EARTHQUAKES

Section 19.1Forces within Earth

• Most earthquakes happen because the Earth’s crust moves along plate boundaries

• Usually the movement is very slight because the rocks resist the movement

• This causes stress to build up

Stress

• Is the total force acting on crustal rocks per unit of area

• Movement occurs when stress overcomes the strength of the rocks

• There are three kinds of stress– Compression is the stress that decreases the

volume of a material– Tension is the stress that pulls a material

apart– Shear is the stress that twists a material

Strain

• The deformation of a material in response to the stress

• Rocks can take bending, twisting and stretching to a point. That critical point is when the rocks break releasing energy (earthquake)

Elastic Deformation

• Caused when a material is compressed, bent or stretched

• Generally is temporary• Once stress is reduced to zero, materials

return to normal

Plastic Deformation

• Caused when stress builds up past a critical point

• Rocks are permanently deformed

• Most materials will display both elastic and plastic deformation behavior—the amount of stress and strain determine which

Faults

• Any fracture or system of fractures along which the Earth moves.

• The surface along which the earth moves is called the fault plane.

• There are several types of fault: reverse, normal and strike slip

San Andreas Fault Line

Reverse Faults

• Form due to horizontal and vertical compression

• Squeezes the rock and causes a shortening of the crust

• Rock on one side pushes up relative to the other side

Normal Faults

• Movement is partly horizontal and partly vertical

• Horizontal movement pulls rocks apart and stretches the crust

• Vertical movement occurs as stretching causes rock on one side to move down relative to other side

Strike Slip Fault

• Caused by horizontal shear• Movement is mainly horizontal and in

opposite directions

Earthquake Waves

• Seismic waves are the vibrations of the ground that occurs during an earthquake

• Three main types of seismic waves– Primary waves– Secondary waves– Surface waves

Primary Waves

• Also called P waves• Squeeze and push rocks in the direction

along which the waves are traveling• Movement is similar to that of a coiled wire

—the movement is back and forth movement

Secondary Waves

• Also called S waves• Slower than P waves so they are felt

second hence the name• Their motion cause rocks to move a right

angles in direction of waves

Surface Waves

• Third type and slowest of waves• Travel only along Earth’s surface • Can cause ground to move sideways and

up and down like ocean waves• Cause the most destruction because they

take the longest time to pass and cause the most movement of the ground

Focus and Epicenter

• The focus is the where the waves originate; usually several km below Earth’s surface

• The epicenter is the point on Earth’s surface above the focus

• Surface waves originate from the epicenter and spread out

Learning Targets

• Define stress and strain as they apply to rocks.

• Distinguish among the three types of movement of faults.

• Contrast the three types of seismic waves.

19.2 Seismic Waves and Earth’s Interior

• Most vibrations of Earth can’t be felt far away from the epicenter

• But a seismometer can detect even slight movement

• A seismometer is a sensitive instrument that measures Earth motion

• Produces a seismogram a paper or computer graph of the movement

Clues to Earth’s Interior

• Seismic waves change speed and direction at the boundaries between different materials

• P waves and S waves initially travel through the mantle following direct paths

• P waves move through mantle and are refracted when they strike the core (they bend)

• S waves will not travel through liquid and do not travel through the core

Learning Targets

• Describe how a seismometer works.• Explain how seismic waves have been

used to determine the structure and composition of Earth.

19.3 Measuring and Locating Earthquakes

• Richter scale• Moment magnitude scale• Modified Mercalli scale

Learning Targets

• Compare and contrast earthquake magnitude and intensity and the scales to measure each.