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1. Measuring Earthquakes
3. Most earthquakes take place along faults in the upper 25 miles of the earth's surface
sudden motion causes shock waves (seismic waves) to radiate from their point of origin called the focus
detected by sensitive scientific instruments called seismographs
4. A seismic wave is simply a means of transferring energy from one spot to another within the earth
P (primary) waves
similar to sound waves,
move back and forth
can travel through
solids and liquids
6 and 13 km/sec
S (secondary) waves
a kind of shear wave,
moves perpendicularly to the
direction the wave is traveling;
only travel through solids
travel between 3.5 and 7.5 km/sec
can be much more destructive
6. A highly simplified simulated recording of earthquake waves (a seismogram)
7. seismograms are made by a seismograph (a pen or needle suspended over a slowly progressing drum covered with paper)
9. severity of an earthquake can be expressed in several ways
magnitude: measure of the amplitude of the seismic waves (usually expressed by the Richter Scale)
intensity: a subjective measure that describes how strong a shock was felt at a particular location (expressed by the Modified Mercalli Scale - values ranging from I to XII )
10. The Richter Scale developed in 1935 by Charles F. Richter of the California Institute of Technology
each whole number increase in magnitude represents a tenfold increase in measured amplitude
terms of energy, each whole number increase corresponds to an increase of about 31 times the amount of energy released
15. The Modified Mercalli Scale measures intensity
a way of measuring or rating the effects of an earthquake at different sites
Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end
20. Strike-slip Fault Example
21. Kobe, Japan 1995
22. Surface Faulting - Landers, CA 1992
25. Damage to the Intercontinental Hotel during Mexico City's 1985 earthquake was severe even though the building was relatively new
26. estimate the magnitude of the tiny earthquake we can make on a lab table
27. you will now conduct a Virtual Earthquake
follow the directions on the website and then answer the following:
Student Discussion Questions:
What do we call the place in the earth's crust where an earthquake originates?
What are the 2 main types of earthquake waves? How do they differ?
What two factors affect the speed at which earthquake waves travel?
Why is the difference in arrival time between S and P-type waves important to seismologists?
Explain why reports from 3 seismic stations are needed to determine an earthquakes epicenter. Why are 2 not enough?