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University of Toronto Scarborough
Environmental Science

An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earth’s surface. Stress causes strain Seismic waves are the waves of energy produced by an earthquake and cause ground to tremble and shake. Breaks between two rock masses is called a fault. Classic explanation for why earthquakes takeplace is called Elastic rebound theory which involves sudden release of progressively stored elastic strain energy in rocks, causing movement along a fault. The deep seated internal forces (Tectonic forces) act on a mass of rock over many decades. Rocks on surface are brittle, rocks deep in the earth are ductile. Volcanic eruptions and mineral transformations may also cause earthquakes, not just fault movements. Focus or Hypocentre is the point of the earthquake where the seismic waves first originate. The epicenter is the point on the Earth’s surface directly above the focus. 2 types of seismic waves- body waves are seismic waves that travel through the Earth’s interior, spreading outward from the focus in all directions. Surface waves are seismic waves that travel on Earth’s surface away from the earth’s epicenter. 2 types of body waves: p wave is a compressional or longitudinal wave in which rock vibrates back and forth parallel to the direction of wave propagation. This is a very fast wave and arrives first. S wave (secondary) is the slower, transverse wave that travels through near-surface rocks. Rocks vibrate perpendicular to the direction of wave propagation. Surface waves in general are the slowest and cause most property damage, also longest to pass. 2 types: Love waves are like S waves and move perpendicular to the direction of propagation and no vertical displacement. These take buildings and bridges down but cannot be felt in water bodies. Rayleigh waves cause ground to move in an elliptical path opposite to the direction the wave passes. These produce the most ground movement and are the longest to pass. (like ocean waves) How do we know where earthquakes occur? Siesmometer is the instrument used to measure seismic waves, Seismograph is the recording device that produces a permanent record of Earth motion detected by a seismometer, in digital form which can be processed on a computer. The paper record of Earth’s vibration is called a seismogram. (can be used to measure strength of the earthquake) The order waves reach the seismograph stations around the world is 1. P waves, 2. S waves, 3. Surface waves A Travel-tme curve plots seismic wave arrival time against distance. There is an increase between the P-S wave arrival intervals with increasing distance. The maximum depth of focus (the distance between the foc
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