar to the modern crocodile, which propelled itself through water with its long hind legs and limber tail. It lived during the early Permian period (286 to 258 million years ago) and its remains are found solely in South Africa and Eastern South America. Now if the continents were in still their present positions, there is no possibility that the Mesosaurus would have the capability to swim across such a large body of ocean as the Atlantic because it was a coastal animal.

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