Teacher Suggestions

1. Explain a logarithmic scale.
 
 The Richter Scale is not an actual instrument. It is a measure of the amplitude of seismic waves and is related to the amount of energy released. This can be estimated from the recordings of an earthquake on a seismograph. The scale is logarithmic, which means that each whole number on the scale increases by 10. A magnitude 6.0 earthquake is 10 times greater than a 5.0, a 7.0 is 100 times greater, and a magnitude
 8.0 is 1,000 times greater.
 
 
2. Explain why P and S waves can be used to estimate the distance of an earthquake.
 
 Although wave speeds vary by a factor of ten or more in the Earth, the ratio between the average speeds of a P wave and of its following S wave is quite constant. This fact enables seismologists to simply time the delay between the arrival of the P wave and the arrival of the S wave to get a quick and reasonably accurate estimate of the distance of the earthquake from the observation station. Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to get the approximate distance in kilometers.
 
 
3. Explain how earthquake locations are estimated using 3 stations’ data.
 
 Given a single seismic station, the seismogram records will yield a measurement of the S-P time, and thus  the distance between the station and the event. Multiply the seconds of S-P time by 8 km/s for the kilometers of distance. Drawing a circle on a map around the station's location, with a radius equal to the
 distance, shows all possible locations for the event. With the S-P time from a second station, the circle
 around that station will narrow the possible locations down to two points. It is only with a third station's
 S-P time that you can draw a third circle that should identify which of the two previous possible points is
 the real one.
 
 
4. Describe how a tsunami is formed.
 
 A tsunami (pronounced tsoo-nah-mee) is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis.
 
 
5. Explain how water waves and tsunamis differ.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water
depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.