nature of earthquakes

types of seismic waves

There are two major types of seismic waves. Body waves travel through the Earths interior. Surface waves travel along the ground surface. In an earthquake, body waves are responsible for sharp jolts. Surface waves are responsible for rolling motions that do most of the damage in an earthquake.

subduction zones

Convergent plate boundaries also produce strong, deadly earthquakes. Earthquakes mark the motions of colliding plates and the locations where plates plunge into the mantle. These earthquakes can be shallow, intermediate or deep focus. The Philippine plate and the Pacific plate subduct beneath Japan, creating as many as 1,500 earthquakes every year. In March 2011, the 9.0 magnitude Tohoku earthquake struck off of northeastern Japan. Damage from the quake was severe. More severe was the damage from the tsunami generated by the quake (Figure 7.25). In all, 25,000 people were known dead or missing. The Cascades Volcanoes line the Pacific Northwest of the United States. Here, the Juan de Fuca plate subducts beneath the North American plate. The Cascades volcanoes are active and include Mount Saint Helens. Major earthquakes occur here approximately every 300 to 600 years. The last was in 1700. Its magnitude was between 8.7 and 9.2. It has now been more than 300 years since that earthquake. The next massive earthquake could strike the Pacific Northwest at any time.

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transform plate boundaries

Transform plate boundaries produce enormous and deadly earthquakes. These quakes at transform faults have shallow focus. This is because the plates slide past each other without moving up or down. The largest earthquake on the San Andreas Fault occurred in 1906 in San Francisco. Other significant earthquakes in California include the 1989 Loma Prieta earthquake near Santa Cruz (Figure 7.23) and the 1994 Northridge earthquake near Los Angeles. There are many other faults spreading off the San Andreas, which produce around 10,000 earthquakes a year (Figure

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tsunami

Earthquakes can cause tsunami. These deadly ocean waves may result from any shock to ocean water. A shock could be a meteorite impact, landslide, or a nuclear explosion. An underwater earthquake creates a tsunami this way: The movement of the crust displaces water. The displacement forms a set of waves. The waves travel at jet speed through the ocean. Since the waves have low amplitudes and long wavelengths, they are unnoticed in deep water. As the waves reach shore they compress. They are also pushed upward by the shore. For these reasons, tsunami can grow to enormous wave heights. Tsunami waves can cause tremendous destruction and loss of life. Fortunately, few undersea earthquakes generate tsunami.

the boxing day tsunami 2004

The Boxing Day Tsunami struck on December 26, 2004. This tsunami was by far the deadliest of all time (Figure registered magnitude 9.1. The quake struck near Sumatra, Indonesia, where the Indian plate is subducting beneath the Burma plate. It released about 550 million times the energy of the atomic bomb dropped on Hiroshima. Several tsunami waves were created. The tsunami struck eight countries around the Indian Ocean (Figure 7.31). About 230,000 people died. More than 1.2 million people lost their homes. Many more lost their way of making a living. Fishermen lost their boats, and businesspeople lost their restaurants and shops. Many marine animals washed onshore, including dolphins, turtles, and sharks.

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focus

The point where the rock ruptures is the earthquakes focus. The focus is below the Earths surface. A shallow earthquake has a focus less than 70 kilometers (45 miles). An intermediate-focus earthquake has a focus between 70 and 300 kilometers (45 to 200 miles). A deep-focus earthquake is greater than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage. This is because the focus is near the Earths surface, where people live.

earthquake focus and epicenter

Where an earthquake takes place is described by its focus and epicenter.

earthquake zones

Nearly 95% of all earthquakes take place along one of the three types of plate boundaries. As you learned in the Plate Tectonics chapter, scientists use the location of earthquakes to draw plate boundaries. The region around the Pacific Ocean is called the Pacific Ring of Fire. This is due to the volcanoes that line the region. The area also has the most earthquakes. About 80% of all earthquakes strike this area. The Pacific Ring of Fire is caused by the convergent and transform plate boundaries that line the Pacific Ocean basin. About 15% of all earthquakes take place in the Mediterranean-Asiatic belt. The convergent plate boundaries in the region are shrinking the Mediterranean Sea. The convergence is also causing the Himalayas to grow. The remaining 5% of earthquakes are scattered around the other plate boundaries. A few earthquakes take place in the middle of a plate, away from plate boundaries.

epicenter

The area just above the focus, on the land surface, is the earthquakes epicenter (Figure 7.22). The towns or cities near the epicenter will be strongly affected by the earthquake.

