Part VIII. Plate Tectonics II:

A. Geologic Structures (Chp. 9).

B. Earthquakes and Plate Tectonics (chp. 8).

1.) Movement on faults causes earthquakes.

a.) Earthquakes = shockwave due to sudden energy release.
b.) Energy released = seismic waves.

i) P-waves (primary): compressional, push-pull motion parallel to direction of wave movement.

• P-waves travel through solid rock+ liquid
• Fastest seismic wave
• Travel inside Earth (body wave)

ii) S-waves (secondary) = shear wave, vibration direction is perpendicular to direction of wave movement.

• S-waves travel though solids, but not liquids
• Overall, slower than P-waves
• Only travel inside Earth (body wave)

iii) Surface waves: travel on surface like a water wave.

• One type moves side-to-side and causes shaking and knocking building off foundations.
• The other type moves surface up and down in rolling, circular fashion like waves on the ocean.
• Surface waves are the most destructive type of seismic wave.
• Surface waves = slowest type .

iv) Measuring earthquakes: device used = seismograph.

Simple sketch of a seismogram.

vi.) Determining earthquake locations.

Three or more seismograms (stations A,B, and C above) from one earthquake event can be used to locate an earthquake epicenter.

The graph above shows the general relationship that results from the faster P-wave travel time relative to the S-wave. The farther the waves travel, the greater the time difference between the P and S waves.

The spacing between the P and S-waves of the seismograms can be aligned with the theoretical separation between the two waves. This allows the distance to the epicenter to be determined for each location. The three distances then can be used to triangulate to the location.

 

C. Earthquake Hazards.

1.) Gauging site.

a.) Magnitude: Richter Scale

i)  Scale : 1-10
ii.) Logarithmic (10x)
iii.) Below 3.4 on a Richter scale, earthquakes is not felt by most  people
iv.) > 8.0, widespread damage; 9.5 max. recorded (1960, Chile)

b.) Intensity scale: Modified Mercalli Scale

i.) Qualitative
ii.) Evaluated historical events.
iii.) Scale I > XII.

Mercalli scale I = barely felt by people….
Mercalli scale VII = chimneys break….

2.) Case Histories: more than 15 million people have died in earthquakes in recorded history.
            * Hazard/ death rate is increasing!

a.) Northridge Earthquake, 1994

i.) North of L. A., 4:30 am, 40 sec. long
ii.) 61 people killed, $10 billion in damage
iii.) mag. 6.4

 

b.) San Francisco, CA: 1906

i.) Magnitude 8.3 (estimated)
ii.) Occurred in early morning
* only 700 people killed
iii.) most damage occurred where buildings built on sediments.
iv.) Broken natural gas lines and water lines resulted in massive, uncontrollable fires that destroyed much of the city.

 

c.) New Madrid, Missouri: 3 events Dec 1811 + Jan + Feb 1812

i.) Largest (in terms of energy released) to hit continental U.S.
ii.) Large Earth Movements:

• ground moved like ocean wave
• Mississippi River changed course

d.) Lisbon, Portugal: 1755

i.) Between 9:00 am®noon on religious holiday (people killed in large church cathedrals)
ii.) > 30,000 killed, many by a Tsunami (“tidal wave”) (Large ocean wave caused by under water shock wave)
iii.) Lisbon quake changed popular beliefs:

* Natural occurrence
* Not due to “ unhappy supernatural” force

D. Worldwide Distribution of Earthquakes/Plate Boundaries.

• What is the relationship between earthquake occurrence and plate boundaries?
• Imaging the Interior of the Earth:

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	Diagram showing the change in P-wave velocity through the crust and mantle. The large drop in velocity that occurs in the upper mantle defines the asthenosphere (also known as the low velocity zone).

 

 

Check out this UC San Diego web site for an explanation of seismic waves (includes animations of the difference wave types)

Frankel and others, US Geological Survey (1997).

To explore how different parts of the U.S. could be affected by earthquakes, check out this U.S. Earthquake Hazards Map Site (from the USGS)

Also, to find more information about recent earthquakes, check out the USGS' Earthquake Page.

• Check out the amazing pictures of the recent earthquake on the Denali Fault in Alaska
• Why are there so many earthquakes in this region?
• Here is information on the 2004 earthquake and tsunami in Indonesia.
• Check out the Glossary for Earthquakes from the U.S. Geological Survey.