Geology Notes Chapter 11: Earthquakes 15:41 What is an earthquake? An earthquake is the vibration of earth, produced by the rapid release of energy Energy released radiates in all directions from its source, the focus Energy is in the form of waves Sensitive instruments around the world record the event Earthquakes and faults Movements that produce earthquakes are usually associated with faults Elastic rebound Rocks on both sides of an existing fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome Earthquake mechanism Slippage at the weakest point (the focus) occurs Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape ( elastic rebound ) Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome Foreshocks and aftershocks Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks , often precede a major earthquake by days or, in some cases, by as much as several years San Andreas: An Active Earthquake Zone San Andreas is the most studied fault system in the world
Displacement occurs along discrete segments 100 to 200 km long Some portions exhibit slow, gradual displacement known as fault creep Other segments regularly slip, producing small earthquakes Displacements along the San Andreas fault Still other segments store elastic energy for hundreds of years before rupturing in great earthquakes Process described as stick-slip motion Sticks for a long time, then slips Great earthquakes should occur about every 50 to 200 years along these sections Seismology The study of earthquake waves, seismology , dates back to almost 2000 years to the Chinese Seismographs are instruments that record seismic waves Records the movement of earth in relation to a stationary mass on a rotating drum or magnetic tape More than one type of seismograph is needed to record both vertical and horizontal ground motion Records obtained are called seismograms Types of seismic waves Surface waves – travel along outer part of the earth Complex motion Cause greatest destruction Exhibit greatest amplitude and slowest velocity Waves have the greatest periods (time interval between crests) Often referred to as long waves, or L waves
Body waves – travel through earth’s interior Two types based on mode of travel Primary (P) waves Push-pull (compress and expand) motion, changing the volume of the intervening material Travel through solids, liquids, and gases Generally, in any solid material, P waves travel about 1.7 times faster than S waves
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