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Ch. 11 & 12 Notes

# Ch. 11 & 12 Notes - 1 Chapter 11(13(Earthquakes a...

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1. Chapter 11 (13) (Earthquakes) a. Introduction a.i. Earthquakes occur where rocks being stressed suddenly break along a new or pre-existing fault. (result of stress builds up over time) a.ii. Seismic waves are ground vibrations caused by rocks slipping along opposite sides of a fault. a.iii. Stress amount of pressure/force applied. Strain deformation on the formation. If no movement occurs, the fracture is called a joint. a.iv. As stress is applied to a body, energy is stored in the body as strain. When sufficiently high stress is applies, the body undergoes brittle failure (fracture). a.v. The stored strain energy is released when fracture occurs. This generates an earthquake. a.vi. Elastic rebound theory – 1. Fault at rest. 2. Buildup of strain. 3. Rupture of the fault. 4. Strain released. b. What is an Earthquake? b.i. What happens in an Earthquake? b.i.1. Focus (hypocenter)– very center of rupture. (Rupture expands circularly on fault plane, sending out seismic waves in all directions) b.i.2. Fault rupture – displacement along the fault trace, crack along the fault plane. (rock surface begin to rebound from their deformed state) b.i.3. Rupture has progressed along the entire length of the fault. The fault has reached its maximum displacement and earthquakes stop. b.ii. Focus and epicenter b.ii.1. Focus (hypocenter) - is the point along the fault at which fracture begins and is where the initial release of energy occurs b.ii.2. Epicenter – the point on the surface vertically above the focus. b.iii. Local buildup and release of stress b.iii.1. Foreshock – smaller earthquakes that occur before the actual movement of the fault/ rupture. b.iii.2. Aftershock – smaller earthquakes that occur after the actual movement, rupture has occurred. b.iv. Buildup and release of stress b.iv.1. Because there are many variables at each potential earthquake site stress may build up more quickly or less quickly and the strength of

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the fault may vary over time causing earthquakes to occur at varying times with varying amounts of slips. c. Studying earthquakes c.i. Seismographs are machines that record the seismic waves generated by earthquakes. c.i.1.
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