Chapter 11 Earthquakes
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 it’s source, the focus
Energy is in the forms of waves, sensitive instrument around the world record the
event
Earthquakes and Faults
Movements that produce earthquakes are usually associated with large fractures in
Earth’s crust called faults
Most of the motion along faults can be explained by the plate tectonics theory
Elastic rebound
Mechanism for earthquakes was first explained by H.F. Reid
• 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) occurs as the deformed rock “springs back” to its original
shape (elastic rebound)
Earthquakes most often occur along existing faults whenever the friction forces on the
fault surface are overcome
Foreshocks and Aftershocks
Adjustments that follow a major earthquake often generates smaller earthquakes called
aftershocks
Smaller 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 studies fault system in the world
Displacement occurs along discrete segments 100 to 200 kilometers long
some portions exhibit slow, gradual displacement known as fault creep
other segments regularly slip producing small earthquakes
Displacement 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
• great...