Earthquake Origin Causes and effects

Lab

goals• In an earthquake, will all parts of a city suffer the same

damage?• What factors may affect the damage caused by an

earthquake?• What is the additional hazard of an underwater

earthquake?• Describe the effects on life and property from

consequences of earthquakes such as landslides, liquification, surface faulting, and tsunamis.

• Cite ways these hazards can be minimized.• Describe ways in which people use historical data, geologic

maps, and technologies to minimize earthquake damage.

Each students gets a white Textbook

• When you learned about the Loma Prieta earthquake, you learned about some of the destruction around the city. In your textbook, turn to page G-158. Look at the three seismograms taken from different areas of the city during that quake

Answer the questions in your journal

1. At which seismic station was the quake felt most intensely? How can you tell?

2. At which seismic station was the least intensity measured? How can you tell?

3. Look at the map next to the seismograms. What is the underlying geology at the station registering the greatest intensity?

4. What is the underlying geology at the station registering the least intensity?

5. Turn to page G-160 and find the Loma Prieta quake in the event column. What geologic changes occurred as a result of this earthquake?

Liquefaction

• Have you ever been to the beach and let the surf wash over your feet? As each wave goes out, it takes the sand under your feet with it. The sand and the water seem to act as one. This is similar to the effect that is noted when earthquake waves pass through loose soil or sand that is saturated with water. The soil and water act as a fluid together in an effect known as liquifaction

Results of liquefaction

• This creates an unstable foundation for buildings, and is responsible for heavy damage to them. Looking back at the Loma Prieta map, you can see that the area of artificial fill near the waterfront showed the most intense quake, and the area of beach dunes showed a similar high intensity.

Part B

• Read pages G-141 to 144 to find out more about earthquake intensity measurement and how the underlying geology of an area contributes to the damage caused by the quake.

Answer these questions in your journal:

1. How does the underlying geology of an area affect the intensity of an earthquake there?

2. Is the Mercalli Scale a good choice for scientists? Why/ why not?

3. Why do scientists use the Modified Richter scale to describe the intensity of an earthquake instead of the original Richter scale?

4. How is the measured amplitude of a quake with a magnitude of 7 different from a quake with a magnitude of 5?

5. What other factors affect the damage done during an earthquake?

Part C

• Why do buildings break during earthquakes but trees do not?

• What can be done to earthquake proof buildings?

Vibrations and natural frequency• Let’s review some of the things we have learned about energy and

materials. We have learned that everything vibrates at its own natural frequency. That frequency depends on the type of material and the structure of the object made from it. A simple object made from a single substance, like a guitar string, can be tuned to vibrate at a specific frequency when plucked. When objects are made from a variety of materials and have complex structures, like buildings, the vibrations become far more complex. A building may have a natural frequency, or parts of the building may have their own natural frequencies. The places where pieces join together become an important part of the structure, and also affect the ability of the whole building to vibrate as a unit or in sections.

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Molecular springs

• We have also learned that molecules are held together by bonds that resemble tiny springs. Some materials have the property we call elasticity that enables the bonds to deform and return to their original shape. Other materials are inelastic. They do not deform readily, and are more likely to break if force is applied to their bonds. Concrete is an example of a material that is inelastic.

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Mass and Motion

• Why do objects move? We know that unbalanced forces are needed to begin motion. We know that the heavier an object is, the more mass it has, and the more force is needed to start it moving. We recognize that the mass of an object is the property that is important in its ability to resist the forces that are applied to it.

Watch video section

• Engineering segment of the video Shock Waves to learn how engineers are developing construction methods to reduce the damage to buildings during earthquakes.

• As you watch the video, look for specific construction techniques used and why they are effective.

Answer these questions in your journal.

1. What are three modifications made to existing structures to help them withstand earthquakes? (How do they help?)

2. What are two types of foundation modifications for new buildings

help prevent damage from vibration due to earthquakes? 3. In the computer simulation of City Hall, how is the building

vibration with the isolators different from its vibration without them?

4. What is the value of a shake table to an engineer? 5. Why would the gas company replace cast iron pipe with plastic

pipe?

Part D: Tsunamis

• December 26, 2004 – the world is shocked at the news of a killer tsunami that engulfed the shores of countries along the Indian Ocean. One survivor’s story warmed the hearts of people everywhere.

• A school girl from Britain saved numerous lives because she recognized the signs of a tsunami and got her family and many others to run for safety in time.

What is a Tsunami

• Imagine a very large basin of water that is suddenly shaken.

• What will happen to the water? • It will slosh from side to side. • When the water level is rising to the edge on

one side of the basin, it is shallower on the other side

Dec 26, 2004

• Now imagine an incredibly big basin, one the size of the Indian Ocean.

• On one side of the ocean, over near Sumatra, a 750 mile section of the ocean bottom suddenly shifted about 50 ft. (about 5 stories on a building) due to an earthquake.

• The water in the ocean sloshed from side to side, creating an enormous wave known as a tsunami.

Energy transfer

• The energy that caused the movement of land is transferred to the waves it created in the water

• It take enormous energy to move land, so an enormous amount of energy is transferred to the Tsunami

Tsunami in the ocean

• Out in the ocean, where the water is deep, the water wave generated by an earthquake does the same thing. If you are on a ship out in the middle of the ocean, you don’t notice the enormous amount of energy being carried by the wave.

As the Tsunami reaches the shore…

• as the wave gets close to shore, it can no longer go as deep as it did before. This creates great crests that tower over the beaches and crash inland. These crashing waves behave like the gentle waves you see at the beach, but they carry much more energy and do far more damage.

The force of a Tsunami wave

• If you have ever been knocked down by a wave at the beach, you have some understanding of the force of a wave. A wave 10 times that size would slam you into the beach so hard you would never be able to get up again. It would tear up trees and knock down buildings. The withdrawing wave would drag out to sea anything that was not extremely well anchored.

What not to do if your at the leading edge of a Tsunami

• Sometimes, people are on the receding side of a tsunami before the wave comes in. People have gathered on the newly open beach to pick up stranded fish and been unable to escape when the wave came in. This occurred in Thailand and Sri Lanka.

Secondary effects of Tsunamis

• In addition to the deaths directly caused by the tsunami, the devastation to water supplies is an invitation to diseases like dysentery and cholera. Standing water provides breeding places for insects that carry other diseases. Salt water inundation of fields causes crop failure. While all tsunamis are not as devastating as this one was, they are a very serious secondary effect of some earthquakes.

Do all earthquakes create Tsunamis?Are all Tsunamis created by

earthquakes?

• Not every earthquake produces a tsunami, and not every tsunami is caused by an earthquake. Any major disturbance of the water can cause such a wave to develop. Volcanic eruptions like the 1906 Mount Pelée event create mudslides that reach the ocean and cause local tsunamis.