1. Earthquakes!
Natural Disasters
Unit Five

2. Earthquakes and the Earth's Interior
The movement of earthquake waves through the Earth’s interior has given us a better picture of the Earth’s interior.

3. P-waves are faster than S-waves
P-waves are faster than S-waves. The density and elasticity of rock affects the speed of the waves.
When P-waves hit the core, they are refracted and slow down. This creates a P-wave shadow 103º -143 º from the focus

4. S-waves can’t travel through liquids.
When S-waves hit the outer liquid core, they stop. This creates a S-wave shadow at locations greater than 103º from the focus of the earthquake.

5. Speed of Seismic Waves:
*Boundary between crust and upper mantle (Moho)
*P-waves travel at 6.75 km/sec in the crust and 8 km/sec below this boundary.
*The depth varies depending if it is under continents ( km) or under ocean floors (5 to 10 km).
*Speed also increases with depth in the mantle except for a low velocity zone at km. This corresponds to the asthenosphere.

6. Determining an Earthquake's Epicenter
Focus: where “slippage” first occurs
Epicenter: directly above the focus on the Earth’s surface
The arrival times of P and S waves determines the distance to an earthquake

7. Seismograph Record

9. Where is the Epicenter?
The difference in arrival times of P and S waves determines the distance of the earthquake to a seismograph station.
A circle representing the distance is drawn around the station.
This is done with three stations, and where the circles cross is the exact location of the epicenter.

10. Earthquakes in the U.S.
But they are clustered along the Pacific Coast. Why?

12. Movement along the San Andreas Fault
Tectonic Creep is the slow continuous movement along a fault zone that is not accompanied by felt earthquakes.
Locked faults are sections that are not moving (locked due to friction). Pressure builds up in these sections until it overcomes the friction and the energy is released in an earthquake (elastic rebound).

14. Offset fence
Point Reyes

15. New Madrid Fault Zone 150 miles long
Illinois, Missouri, Arkansas, Kentucky, Tennessee
Faults formed when N.A. began to split (failed)
Flooded by ancient sea (covered by sediment)
Thousands of E-Q since 1970’s
: 4 of largest NA major E-Qs felt

16. Depth: C-O and O-O Convergent Boundaries
 Two lithospheric plates come together
Yellow squares indicate earthquake depth
Oceanic plate subducts beneath continental (more dense)
Older oceanic plate subducts beneath younger oceanic plate
Shallow to deep earthquakes

17. Depth: C-C Convergent and Transform
No subduction occurs when a continent collides with another continent (same density); folding and buckling occurs along the continental margins
Shallow earthquakes
Transform boundaries (plates slide past one another) also create relatively shallow earthquakes

18. Depth: Divergent Boundaries
Plates separate
Mid-Atlantic Ridge (MAR)
New crust formed
Shallow earthquakes
Generally less frequent

20. Intensity and Magnitude
Measuring Earthquake
Intensity and Magnitude
The strength of an earthquake can be measured in two different ways:
Intensity: qualitative assessment of the extent of damage done by an earthquake; subjective;
Modified Mercalli Intensity scale ( I to XII)
Magnitude: quantitative measurement of the amount of energy released by an earthquake;
Richter Magnitude scale (1 to 10);
each step in the Richter Scale is an increase
of 10-fold in movement and a 30-fold
increase in energy

21. Earthquake Effects

22. Fault Scarp -Central California
Linear valley
Linear ridge
Offset stream

23. Earthquake Damage depends on many factors:
The size of the Earthquake (magnitude)
The distance from the focus of the earthquake
The types and properties of the materials at the site
The nature of the building

24. Soil and Earthquake Damage:
Soil thickness: shallow soils may not perform as well as deep
Soil saturation: saturated soils perform less well than dry
Soil grain size and sorting: well-sorted, fine grained sands and silts are the most likely to liquefy
Types of bedrock: unweathered igneous rocks are better than weak fractured rock
Areas where ground may settle or slide
Nature of building and Earthquake Damage:
Type of construction (size and use of building)
Seismic design considerations
Architectural simplicity of building

25. Landslides Rock, earth or debris flows on slopes due to gravity
Major geologic hazard because widespread
$1-2 billions/year in damage
Fisheries, tourism, timber harvesting, mining and energy production impacted
As development , damage 

27. Fire
Appliances, furniture and gas, chemical and electrical hazards present
Water pools can  fire hazard
Leaking gas lines, damaged or leaking propane containers, and leaking gas tanks

28. San Francisco, 1906 Earthquake struck at 5:13 AM
“Liquified” ground beneath the city exacerbated potential for fires
Fires burned for four days

29. Tsunamis Occur when faulting or landslide occur on the ocean floor
Seafloor permanently uplifted or down-dropped, creating “sloshing” of water
Displaced water gains height as approaches shore
Floating debris and strong currents create damage

30. Indonesia, 12/26/2004
Picture taken by newlyweds vacationing when tsunami stuck; both were killed when tsunami came inland
The destruction of coral reefs along shorelines (for shrimp “farms”) increased destruction from tsunami as waves took longer to “break”

32. Earthquake Prediction
It is difficult to accurately predict earthquakes.
Analysis of past earthquake patterns, measurement of movement, and the distribution of faults have allowed scientists to create seismic risk maps.
Along the San Andreas fault, geologists are looking for areas along faults that are currently inactive. This probably means that these regions are locked and that energy is building up.
Tilting of rocks on either side of fault-lines is also a sign of pressure build-up.

34. Earthquake risk in the Bay Area
Green:
Stable bedrock
Orange:
Unstable bedrock
Yellow:
Unconsolidated
Soil
Red:
Mud and Fill