1. Features of Earthquakes
Chapter 11, Section 2

2. Focus
The point on a fault where rocks “break” and energy is released.

3. Epicenter
Point on the surface of the Earth directly above the focus.

4. Seismograph
Instruments that register seismic waves and record horizontal and vertical ground movement.
Produce a paper record of the seismic event that is called a seismogram.

5. Magnitude
The height (or amplitude) of the lines on a seismogram indicate the magnitude or strength of an earthquake.
Most commonly measured with the Richter magnitude scale.
For each increase of 1.0 on the scale:
The height of the line on the seismogram is 10x greater.
The amount of released energy is 32x greater.
A magnitude 9.5 earthquake is the largest ever recorded (southern Chile – 1960).

6. Locating an Epicenter
Because s and p waves travel at different speeds, they each reach a seismograph station at different times.
The difference in time can be converted into a distance.
Using three seismograph stations, the location of the epicenter can be determined through triangulation.

7. Tips for Your Assignment:
Finding amplitude of S-wave:
The distance between the center line and the highest peak on the seismogram.

8. Tips for Your Assigment:
Finding S-P lag time:
Time between the start of the p-wave and the start of the s-wave.

9. Tips for Your Assignment:
Finding distance:
Use the chart to convert s-p lag time into distance.

10. Tips for Your Assignment:
Finding magnitude:
Line up distance and s-wave amplitude to determine magnitude.

11. Tips for Your Assignment:
Finding epicenter location:
Draw a circle from each station with a radius equal to the distance of the station from the epicenter.
Epicenter is located where the three circle cross.