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Chapter 3

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Environmental Science
Nick Eyles

C HAPTER 3: E ARTHQUAKES What causes Earthquakes? * Earthquake – a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the surface o Forces acting deep in the Earth may put a stress on the rock, which may bed or change the shape (strain) * Seismic waves – waves of energy produced by an earthquake o Energy released when a rock breaks  The break between two rock masses is known as a fault o Cause the ground to tremble and shake during an earthquake * Elastic reboundtheory - involves the sudden release of progressively stored elastic strain energy in rocks, causing movement along a fault 1. Internal (tectonic) forces act on a mass of rocks over years 2. Rock bends, lifts or stretches initially, but does not break 3. Energy is stored in the rock as bending becomes more severe 4. Energy stored in the rock exceeds the breaking strength of the rock and the rock breaks suddenly, causing an earthquake o The released energy is expended by moving the rock into new positions and by creating seismic waves o Previously existing ideas suggested that a very large stress must act to break rocks along a fault, but new evidence suggests that faults are weak and only require a small stress to cause rupture and an earthquake o Some earthquakes are not connected movements on faults (most North American earthquakes) * The brittle/breaking behavior is characteristic of rocks near the earth’s surface while rocks at depth deform plastically (ductile) o Occurs due to increased temperature and pressure at depths * Another cause has recently been postulated for deep earthquakes, essentially all of which are found on cold, subducting plates sliding down into the mantle. o The suggested cause of deep quakes is mineral transformations within the down going rock, as pressure collapses one mineral into a denser form. Why do Earthquakes cause so much damage? * Focus (hypocenter) – point within the Earth where seismic waves first originate o Center of the earthquake, the point of initial breakage and movement on a fault o Epicenter – point on the Earth’s surface directly above the focus * Two types of seismic waves are generated during Earthquakes: o Body waves – seismic waves that travel through the earth’s interior, spreading outward from the focus in all directions  Two types of body waves:  P wave (primary) – a compressional (or longitudinal) wave in which rock vibrates back and forth parallel to the direction of wave propagation o Travels through near-surface rocks at speeds of 4-7 km/s o First wave to arrive at a recording station following an earthquake o Can pass through solid rock and fluid (gas or liquid)  S wave (secondary) – a slower transverse wave that travels through the near surface rocks at speeds of 2-5 km/s o Propagated by a shearing motion much like that in a stretched, shaken rope o Vibrates perpendicular to the direction of wave propagation o Can only pass through solid rock  o Surface waves – seismic waves that travel on Earth’s surface away from the epicenter  Slowest waves set off by earthquakes  Cause more property damage than body waves because surface waves produce more ground movement and travel more slowly, so they take longer to pass  Two types of surface waves:  Love wave – are most like S waves that have no vertical displacement o Groun
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