Earthquakes

An earthquake is shaking or movement of the Earth.

They are caused by plate tectonics.

Faults

Earthquakes occur on faults. Faults are fractures along blocks of rock

Faults allow the blocks to move relative to one another

Types of Faults

The direction and angle of the slip determines the type of fault.

The type of fault determines the type of earthquake.

During an earthquake, the rock on one side of a fault moves in relation to the other side. This is called slip.

Most faults produce repeated displacement of blocks over geologic time

Strike-Slip Faults

Strike-slip faults are nearly vertical fractures where the blocks have moved mostly horizontally.

strike slip animation

Strike-Slip Fault Example

Strike-slip faults occur in regions with transform boundaries, such as , the San Andreas Fault.

Bigger Faults=Bigger Earthquakes

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Normal FaultsNormal faults are faults

where one block has moved downward.

These faults are seen in areas where there are divergent boundaries

Normal Fault Animation

Normal Fault Examples

Valley-Fairview Peaks, Nevada earthquakeDecember 16, 1954

Reverse FaultReverse faults are

faults where one block moves upwards

Thrust faults are reverse faults movement of 45 degrees or less

Reverse Fault Animation

Reverse Fault Example

Reverse faults can be found in areas with convergent boundaries. There needs to be subduction zones for reverse faults to occur.

Magnitude

The magnitude of an earthquake is a measure of the amount of energy released by the earthquake.

Amount of Shaking

The amount of shaking is controlled by:

MagnitudeMore energy released

DistanceShaking decays with distance

Local soilsamplify the shaking

Seismic Waves

When an earthquake occurs, it releases energy in the form of seismic waves.

The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities.

Types of Waves

There are two categories of waves: Body waves travel through the

Earth’s interior

Surface waves travel along or near the earth’s surface.

Body WavesA P wave, or

compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

P Waves

The fastest wave, and therefore the first to arrive at a given location, is called the P wave. The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling

Body Waves

An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.

S Waves

The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling.

Types of Waves

There are two categories of waves: Body waves travel through the

Earth’s interior

Surface waves travel along or near the earth’s surface.

Surface Waves: Surface waves follow the P and S waves. A Rayleigh wave is

a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion.

Surface Waves

A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Magnitude

An estimate of the energy of an earthquake is called the Seismic Moment (Mo). Typically given in units of Newton-meters (Nm)

What factors contribute to the amount of energy released by an earthquake?

S = Surface Area of Fault (m2) (Fault length x Fault width)

D = Displacement or slip (m) µ = Rigidity of rock along the fault (N/m2)

Mo = S*D* µ