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Faults & Earthquakes Deforming the Earth’s Crust
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Faults & Earthquakes Deforming the Earth’s Crust.

Jan 12, 2016

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Lester Flynn
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Page 1: Faults & Earthquakes Deforming the Earth’s Crust.

Faults & Earthquakes

Deforming the Earth’s Crust

Page 2: Faults & Earthquakes Deforming the Earth’s Crust.

Deformation• The process by which the shape of a rock

changes because of stress is called deformation.

• When rock deforms in a plastic manner, it folds like a piece of molded clay.

• With elastic deformation, the rock stretches like a rubber band until it breaks.

• Elastic deformation can lead to earthquakes.

• There are two basic types of deformation:– Plastic deformation– Elastic deformation

Page 3: Faults & Earthquakes Deforming the Earth’s Crust.

Stress

• Stress is a force that acts on rock to change its shape or volume.

• Because stress if a force, it adds energy to the rock.

• This energy is stored in the rock until the rock either breaks, or changes shape.

Page 4: Faults & Earthquakes Deforming the Earth’s Crust.

There are three types of stress that occur in the Earth’s crust:

• Compression

• Tension

• Shearing

Page 5: Faults & Earthquakes Deforming the Earth’s Crust.

Compression• The stress force

called compression squeezes rock until it folds, or breaks.

• Compression makes a mass of rock occupy a smaller space.

• When compression occurs at a convergent boundary, large mountain ranges can form.

Page 6: Faults & Earthquakes Deforming the Earth’s Crust.

Tension

• The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle.

• Tension occurs where two plates are moving apart, such as mid-ocean ridges, or rift valleys.

Page 7: Faults & Earthquakes Deforming the Earth’s Crust.

Shearing• Stress that pushes

a mass of rock in two opposite, horizontal directions is called shearing.

• Shearing can cause rock to break and slip apart.

• Shearing occurs at transform boundaries.

Page 8: Faults & Earthquakes Deforming the Earth’s Crust.
Page 9: Faults & Earthquakes Deforming the Earth’s Crust.

Folding• The bending of rock

layers because of stress in the Earth’s crust is called folding.

• Undisturbed rock layers are horizontal, so when we see a fold we know that deformation has taken place.

Page 10: Faults & Earthquakes Deforming the Earth’s Crust.

Folds

• The two most common types of folds are:

– Anticlines

– Synclines

• Anticlines are upward-arching folds.

• Synclines are downward, trough-like folds.

• Another type of fold is a monocline. • In a monocline, both ends of the fold

are horizontal.

Page 11: Faults & Earthquakes Deforming the Earth’s Crust.

Faults• When the stress on rocks causes them to break and

slip past each other, a fault is formed.

• The blocks of crust on each side of the fault are called fault blocks.

• When faults are not vertical one side of the fault block will be called a hanging wall and the other the footwall.

• The position of the fault block will determine which it is.

Page 12: Faults & Earthquakes Deforming the Earth’s Crust.

Faults• There are 3 main

types of faults:

– Normal fault

– Reverse, or thrust fault

– Strike-slip

Reverse Fault

Page 13: Faults & Earthquakes Deforming the Earth’s Crust.

Normal Faults• Tension forces cause normal faults.

• The hanging wall lies above the fault and the footwall lies below the fault.

• When movement occurs along the fault line, the hanging wall slips downward.

• Normal faults are found at divergent plate boundaries, where plates pull apart.

Page 14: Faults & Earthquakes Deforming the Earth’s Crust.

Reverse Faults• Compression forces produce reverse faults.

• Reverse faults have the same basic structure as a normal fault, but the blocks move in the opposite direction.

• When movement occurs along the fault line, the hanging wall slides up and over the footwall.

• Reverse faults are found at convergent plate boundaries, where plates are pushed together.

Page 15: Faults & Earthquakes Deforming the Earth’s Crust.

Strike Slip Faults

• Shearing creates strike-slip faults.

• The rocks on either side of the fault slip past each other sideways with little up or down motion.

• A strike-slip fault that forms the boundary between two plates is called a transform boundary.

Page 16: Faults & Earthquakes Deforming the Earth’s Crust.

Fault Block Mountain

• When the tension in a normal fault uplifts a block of rock, a fault-block mountain forms.

• The Grand Tetons in Wyoming are an example of a fault-block mountain range.

Page 17: Faults & Earthquakes Deforming the Earth’s Crust.

Folded Mountains

• Folded mountains form at convergent boundaries where continents have collided.

• The Appalachian Mountains, the Alps, and the Himalayas are examples of folded mountains.

Page 18: Faults & Earthquakes Deforming the Earth’s Crust.

Earthquakes

• An earthquake is the shaking and trembling that results from the movement of rock beneath Earth’s surface.

