# I. How do Earthquakes happen?

### A. Most earthquakes happen when rocks fracture deep within Earth

1. When stress exceeds strength of the rocks, a fracture occurs

a. compression is stress that decreases the volume of the rock

b. tension is stress that pulls the rock apart

c. shear is stress that causes a material to twist

2. Fault: fracture in Earth's crust that occurs when stress is applied too quickly or stress is too great

a. reverse faults result from compressional stress (stress pointing toward the fault line from opposite sides)

b. normal faults result from tension stress (stress pulls rock material apart from opposite sides)

c. strike-slip faults result from shear stress. The movement of the surface of the Earth is generally horizontal. The San Andreas Fault in California is a strike-slip fault.

### B. Seismic waves

1. primary waves: P-waves cause rock particles to move back and forth as the wave passes (occur in the Earth's interior)

2. seconday waves: S-waves cause rocks to move at right angles to the direction of the wave (occur in the Earth's interior)

3. surface waves: particles move up and down and side to side (occur at the Earth's surface)

4. focus: point where earthquake originates, all waves spread out from this point

5. epicenter: Earth's surface directly above the focus on the fault line

# II. Measuring Earthquakes

### A. Seismometers amd siesmograms are instruments used to detect and record earthquake activity

1. P and S-waves are recorded with these instruments, P-waves always are detected first

2. Detection of P and S-waves at sites far from the epicenter of a quake have lead scientists to some information about the core of the Earth

a. P-waves are refracted when they hit the outer and inner core (indicates the core is liquid)

b. S-waves do not enter Earth's core (again, indicates the core must be liquid)

### B. Earthquake Magnitude and Intensity

1. Magnitude: the amount of energy released during an earthquake

a. Richter scale describes the magnitude of an earthquake with the increase in number on the scale corresponding to an increase in magnitude of 10-fold

b. Moment magnitude scale is used by siesmologist to measure magnitude

2. Intensity of an earthquake is the amount of damage done to structures

a. modified Mercalli intensity scale rates the types of damage and other effects as noted by observers during and after an earthquake

b. intensity depends on the amplitude of the surface waves

### C. Locations of Earthquakes

1. Most earthquakes occur at tectonic plate boundaries

2. A small percent occur far from plate boundaries

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