1. Earthquakes
Chapter 8

2. 8.1 What is an Earthquake?
Earthquake: vibration of Earth produced by the rapid release of energy
Most (NOT ALL) occur at faults
most faults and stress occurs along active plate tectonic boundaries
Focus: the point within Earth where the EQ starts; the source of the earthquake
Epicenter: location on the surface directly above the focus

3. Fault Types
Normal
Strike-Slip

4. Focus, Epicenter, and Fault

5. Causes of Earthquakes Elastic Rebound Hypothesis:
Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces
As rock is stressed, it bends, storing elastic energy. Once the rock is strained beyond its breaking point, it ruptures and releases the stored energy in the form of vibrations (seismic waves) of earthquakes

6. Elastic Rebound Hypothesis

7. Other causes of earthquakes:
Landslides, rockslides, or slumping of rocks.
Movement of magma, gases, or rocks associated with volcanism

8. Aftershocks and Foreshocks
Foreshocks: small earthquakes before a major earthquake
Can happen days or years before the major quake
Main shock: is the main earthquake disturbance generated at the focus
Aftershocks: movements that follow a major earthquake
Smaller than the major EQ
Can sometimes destroy structures weakened by the major earthquake

9. Where do earthquakes occur
Time for Excel Practice!
Log on to computers, open up excel spreadsheet from classroom website
Follow directions of excel spreadsheet
Do not worry about printing map, just call me over when you are done so I can check over
WHERE ARE MOST OF THE EARTHQUAKES OCCURRING?

10. Earthquake Zones 80% of all occur in circum-Pacific belt
Most result from convergent margin activity
15% occur in the Mediterranean –Asiatic belt
5% occur in the interiors of plates and on spreading ridge centers

11. Measuring EQs Seismographs: instruments that record EQ waves
Seismograms: traces of amplified, electronically recorded ground motion made by seismographs

12. Earthquake Waves Surface Waves (L waves):
Travel along Earth’s outer layers
Especially damaging to buildings
Most destructive of the three types of waves
Rolling and side-to-side movement (think of ocean wave movement)

13. Body Waves-Travel through Earth’s Interior
P (primary) waves
Push-pull motion
Compression waves- material is moved in the same direction as the wave moves
Fastest moving wave
Travel through solids, liquids, or gases
S (secondary) waves
Slower than p waves, faster than surface waves
Travel through solids only
Move material perpendicular (90 deg) to wave movement

14. Earthquake Waves

15. How is an Earthquake’s Epicenter Located?
Seismic wave behavior
P waves arrive first, then S waves, then L
Average speeds for all these waves is known
After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

16. Locating an Earthquake
Earthquake Distance:
Epicenter is located using the difference in the arrival times between P & S wave recordings, which are related to distance
We will use a reference table for these measurements
Earthquake Direction
Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter
We will practice this using a compass

17. Seismograph example
2:33:00
2:35:30
2:35:30 – 2:33:00 =
00:02:30

18. 2:35:10
2:39:20
2:39:20 – 2:35:10 =
00:04:10

19. We don’t know when the EQ started.
But we know how much time there was between the P&S wave arrivals.
Let’s say the difference is 0:04:00

20. We have to find a spot on the graph where P&S (careful
We have to find a spot on the graph where P&S (careful!) are separated by 4:00.

21. Then drop straight down to see the distance to the epicenter.
2,600km

22. Now that we know the EQ was 2,600km away, when did it start?
To travel 2,600km, a P-wave…
needs 5:00 minutes

23. 1,400km

24. The EQ happened somewhere on this line

25. Measuring Earthquakes
Two different types of measurements: intensity and magnitude
Intensity: based on observed effects of ground shaking on people, buildings, and natural features.
Varies from place to place w/in disturbed region depending on the location of the observer with respect to the EQ epicenter
Magnitude: related to the amount of seismic energy released at the hypocenter of the earthquake
Based on amplitude of earthquake waves recorded on instruments

26. Scales to Measure Earthquakes
Richter Magnitude Scale
Modified Mercalli Intensity Scale
Seismic waves are the vibrations from earthquakes that travel through the Earth
Recorded on seismographs
Richter Scale-1935
Base-10 logarthmic scale
Each unit=32 fold energy increase
Calculate combined horizontal amplitude
Range from 0-10
<2.0 not felt or recorded
: strong
: major
: great
10+: Epic (never recorded)
DOES NOT ADEQUATELY ESTIMATE THE SIZE OF VERY LARGE EQS!!
Developed in 1931
12 different levels
I-not felt except by very few
II-felt only by a few at rest
III-felt noticeably by persons indoor
IV-felt indoors by many, outdoors by a few
V-felt by nearly everyone, many awakened
VI-felt by all, some heavy furniture moved
VII-damage negligible in well-designed buildings
VIII-some chimneys broken
IX-buildings shifted off foundations
X-some well build wooden structures destroyed
XI-few structures remain standing
XII-total damage

27. Moment Magnitude
Derived from the amount of displacement that occurs along the fault zone
Most widely used measurements for EQs because it is the only magnitude scale that estimates energy released by earthquakes
Measures very large earthquakes
Calculated by different factors including:
Avg amt of movement along the fault (a)
Area of the surface break (b)
Strength of broken rock (c)
a x b x c=measure of how much energy rock can store before it slips and releases energy during an earthquake

28. Moment Magnitude Chart

29. Notable Earthquakes

30. Destruction from Earthquakes

31. Seismic Vibrations
The damage to buildings and other structures from earthquake waves depends on several factors.
Factors include:
intensity and duration of the vibrations
nature of the material on which the structure is built
design of the structure

32. Building Design Factors that determine structural damage
Intensity of EQ
Unreinforced stone or brick buildings
Most serious safety threat
Nature of material upon which structure rests
Design of the structure

33. Seismic Vibrations Liquefaction
Saturated material turns to fluid
Underground objects may float to surface
Occurs when loosely consolidated soils saturated with water are shaken by EQ waves

34. Tsunamis Japanese for seismic sea waves Causes: Triggered by an EQ
Occurs where slab of the ocean floor is displaced vertically along a fault
Can also occur when the vibration of a EQ sets an underwater landslide in motion

35. Movement of Tsunamis
A tsunami is generated by movement of the ocean floor. The speed of a wave moving across the ocean is related to the ocean depth

36. Tsunami Warning System
Large earthquakes are reported to Hawaii from Pacific Seismic Stations
Although tsunamis travel quickly, there is sufficient time to evacuate all but the area closest to the epicenter
On average, only 1-2 destructive tsunamis worldwide per year
On average only 1 tsunami every 10 years causes major damage and loss of life

37. Predicting Earthquakes
Short-Range Predictions
Not successful yet 
Long-Range Forecasts
Data can be important for updating building codes
Probability of EQ occurring within years
Scientists don’t yet understand enough about how and where earthquakes will occur to make accurate long-term predictions.
A seismic gap is an area along a fault where there has not been any EQ activity for a long period of time

38. Other Dangers Landslides Fire
Results from the violent shaking of EQs, causing the soil and rock on slopes to fall
The greatest cause of structural damage
Fire
In the San Francisco EQ of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut

39. US Earthquakes