1. EARTHQUAKES Part II 

2. Measuring Earthquakes
Earthquake Waves
Measuring Earthquakes
 Seismographs are instruments that record earthquake waves.
 Seismograms are traces of amplified, electronically recorded ground motion made by seismographs.

3. Measuring Earthquakes
Locating an Earthquake
Measuring Earthquakes
 Earthquake Distance
• The epicenter is located using the difference in the arrival times between P and S wave
recordings, which are related to distance.
 Earthquake Direction
• Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter.
 Earthquake Zones
• About 95 percent of the major earthquakes occur in a few narrow zones.

4. Measuring Earthquakes
Historically, scientists have used two different types measurements to describe the strength of an earthquake
intensity (energy released)
magnitude (amount of motion produced)

5. Measuring Earthquakes
The Richter Scale
Developed in the 1930’s to measure the strength of an earthquake based on the amplitude (maximum movement) of the waves produced.
Every unit of magnitude represents 10X more movement
Not used much anymore because it didn’t really deal with the energy released during an earthquake and it topped out at a magnitude of 7, which was too small for large earthquakes.

6. Measuring Earthquakes
Moment Magnitude Scale
The Moment Magnitude Scale was developed in the 1970’s to more accurately describe the strength of an earthquake based on the energy released.
Like the Richter Scale each increase of 1 on the Moment Magnitude Scale is equivalent to a 10X increase in motion caused by the quake
Unlike the Richter Scale, the Moment Magnitude Scale gives a measure of energy.
An increase of 1 on the Moment Magnitude Scale corresponds to a 30X increase in energy released.
This scale also could be used to measure large earthquakes.

7. Earthquake Magnitudes
If your were comparing two earthquakes, one with a magnitude of 2 and another with a magnitude of 3 on the Moment Magnitude Scale, the larger earthquake would have 10X the motion as the smaller one and release 30X more energy.
If you were comparing an earthquake with a magnitude of 2 to an earthquake with a magnitude of 4 on the Moment Magnitude Scale, the larger earthquake would produce 100X more motion than the smaller one and release 900X more energy.

8. Earthquake Magnitudes