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An earthquake is a sudden, rapid shaking of the earth caused by the release of energy stored in its crust. The term “earthquake” means any seismic event, whether natural (from the rubbing together of tectonic plates) or caused by humans. In this lesson, students explore how earthquakes happen and what their effects are, by building models, making seismographs, and acting out tectonic movement in a game.
When we look at the earth, it appears to have a smooth surface made up of one piece. Actually, the surface (Earth's outermost layer, comprising the crust and upper mantle) is made up of many large, flat pieces of rock called tectonic plates.
The plate tectonics concept tells us that the earth's rigid outer shell (lithosphere) is broken into a mosaic of plates that can slide over the uppermost layer of the mantle. The plates are in constant motion, moving at a rate between 2 to 12 centimetres per year. Heat cycles in the fluid layer beneath the earth's crust create convection currents that lead to the constant motion of the tectonic plates. There are three common ways in which tectonic plates interact with each other at their boundaries, or faults: they may be convergent plates (when the plates crash together), divergent (when the plates move apart), and transform (when the plates slide past each other).
At some places the motion of plates isn’t smooth — some parts of the plates get stuck against each other, while other parts keep moving. When the stress and strain of these stuck parts becomes too great, rocks break and the plates move suddenly. The sudden motion and resulting shaking is an earthquake.
Often earthquakes are barely noticeable. Occasionally, however, they can be strong enough to knock down buildings, break trees in half, and do other major damage. Earthquakes are commonly measured using the exponential Richter scale. A level 6 earthquake is 10 times as strong as a level 5 quake, which is 10 times as strong as a level 4, and so on.
The largest earthquake ever recorded was a magnitude 9.5! This earthquake struck just off the coast of southern Chile on May 22, 1960. It broke apart a segment of fault more than 1,300 kilometres long and caused a tsunami that wreaked destruction throughout the Pacific Ocean area, including places as far away as Hawaii and Japan. In this case, the earthquake occurred in a complex tectonic region called the Chile Triple Junction, where the interactions between three tectonic plates results in an oceanic plate being pushed under a continental plate!
Only a few people will feel a level 1 earthquake. In a level 2 earthquake, a few people who are resting may feel it, especially if they are near the top of a tall building. Nearly everyone will feel a level 5 earthquake, and some dishes and windows will break. Areas with poorly constructed buildings could experience major damage. A level 8 earthquake can cause damage in areas as large as 2,000 kilometres across!
Earthquakes occur throughout the world. However, they are most common where two (or more) plates meet each other. For example, there are so many earthquakes and volcanoes around the edges of the Pacific Plate that it is nicknamed "the Ring of Fire".
What should students do during an earthquake?
Drop, Cover and Hold is the most appropriate response to earthquake-shaking in British Columbia. Drop under some heavy furniture or into an alcove, cover your head and torso to prevent being hit by falling objects and breaking glass, and hold on so that you remain covered.
(from the Provincial Emergency Program of the Ministry of Public Safety)
tectonic plate: One of a collection of pieces that make up the earth’s surface. There are 7 or 8 major plates and many smaller ones. They are constantly in motion, moving, on average, a few centimetres per year. As they move, they bump into, move away from, or slide under each other, at boundaries called faults. These interactions create features such as mountains and volcanoes, and geologic events such as earthquakes.
Richter scale: A logarithmic scale used to determine the strength of earthquakes. A level 6 earthquake is 10 times as strong as a level 5, which is 10 times as strong as a level 4, and so on.
seismic waves: The energy that radiates out from the centre of an earthquake (the epicentre). They consist of body waves (P- and S-waves) and surface waves (L-waves).
P-waves (Primary or compressional): Waves that are able to move through solids, liquids, and gases at speeds ranging between 300-5,000 metres per second. As they travel through rock, they move particles back and forth in the same direction that the wave is moving.
S-waves (Secondary or shear): Occur just after the P-waves. As these waves move, they displace rock particles perpendicular (i.e. up and down) from the wave direction. They can only move through solids.
liquefaction: Describes the transition from a solid state to that of a liquid. In relation to earthquakes, it describes the behaviour of soils that, when loaded (with mechanical stress), suddenly have the consistency of a heavy liquid.
natural frequency: The consistent rhythm at which objects move when shaken.
resonance: The tendency of an object to move at a larger amplitude when there are many smaller forces occurring at the same time and direction.
seismograph: An instrument for detecting and measuring the intensity, direction and duration of ground movements (i.e. an earthquake).
» Kidipede: Science for Kids | Earthquakes
» Wikipedia | Earthquake
» How Stuff Works | How Earthquakes Work
» San Diego Supercomputer Center | TeacherTECH Science Series | What's Shaking? Hands-on Activities in Earthquake Science
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