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This is Olivia's earthquake page. She has done extra work so you can have a look at her page called 'Olivia's Extra earthquakes page' How do Earthqu﻿akes Start?

An earthquake is when the surface of the earth is shaking because of rapid movement of the rocky outer layer of the earth. The plates under the earth move rapidly and when they crash together it makes strain in the rocks. If the energy that is stored inside the earth -the strain in the rocks- is released suddenly an earthquake occurs. When this happens, earthquake waves send the released energy to the surface of the earth.

Where do Earthquakes come from? Earthquakes come from the plates under the earth that move and then suddenly slip past each other and crash. The suface where they slip is called the fault or fault plane. The location under the earth where the earthquake starts is called the hypocenter and the location just above it is called the epicenter. Sometimes earthquakes have foreshocks and aftershocks. Foreshocks are mini earthquakes that happen in the same place as the big earthquake is going to happen in but before the big earthquake happens. Aftershocks are mini earthquakes that happen in the same place as the big earthquake but after the big earthquake. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earth- quakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

How do the plates move?

Plates at earths surface move because of the *intense heat in the earths core that causes molten rock in the mantle layer to move. It moves in a pattern called a convection cellScientists once thought that Earth's plates just surfed on top of the mantle’s giant convection cells, but now scientists believe that plates help themselves move instead of just surfing along. Just like convection cells, plates have warmer, thinner parts that are more likely to rise, and freeze, *denser parts that are more likely to sink. This image is a cross section through the Earth showing the convection cells of the mantle. Ridge push happens at spreading centers where plates are moving apart. Slab pull happens at subduction zones where one plate is pulled down into the mantle.

*intense - intense means strong

*denser - denser means crammed

How deep do cracks go under the earth? Deep earthquakes were discoverd in the 1820's, but they remain a subject of contention today. The reason is simple, they aren't supposed to happen. Yet they account for more than 20% of all earthquakes. Shallow earthquakes need solid rocks to occur-more specifically, cold, brittle rocks. Only these can store up elastic strain along a geologic fault until the strain lets loose in a violent rupture. The Earth gets hotter by about 1 degree C with each 100m of depth on average. Combine that with high pressure underground and its clear that about 50 kilometres down, on average the rocks should be too hot and squeezed too tight to crack and grind the way they do at the surface. The big cracks go down to at least 6 378km. How far can Earthquakes spread?

At the instant an earthqua ke occurs,*P, *S and L waves immediately begin racing outward in all directions, losing energy as they spread out. If they encounter no interference, P and S waves for a large earthquake should quickly travel all of the way through the middle of the earth and faintly arrive on the opposite side of the globe. An earthquake at the south pole, for eample, would shake the north pole in less than half an hour (though the vibrations would be very weak). This is what P and S wavesshould do. However, the P and S waves do not always make it to the opposite side because when they crash into other places it slows them down. Earthquakes keep on spreading until they hit another place (State/Country).

*P Wave:The fastest wave, and therefore the first to arrive at a given location - also known as compressional waves, the P wave alternately compresses and expands material in the same direction it is traveling - it can travel through all layers of the Earth - it is generally felt by humans as a bang or thump. *S Wave:The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth - also known as shear waves - these waves move more slowly than P waves, but in an earthquake they are usually bigger - S waves cannot travel through the outer core because these waves cannot travel through fluids, such as air, water or molten rock. How long do earthquakes go for?

Average Earthquakes actually only last a few seconds! But the aftershocks, often lighter than the earthquake itself that cause things weakened by the earthquake,like trees, to fall. It depends on how big the earthquake is but most of the time earthquakes are 15 seconds.The longest earthquake that has ever happened was 4 minutes. How much damage can Earthquakes do? Earthquake damage depends on what area is hit. If an unpopulated region is struck, there will be low loss of life or property. If it hits a large city, there may be many injuries and destruction. Many of the areas at risk on the Ring of Fire are largely populated. Major earthquakes hitting those areas today could produce terrible damage.

Most global cities have at least tripled their populations in the last hundred years, so a major earthquake could collapse skyscrapers, factories, and power plants. Millions of people could feel the shock waves, not only of the initial tremor, but of the aftershocks, tsunamis, landslides, floods, and social effects.

Earthquakes have the power to uproot trees and send them crashing into buildings. They can trigger landslides and avalanches, and cause flooding and tsunamis. Human structures are also at risk. It is interesting to note that tall buildings will sustain the least damage if they are located directly at the epicenter. This is because they can withstand the up-and-down motion of P-waves. S-waves, on the other hand, occur far away from the epicenter, and cause the greatest stress by shaking buildings from side to side. These buildings are often knocked off their foundations.

Buildings with thick, heavy walls do not resist shock waves very well. Violent earthquakes often cause structures to collapse, burying people underneath. Brick buildings are the most vulnerable. Chimneys and heavy roof tiles are often shaken off during the tremors, and can crash into bystanders or passerbys.

