Types of Faults and seismic waves

What is a fault?

• A fault is a break in the rocks that make up the Earth’s crust.
• Plate boundaries count as faults.
• The way rocks move along faults are different.

GEOLOGICAL CONCEPTS�TYPES OF FAULT�Footwall/Hanging wall from Mining geology

Miners put their lamps on the hanging wall and

walk on the foot wall.

Three major Faults

Strike-slip

Reverse

Normal

Normal Fault

The hanging wall moves down relative to the foot wall.

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This fault type is caused by tensional stress.

Normal Fault

Normal Fault

Normal Fault

Reverse Fault

The hanging wall

moves up relative to

the foot wall.

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This fault is caused by

Compressional stress.

Reverse Fault

Reverse Fault

Reverse Fault

Strike-Slip Fault

The fault exists

between two pieces

of crust and the

Movement occurs

horizontally where

the sides slide past

each other.

This fault type is caused by shear stress.

Strike-Slip Fault

Strike Slip Fault

Strike-Slip Fault

Fault Movement = Earthquakes

• An earthquake is caused by a sudden slip on a fault. When the stress is applied and stored and suddenly released, there is an earthquake.
• Earthquakes release energy in waves that travel through the earth's crust and cause the shaking that we feel. �

Focus- where the slip happens below ground

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Epicenter-where the shaking is first felt above

ground directly above the focus.

• Slippage along fault initiates seismic waves in all directions radiating from the focus.
• Seismic waves are vibrations that travel through the Earth carrying the energy released during earthquakes

• There are two categories of seismic waves:
• Body Waves: Can travel through Earth’s inner layers (core, mantle, crust)-
• Primary and Secondary waves
• Surface Waves: Can only travel through the surface layers (crust)

Primary Waves

P waves- are the first waves to arrive at a seismograph station.

-fastest form of wave

• Moves through both liquids and solids.
• Waves rock particles to move back and forth in a push-pull.

Secondary Waves

S-Waves- arrive after the primary waves at the seismograph station.

-Moves through solids only.

-Rocks move back and forth at right angles to the line of wave movement

Surface Waves

• Can roll or shake

Measuring Earthquakes

• Magnitude- the amount of energy released during an EQ
• Richter Scale- based on the largest seismic waves generated by the EQ on a factor of 10.
• Moment Magnitude Scale- considers the size of the fault rupture, the amount of movement, and rock’s stiffness

SEISMOLGY�EARTHQUAKE LOCATION�Travel time versus distance plots

Can graph distance versus travel time for earthquakes

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Longer the distance, larger difference in arrival times between P wave and S wave.

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The 3 min 45 sec difference between the arrival time of the P and S waves corresponds to a distance of 2000 km.

SEISMOLOGY�LOCATING EARTHQUAKES�Triangulation

-Need three different seismic stations to locate an earthquake.

-Measure the P-S arrival times and convert these to distance (radius).

-Triangulate by drawing circles around three sites.

-Epicenter of earthquake is where all three circles connect