SeismologySeismology

I. Earthquakes

Rocks will bend and stretch up to a limit undergoing elastic deformation.

A fault is created when the rock finally breaks.

An earthquake is the vibrations produced by the breaking of the rock.

1. Normal Faults

Tensional forces pull the rock apart.

Rock above the fault surface moves downward compared to the rock below the fault surface.

A. Types of Faults

2. Reverse Fault

Compression forces squeeze the rock.

Rock above the fault surface moves upward and over the rock below the fault surface.

3. Strike-Slip Fault

Shear forces tear the rock.

Rocks on either side of the fault are moving past each other without upward or downward movement.

B. Seismic Waves

Waves generated by earthquakes that travel through the Earth.

1. Origin of Seismic Waves

a.) Focus- the point where the energy is first released.

Below the surface along the fault.

b.) Epicenter- point on the surface of the Earth directly above the focus

Epicenter

Focus

Fault line

2. Types of Seismic Waves

a.) Primary Waves (P-Waves)

• Particles in the rock move back and forth in the same direction the wave is traveling.

• Travels trough solids, liquids, & gases

• Fastest seismic waves

b.) Secondary Waves (S-Waves)

• Particles in the rock move up and down at right angles to the direction of wave travel.

• Travels through solids only.

c.) Surface Waves (L-Waves)

• Particles in the rock move in a backward, rolling motion and a side-to-side, swaying motion.

•Causes the most damage.

•Originate from the epicenter.

3. Locating an Epicenter

a.) Seismograph

•Register the waves & record the time that each arrived.

P WavesS Waves

L Waves

Time between P & S waves = 3 minutes

Time for entire quake = 8 minutes

1 min

Seismogram

• Data from three separate

stations are required to

locate the epicenter of

an earthquake.

• The epicenter distance

is figured out using the time

difference between the P & S

waves graph.

II. Earthquake Activity

A. Magnitude• The amount of energy released in an

earthquake.

•Richter Scale- a numeric scale used to describe the strength (magnitude) of an earthquake.

•Mercalli Scale- scale that describes the amount of damage.

B. Liquefaction•Wet soil can be strong most of the

time, but the shaking from an earthquake can cause it to act like a liquid.

• This causes soil to sink into the ground and cause things to collapse.

C. Tsunamis•Under water earthquakes cause the

water to shake forming large waves.

• Seismic sea waves

•Wave crests can reach 30m in height by the time it reaches shore.