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intraplate earthquakes

About 5% of earthquakes take place within a plate, away from plate boundaries. These intraplate earthquakes are caused by stresses within a plate. The plate moves over a spherical surface, creating zones of weakness. Intraplate earthquakes happen along these zones of weakness. A large intraplate earthquake occurred in 1812. A magnitude 7.5 earthquake struck near New Madrid, Missouri. This is a region not usually known for earthquakes. Because very few people lived here at the time, only 20 people died. The New Madrid Seismic Zone continues to be active (Figure 7.26). Many more people live here today.

seismic waves

Seismic waves are the energy from earthquakes. Seismic waves move outward in all directions away from their source. Each type of seismic wave travels at different speeds in different materials. All seismic waves travel through rock, but not all travel through liquid or gas. Geologists study seismic waves to learn about earthquakes and the Earths interior.

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causes of earthquakes

Almost all earthquakes occur at plate boundaries. All types of plate boundaries have earthquakes. Convection within the Earth causes the plates to move. As the plates move, stresses build. When the stresses build too much, the rocks break. The break releases the energy that was stored in the rocks. The sudden release of energy creates an earthquake. During an earthquake the rocks usually move several centimeters or rarely as much as a few meters. Elastic rebound theory describes how earthquakes occur (Figure 7.21).

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divergent plate boundaries

Earthquakes also occur at divergent plate boundaries. At mid-ocean ridges, these earthquakes tend to be small and shallow focus because the plates are thin, young, and hot. Earthquakes in the oceans are usually far from land, so they have little effect on peoples lives. On land, where continents are rifting apart, earthquakes are larger and stronger.

body waves

Primary waves (P-waves) and secondary waves (S-waves) are the two types of body waves (Figure 7.28). Body waves move at different speeds through different materials. P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles) per second. Primary waves are so named because they are the first waves to reach a seismometer. P-waves squeeze and release rocks as they travel. The material returns to its original size and shape after the P-wave goes by. For this reason, P-waves are not the most damaging earthquake waves. P-waves travel through solids, liquids and gases. S-waves are slower than P-waves. They are the second waves to reach a seismometer. S-waves move up and down. They change the rocks shape as they travel. S-waves are about half as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves can only move through solids. This is because liquids and gases dont resist changing shape.

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continentcontinent collisions

The collision of two continents also creates massive earthquakes. Many earthquakes happen in the region in and around the Himalayan Mountains. The 2001 Gujarat, India earthquake is responsible for about 20,000 deaths, with many more people injured or made homeless.

tsunami warning systems

Most of the Indian Ocean tragedy could have been avoided if a warning system had been in place(Figure 7.32). As of June 2006, the Indian Ocean now has a warning system. Since tsunami are much more common in the Pacific, communities around the Pacific have had a tsunami warning system since 1948. Warning systems arent always helpful. People in communities very close to the earthquake do not have enough time to move inland or uphill. Farther away from the quake, evacuation of low-lying areas saves lives.

surface waves

Surface waves travel along the ground outward from an earthquakes epicenter. Surface waves are the slowest of all seismic waves. They travel at 2.5 km (1.5 miles) per second. There are two types of surface waves. Love waves move side-to-side, much like a snake. Rayleigh waves produce a rolling motion as they move up and backwards (Figure 7.29). Surface waves cause objects to fall and rise, while they are also swaying back and forth. These