Constructions on soft or filled-in soil suffer the most because they feel shock waves most directly. Buildings on bedrock suffer less damage because the ground is firmer.

The shaking of the earth is sometimes not the greatest disaster. It is in the ensuing fires and floods that often the greatest damage occurs. In the 1906 earthquake, it was the subsequent fires that did the majority of the damage. An earthquake can also destroy dams high above a city or valley, causing floods to sweep down and sweep away everything in their path.

A 60-second or less earthquake can cause devastation that continues for years after the first tremor. In 1972, a series of severe earthquakes struck Managua, Nicaragua. Fifteen years later, the city still looked the way it had a week after the earthquake hit, because the country did not have the money necessary to rebuild.

What causes the ground to shake? While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us! Is a volcano related to an earthquake?

Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to manmade structures. There are two general categories of earthquakes that can occur at a volcano: volcano-tectonic earthquakes and long period earthquakes. Earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma (molton rock) are called volcano-tectonic earthquakes. These earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don't indicate that the volcano will be erupting but can occur at anytime. The second category of volcanic earthquakes are long period earthquakes which are produced by the injection of magma into surrounding rock. These earthquakes are a result of pressure changes during the unsteady transport of the magma. When magma injection is sustained a lot of earthquakes are produced. This type of activity indicates that a volcano is about to erupt. Scientists use seismographs to record the signal from these earthquakes. This signal is known as volcanic tremor. People living near an erupting volcano are very aware of volcanic earthquakes. Their houses will shake and windows rattle from the numerous earthquakes that occur each day before and during a volcanic eruption. Residents in Pompeii felt earthquakes daily before Vesuvius erupted in A.D. 79 but continued to go about their daily routines. When Mount Pinatubo in the Philipines erupted in 1991, nerves were rattled as much as windows by volcanic earthquakes. Earthquakes exhibiting volcanic tremor warn of an impending eruption so that people can be evacuated to areas of safety. The volcanic tremor signal has been used successfully to predict the 1980 eruptions Mount St. Helens and the 1991 eruption of Pinatubo. Volcano-tectonic earthquakes can cause damage to manmade structures and landsliding. To prevent damage from being done, structures should be built according to earthquake standards, building foundations should be constructed on firm ground and not unconsolidated material which may amplify earthquake intensity, and buildings should be constructed on stable slopes in areas of low hazard potential.

What is the epicentre of an earthquake?

The place under the earth's surface, the hypocentre is where the earthquake starts and place above the hypocentre is the epicentre. The epicentre is the surface where we live and walk, so it is the ground. The epicenter is the point where an earthquake or underground explosion originates. The word derives from the New Latin noun epicentrum, the latinisation of the Greek adjective ἐπίκεντρος (epikentros), "occupying a cardinal point, situated on a centre", from ἐπί (epi) "on, upon, at" and κέντρον (kentron) "centre".

In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone. For example, in the magnitude 7.9, 2002 Denali earthquake in Alaska, the epicenter was at the western end of the rupture, but the greatest damage occurred about 330 km away at the eastern end of the rupture zone.

How do aftershocks occur? Aftershocks usually occur within the next two days, but have also been recorded to happen for years. Their size, strength, and frequency usually diminish with time. (Incidentally, there is also such a thing as a foreshock, a small tremor that comes before a major earthquake.) Even though the major strain between two plates is released by the initial quake, their touching edges still need to adjust to new positions. The edges may not be able to pass each other smoothly, and this additional realignment creates the smaller shocks.

When the earth’s plates finally stop grinding against each other, the surface landscape may have changed over huge areas around the earthquake’s center. This happened after the 1906 earthquake in San Francisco, California (United States), when a piece of land 430 kilometers (267 miles) long shifted north by 6 meters (20 feet). Aftershocks are able to do serious damage, so don’t look them over lightly! They have the power to crumble buildings, roads, and bridges already weakened by the initial earth movement.

How do seismographs work?

Seismographs are instruments used to record and measure earthquakes. During an earthquake, vibrations initiated by the breakage of rock along the fault zone radiate outward from the point of fracture. Seismographs detect, amplify, and record these vibrations. The visual record produced by a seismograph is called a seismogram.The motion of the earth during an earthquake is measured in terms of its movement relative to some object that remains independent of the ground motion. In a seismograph this object consists of a mass suspended on springs within a case. The unit is called a seismometer. During an earthquake the mass remains still, while the case around it moves with the ground motion. Most modern seismographs work electromagnetically. A large magnet is used for the mass and the outside case contains numerous windings of fine wire. Movements of the case relative to the magnet generate small electric signals in the wire coil. These signals are then amplified electronically and stored digitally on a computer or played out on a recording drum called a seismogram.One seismograph station, having three different pendulums sensitive to the north-south, east-west, and vertical motions of the ground, will record seismograms that allow scientists to estimate the distance, direction, Richter Magnitude, and type of faulting of the earthquake. Seismologists use networks of seismograph stations to determine the location of an earthquake, and better estimate its other parameters.