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.

Magnitude For 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!