Chapter 2: Earthquakes

THE BIG IDEA:  

Earthquakes release stress that has built up in rocks.

KEY CONCEPTS:

2.1 Earthquakes occur along faults.

2.2 Earthquakes release energy.

2.3 Earthquake damage can be reduced.

2.3 Earthquake damage can be reduced.

I. Earthquakes can cause severe damage and loss of life.

A. Every year an extremely powerful earthquake, with a magnitude of 8 or higher, strikes somewhere on Earth.

B. Earthquakes are most dangerous when the epicenter is near a heavily populated area.

1. Most injuries are not caused by the movement of the ground.

2. They are caused by collapsing buildings and structure fires due to broken natural-gas lines, electrical lines, etc.

II. Earthquake Magnitude

A. Measured using one of two scales.

1. Richter Scale- measures magnitude by how fast the ground moves at a seismic station

2. Moment magnitude scale- based on the total amount of energy released by earthquakes; more accurate than Richter (page 61)

III. Damage from Earthquakes

A. Structures weakened during earthquakes collapse during shaking caused by aftershocks.

1. Aftershock- a smaller earthquake that follows a more powerful earthquake in the same area

2. Fires that break out can be big problems if water lines that firefighters use to get water are broken. 90% of the buildings damaged in the 1906 San Francisco earthquake were burned.

B. Landslides often occur as a result of earthquakes.

C. Liquefaction- a process in which shaking of the ground causes soil to act like a liquid

IV. Damage from Tsunamis

A. Tsunami- a water wave triggered by an earthquake, volcanic eruption, or landslide

1. These special waves will rise more than the height of a 20 story building.

2. Sometimes called tidal waves, but they are not caused by the same forces that cause the tides.

B. Tsunamis can travel thousands of miles without weakening.

C. They can reach speeds of 430 mi/hr.

D. In the open ocean the tsunami may only reach about 3 ft. in height. As they get closer to the shore they slow down and gain height. (page 63, Alaskan earthquake)

E. The Pacific Tsunami Warning Center, Hawaii monitors earthquakes and issues warnings to areas that could be struck.

V. Scientists work to monitor and predict earthquakes.

A. Scientists cannot yet predict the day or even the year when an earthquake will strike.

B. Sometimes there are warning signs, sometimes none at all.

C. Scientists give long term predictions. They may state that an area has a 60% chance of having a magnitude 7 or higher earthquake in the next 25 years.

D. U.S. earthquake risks

E. Scientists study seismic activity along faults all over the world. They monitor stress that is building up in rocks.

F. Signs of stress buildup

1. Tilts or changes in elevation of the ground.

2. Slow movements or stretching of rocks.

3. Development of small cracks in the ground.

VI. Structures can be designed to resist earthquake damage.

A. Read pages 66-67 in book

1. Describe two methods used to make buildings stronger.