• Not all Earthquakes occur at plate boundaries. Sometimes they happen in the middle of a tectonic plate.

• Earthquakes can happen both near the Earth’s surface or far below it.

Page 19: Faults & Earthquakes Deforming the Earth’s Crust.

• Earthquakes always begin in rock below the surface.

• Most earthquakes begin in the lithosphere within 100 kilometers of the surface.

• The focus is the point beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake.

• The point on the surface directly above the focus is called the epicenter.

Page 20: Faults & Earthquakes Deforming the Earth’s Crust.

Seismic Waves• Seismic waves are vibrations that travel through Earth

carrying the energy released during an earthquake.

• Seismic waves that travel through the Earth are called body waves.

• Seismic waves that travel along Earth’s surface are called surface waves.

• Each type of seismic waves travels through Earth’s layers in a different way and at a different speed.

Page 21: Faults & Earthquakes Deforming the Earth’s Crust.

Body Waves

• There are two types of body waves:– P waves– S waves

Page 22: Faults & Earthquakes Deforming the Earth’s Crust.

P-waves

• The first waves detected in an earthquake are p waves, or pressure waves.

• P waves compress and expand the ground like an accordion.

• P waves can travel through solids, liquids, or gases.

Page 23: Faults & Earthquakes Deforming the Earth’s Crust.

S waves

• After P waves come secondary waves, or S waves.

• S waves are earthquake waves that vibrate from side to side and thrust the ground up and down, or back and forth.

• When S waves reach the surface, they shake structures violently.

• S waves cannot move through liquids.

Page 24: Faults & Earthquakes Deforming the Earth’s Crust.

Surface Waves• When P and S waves

reach the surface, some of them are transformed into surface waves.

• Surface waves move more slowly than P and S waves, but they produce the most severe ground movements.

• They can actually make the ground roll like ocean waves.

• Other surface waves shake the ground from side to side.

Page 25: Faults & Earthquakes Deforming the Earth’s Crust.

All 3 waves

Page 26: Faults & Earthquakes Deforming the Earth’s Crust.

• Geologists use a seismograph to record and measure the vibrations of seismic waves.

Seismograph

Seismogram

Detecting Seismic Waves

• When the waves reach a seismograph, the instruments creates a seismogram.

• A seismogram is a tracing of earthquake motion.

• Until recently, scientists used mechanical seismographs, like the one in the picture.

• Today they use electronic seismographs that convert ground movements into a signal that can be recorded and printed.

Page 27: Faults & Earthquakes Deforming the Earth’s Crust.

Finding the Epicenter• Scientists use seismograms to find the earthquakes epicenter.

• One method they use is called the S-P Time Method.

• They collect readings for the same earthquake from seismographs stations at different locations.

• They then use this data to determine the distance each station is from the earthquake.

• They can then triangulate the results to find the epicenter.

• It takes a minimum of 3 seismograph readings to find the epicenter of and earthquake.

Page 28: Faults & Earthquakes Deforming the Earth’s Crust.

Richter Scale• There are many ways that scientists can measure an earthquake.

• Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults.

• Charles Richter created the Richter magnitude scale in the 1930s to compare earthquakes by measuring ground motion and adjusting for distance to find their strength.

• When magnitude increases by one unit the measured ground motion becomes 10 times larger on the Richter scale.

• The Richter scale provides accurate measurements for small, nearby earthquakes, but the scale does not work well for large, or distant earthquakes.

Page 29: Faults & Earthquakes Deforming the Earth’s Crust.

Richter Scale no.

No.of earthquakes per

year

Typical effects of this magnitude

< ¾ 800,000 Detected only by seismometers

3.5 - 4.2 30,000 Just about noticeable indoors

4.3 - 4.8 4.800 Most people notice the, windows rattle.

4.9 – 5.4 1,400 Everyone notices the, dishes may break, open doors swing.

5.5 – 6.1 500 Slight damage to buildings, plaster cracks, bricks fall.

6.2 – 6.9 100 Much damage to buildings; chimneys fall, houses move on foundations.

7.0 – 7.3 15 Serious damage; bridges twist, walls fracture, buildings may collapse.

7.4 – 7.9 4 Great damage, most buildings collapse.

> 8/0 One every 5 to 10 years

Total damage, surface waves seen, objects thrown in the air.

Page 30: Faults & Earthquakes Deforming the Earth’s Crust.

Mercalli Scale

• Seismologists can also measure the intensity of an earthquake.

• The Mercalli scale uses Roman numbers from I to XII to describe increasing earth quake intensity levels.

• The Modified Mercalli Intensity Scale is used to measure the degree to which an earthquake is felt by people and the amount of damage done by it.