Geography Essay Earthquakes

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EARTHQUAKES

Introduction and Terminology

• Vibrations in the earth’s crust that occur when strain in the crust is suddenly released by displacement along a fault line • Focus = point at which an earthquake originates inside the earth. May be many km below surface • Epicentre = the point on the earth’s surface directly above the focus. Strongest shock normally felt here. Most damage usually occurs here. • Seismology = the study of earthquakes – from Greek – seio – to shake • Seismologists record earth tremors and earthquakes on seismographs • These record the passage of seismic waves through the ground • An analysis of the pattern reveals a lot about the underlying rock structures • Primary waves (p) – fastest – 7.5km/sec

• Secondary waves (s) – bigger and slower – 4.3km/sec • Time difference between p and s waves reflects distance between seismograph and epicentre of earthquake • If this distance can be calculated for at least 3 widely spaced seismographic sites – precise location of epicentre can be determined

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Earthquakes – Causes and Mechanisms

• Explained in terms of Plate Tectonics and associated movement of fault lines e.g. San Andreas Fault • When movement is restricted – fault is locked and stores up strain energy • This energy is ultimately released as an earthquake

• Immediate cause = sudden breaking and displacement of rock • As stress area ruptures – strain is relieved causing sudden ground movement around the fault • Aftershocks occur as fault settles back

Locations

• Major zone = around Pacific – Andes, California, Alaska, Japan, New Zealand • Other main areas of deep earthquakes usually close to crustal boundaries e.g. Great African Rift System

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Earthquakes – Magnitude and Intensity

1. Magnitude usually expressed using the Richter Scale(Charles Richter-1935) see below 2. Intensity measures the effect of an earthquake on people and structures. It is measured using the Mercalli Scale • Epicenter = the zone of maximum intensity

• Seismic waves decrease in size outwards from there • Each earthquake has one magnitude = one Richter number • Intensity varies from place to place => Mercalli number changes depending on location relative to epicentre Seismologists have devised two scales of measurement to enable them to describe earthquakes quantitatively. One is the Richter scale—named after the American seismologist Charles Francis Richter—which measures the energy released at the focus of a quake. It is a logarithmic scale that runs from 1 to 9; a magnitude 7 quake is 10 times more powerful than a magnitude 6 quake, 100 times more powerful than a magnitude 5 quake, 1,000 times more powerful than a magnitude 4 quake, and so on. An estimated 800 quakes of magnitudes 5 to 6 occur annually throughout the world, in comparison with about 50,000 quakes of magnitudes 3 to 4, and only about one earthquake of magnitudes 8 to 9. The other scale, introduced at the turn of the 20th century by the Italian seismologist Giuseppe Mercalli, measures the intensity of shaking with gradations from I to XII. Because seismic surface effects diminish with distance from the focus of the quake, the Mercalli rating assigned to the quake depends on the site of the measurement. Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale can be correlated with magnitudes 8 to 9 on the Richter scale. [pic]

Earthquake Damage

• Most problems with damage to buildings, structures and transport systems • Aftershocks (more subdued but longer lasting and more frequent than initial...
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