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!
