1. 9.3 Earthquake damage can be reduced
Chapter 9: Earthquakes
9.3 Earthquake damage can be reduced

2. Examine P and S waves moving through Earth's interior.

3.                                                                                                                                                                                                                                    
Figure 1: Left panel demonstrates elastic rebound. As rock is stressed, it bends, storing elastic energy. Once the stress acting on the fault is sufficient to overcome friction, the fault slips and the stored energy is released in the form of earthquake waves (graphic from Earth by Tarbuck & Lutgens). Photograph on right shows fence in Marin County offset by slip in the 1906 earthquake.

4. Earthquakes can cause sever damage and loss of life
On average, on 8 magnitude or higher earthquake strikes somewhere on Earth once a year
This destroys almost all buildings near its epicenter, plus a great loss of life
Damage and loss of life are greater where more people live, part due to injuries caused by collapsing buildings/structures and fires
Fires: broken natural-gas lines, electrical power lines, overturned stoves, etc.
Sand vs rock washer

5. Earthquake Magnitudes
One very powerful earthquake can release the equivalent of 1 million weak ones combined
Charles Richter: in the 1930’s developed the 1st earthquake scale: the Richter scale
Based on how fast the ground moves at a seismic station
Newer, more accurate scale: Moment Magnitude scale (developed in 1977)
Based on the total amounts of energy released

6. Earthquake Magnitude
Both Richter and moment magnitude scales are often shown with a top value of 10, but neither has a maximum value
Magnitude 5 releases 32 times as much energy as a magnitude 4 and about 1000 times as much as a magnitude 3
Magnitude needed to be noticed by people? 4
At what magnitude does an earthquake cause total destruction of even well-built structures?
8.0 and above
Show scales

7. Earthquake Magnitude Two advantages of moment magnitude scale:
Is more accurate for larger earthquakes than the Richter scale
Can be used for past earthquakes, before seismographs, by measuring the strength of the rocks and the length they moved along a fault

8. Damage from earthquakes
Blocks of rocks moving on either side of a fault can crack roads, buildings, dams…
Sometimes and earthquake can crack a building, and an aftershock can take it down
Aftershock: A smaller earthquake that follows a more powerful earthquake in the same area
Fires: 1906 San Francisco earthquake: fires caused more than 90% of the building damage

10. Damage from earthquakes
Landslides often occur following an earthquake
Can be through soil liquefaction (shaking of the ground causes soil to act as a liquid)
Occurs only in areas where the soil is made of loose sand and silt and contains a large amount of water
Structures either sink into the soil or float away
Five ways earthquakes can cause damage?
Movement of the ground, collapse of structures, fire, landslides, liquefaction

11. Formation of a Tsunami
When two plates come into contact at a region known as a plate boundary, a heavier plate can slip under a lighter one: subduction
Can form deep ocean trenches along the seafloor
In some cases of subduction, part of the seafloor connected to the lighter plate may "snap up" suddenly due to pressure from the sinking plate
This results in an earthquake
When this piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water
The energy pushes the water upward above normal sea level
The earthquake that generated the December 26, 2004, tsunami in the Indian Ocean was a 9.0 on the Richter scale -- one of the biggest in recorded history.

12. Damage from Tsunamis Waves of Destruction
A special type of wave can make water rise more than the height of a 20-story building
Tsunami: a water wave triggered by an earthquake, volcanic eruption, or landslide
Sometimes called “tidal waves” though are not caused by the same forces that produce tides
Can arrive as several different waves, heights, and hours apart
Can travel thousands of kilometers without weakening
Up to 700 km/hr (430 mi/hr) in deep water, and may only be 1 meter (~3 feet) in height
Slows when reaching shallow water, its height gradually increasing

13. Damage from Tsunamis
1946 earthquake on Alaska’ coast caused a tsunami that swept across the entire Pacific Ocean
Damaged a U.S. Coast Guard lighthouse in Alaska, which was then unable to send warnings
Five hours later reached Hawaii as a series of waves, the highest about 17 meters (55 ft)
Know one knew of the danger: 159 killed
Areas around the Pacific are likely to be hit, as many earthquakes occur in the region
Pacific Tsunami Warning Center (Hawaii, est. 1949)

14. Scientists work to monitor and predict earthquakes
Cannot yet predict the day or even year that an earthquake will occur
Can have signs even years before a strike
Typically give long-term predictions:
Ex: San Francisco has a 60% chance of being hit by a magnitude 7.0 or higher within the next 25 years
Map in book: risk of earthquake in US in next 50 years – based on past occurrences
Where is the highest risk?
Map of earthquake risk in US in next 50 years – based on past info
Map of earthquake risk in US in next 50 years – based on past info

15. Scientists work to monitor and predict earthquakes
Scientists monitory stress building along faults
Signs:
Tilts or changes in elevation of the ground
Slow movements or stretching in rock
The development of small cracks in the ground
Increase in small earthquakes can indicate stress is building up along a fault and a large one is likely
Can also indicated a fault releasing stress bit by bit
Look at a previously active fault for a lack of strikes
Seismic gap: an area of no earthquakes near many strikes
Can indicate a location where a fault is stuck, leading to stress

16. Taiwan
Seismic gap
Subduction

17. Question? / Answer.
Seismic gap image in book: What does the illustration on the left show?
A fault along which many earthquakes have occurred, except for one section
What is important about the section with little earthquake activity?
It might be stuck. A lot of stress may be building up there
What does the illustration on the right show?
A large earthquake and its aftershocks have filled in the gap

18. Structures can be designed to resist earthquake damage
Safety: good to be away from buildings, but we don’t always have the time
Structures are built appropriate for an area – to withstand flooding, landslides, or liquefaction, for example
Buildings can be shaken off their foundations
Firmly fasten them to foundations to prevent damage

19. Structures can be designed to resist earthquake damage
Need strong walls
add (steel) wall supports, especially to brick walls
Need to minimize amount a building shakes
Base isolators are placed between a building and its foundation
Are flexible materials stacked in layers like pancakes
Absorb much of the ground motion
Building may also have a moat (space) around it, allowing to shake more gently than the ground
Special walls: shear walls: add strength
Contain steel supports, or a shear core in the center
Strengthen walls: add braces in an X shape: “cross braces”

20. Q/A
What three special structures add support and strength to a building?
Shear walls, shear core, cross braces
What is a moat?
An open space around a building
Describe two methods used to make buildings stronger
Ex: buildings fastened to their foundations, wall supports, base isolators, moats, shear walls, shear cores, cross bases

21. Earthquake Energy: Math Skills
Seismologists use the moment magnitude scale to describe the energies of earthquakes
Compares heights of waves on a seimogram
To account for very weak and very strong earthquakes, the scale is designed to cover a wide range of energies
Each whole number increase in magnitude (1 unit of magnitude) represents release of about 32 times as much energy
0.1 increase in magnitude = 1.4 x more energy
Ex: a magnitude 5 earthquake releases about 32 times as much energy as a mag. 4
1  2  3  4  5  6  7  8  9  10
x x x32 x32 x32 x32 x x32 x32
Practice Problems 1-3, Challenge