# Earthquakes

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• Created by: Jelena
• Created on: 15-05-13 15:09

## General Terminology

• Earthquake occurs when rocks being stressed suddenly break along a new or pre-existing fault
• The two blocks of rocks on either side of the fault slip suddenly, setting off ground vibrations or seismic waves
• Focus = the point at which the earthquake begins
• Epicenter = point on Earth's surface directly above the focus
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## Stress and Strain

• Fractures form when stress (the forces per unit area acting on a material) exceed the strength of the rocks involved
• 3 types of stress:
• 1. Compression: decreases the volume of a material
• 2. Tension: pulls a material apart
• 3. Shear: causes a material to twist
• Strain = the deformation of materials in response to a stress
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## Faults

• Fault = a fracture (crack) in the Earth's crust along which movement occurs
• Key terms:
• 1. Fault Plane = surface along whcih movement takes place
• 2. Hanging Wall = occurs above the fault plane
• 3. Foot Wall = occurs below the fault plane
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## 3 Types of Faults

• 1. Normal fault = the crust is being pulled apart (tensional force); hanging wall drops down
• 2. Reverse fault = form as a result of horizontal compression; hanging wall mvoes up
• 3. Strike-slip fault = fractures caused by horizontal shear; walls move sideways, rather then up or down. Fault plane is vertical, so there is no hanging wall or foot fall.
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## Seismic Waves

• The vibrations of the ground during an earthquake are called seismic waves
• Every earthquake generates 3 types of seismic waves
• 1. Primary waves (P-waves) = compressional waves --> they travel through solid, liquid, or gaseous materials as a succession of compressions and expansion
• 2. Secondary waves (S-waves) = shear waves --> they displace materials at right angles to their path of travel and do not travel through liquids or gases
• 3. Surface waves = one type of surface material sets up a rolling motion in the ground; another type shakes the ground sideways
• P-waves and S-waves pass through the Earth's interior and are called BODY WAVES
• P-waves are the fastest to travel through solid rock of Earth's crust at 6 km/sec, followed by S-waves (about 3 km/sec), followed by surface waves
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## Earthquakes and Plate Tectonics

• Earthquakes are concentrated along the major plate boundaries; these areas are called earthquake belts
• Most earthquakes occur at divergent boundaries, tranform-fault boundaries and convergent boundaries
• A small percentage of earthquakes occur far from plate boundaries --> called intraplate earthquakes
• Many intraplate earthquakes occur on old faults that were once part of ancient plate boundaries. The faults no longer form plate voundaries but remain zones of crustal weakness
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