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tilly smith hero

Like other waves, a tsunami wave has a crest and a trough. When the wave hits the beach, the crest or the trough may come ashore first. When the trough comes in first, water is sucked out to sea. The seafloor just offshore from the beach is exposed. Curious people often walk out onto the beach to see the unusual sight. They drown when the wave crest hits. One amazing story from the Indian Ocean tsunami is that of Tilly Smith. Tilly was a 10-year-old British girl who was visiting Maikhao Beach in Thailand with her parents. Tilly had learned about tsunami in school two weeks before the earthquake. She knew that the receding water and the frothy bubbles at the sea surface meant a tsunami was coming. Tilly told her parents, who told other tourists and the staff at their hotel. The beach was evacuated and no one on Maikhao Beach died. Tilly is credited with saving nearly 100 people!

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wave structure

Seismic waves are just one type of wave. Sound and light also travel in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The horizontal distance between waves from crest to crest (or trough to trough) is its wavelength (Figure 7.27).

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instructional diagrams

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questions

The energy released by an earthquake travels in seismic waves.

-->  a. true

b. false

Only transform plate boundaries have earthquakes.

a. true

-->  b. false

Earthquakes deep underground cause the most damage.

a. true

-->  b. false

Earthquakes at mid-ocean ridges tend to be small and shallow.

-->  a. true

b. false

Seismic waves travel outward in all directions from their source.

-->  a. true

b. false

All seismic waves travel at the same speed through solid rock.

a. true

-->  b. false

P-waves are the first seismic waves to reach a seismometer.

-->  a. true

b. false

All undersea earthquakes generate tsunamis.

a. true

-->  b. false

The deadliest tsunami of all time occurred in 2004 in Indonesia.

-->  a. true

b. false

Tsunamis are more common in the Atlantic Ocean than the Pacific Ocean.

a. true

-->  b. false

high point of a wave

a. earthquake

b. body wave

-->  c. crest

d. surface wave

e. epicenter

f. focus

g. trough

seismic wave that travels along the surface of the ground

a. earthquake

b. body wave

c. crest

-->  d. surface wave

e. epicenter

f. focus

g. trough

point below the surface where an earthquake begins

a. earthquake

b. body wave

c. crest

d. surface wave

e. epicenter

-->  f. focus

g. trough

low point of a wave

a. earthquake

b. body wave

c. crest

d. surface wave

e. epicenter

f. focus

-->  g. trough

ground movement caused by the sudden release of energy stored in rocks

-->  a. earthquake

b. body wave

c. crest

d. surface wave

e. epicenter

f. focus

g. trough

point on the surface above the focus of an earthquake

a. earthquake

b. body wave

c. crest

d. surface wave

-->  e. epicenter

f. focus

g. trough

seismic wave that travels below Earths surface

a. earthquake

-->  b. body wave

c. crest

d. surface wave

e. epicenter

f. focus

g. trough

According to elastic rebound theory, earthquakes occur when stresses on rocks become too great and the rocks

a. become deformed.

b. fracture and form a joint.

-->  c. return to their original shape.

d. are permanently stretched out of shape.

The focus of a shallow earthquake is

-->  a. less than 70 km below the surface.

b. between 70 and 300 km below the surface.

c. more than 300 kilometers below the surface.

d. none of the above

About 80 percent of all earthquakes take place

a. in the state of Californi

b. at divergent plate boundaries.

-->  c. along the Pacific Ring of Fire.

d. far from plate boundaries.

The wavelength of a wave can be measured by finding the distance between

a. the focus and the epicenter.

b. a P wave and an S wave.

c. a crest and a trough.

-->  d. two adjacent crests.

Seismic waves that do the most damage are

a. body waves.

-->  b. surface waves.

c. primary waves.

d. secondary waves.

Surface waves that produce a rolling motion are

a. P waves.

b. S waves.

c. love waves.

-->  d. Rayleigh waves.

Tsunamis may be caused by

a. landslides.

b. meteorites.

c. nuclear explosions.

-->  d. all of the above

diagram questions

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