Earthquakes

Earthquake Terms• An earthquake is a trembling of the

Earth caused by a sudden release of energy stored in subsurface rock units (on the Moon these are called moonquakes).

• Earthquake activity is also referred to as seismic activity.

• The energy is generally released along fault lines as sections of ground move.

Earthquake Terms

• The point of energy release is called the focus. The point on the Earth directly above the focus is called the epicenter.

• Surface waves move along the surface of the Earth while body waves penetrate the Earth’s interior. Surface waves cause most of the structural damage.

Waves

• The types and sizes of waves generated by an energy release can provide information about the location of the quake and the magnitude.

• Earthquakes are recorded by an instrument known as a seismograph.

Types of Waves

• Primary or P-waves – cause compression and dilation along axis of propagation

Types of Waves• Shear or S-waves – cause vertical

motion along axis of propagation

Seismographs• P-waves travel faster than S-waves.

Seismographs

• The time between the arrival of the P wave and S-wave is the S-P time interval. It is related to the distance of the detecting device from the epicenter.

S-P Interval

If you know the S-P interval you can determine the distance from the recording station to the epicenter.

When you find the distance from the epicenter for 3 stations, you can pinpoint the epicenter. You draw a circle from each station with a radius that is the distance to the epicenter for each station. Where the 3 circles intersect is the epicenter.

Magnitude• To determine the magnitude of an

earthquake you need the distance to the epicenter and the strength (amplitude) of the S-wave from the seismogram.

MagnitudeFor any seismic station, the distance to the epicenter and the amplitude of the S-wave will estimate the magnitude.

Using the nomogram shown here, draw a line connecting the distance and amplitude for a the station. Where the line crosses the magnitude scale is the approximate magnitude.

For a distance of 220 miles and an amplitude of 50 mm, the magnitude is 5.3.

Magnitude and the Richter Scale

Now…

• Back to the “Shake, Rattle, and Roll” activity and locate an earthquake yourself!