Transcript Earthquakes
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!