Earthquake

EARTHQUAKESEARTHQUAKES

Earthquakes are the shaking, Earthquakes are the shaking, rolling or sudden shock of the rolling or sudden shock of the earth’s surface. Earthquakes earth’s surface. Earthquakes happen along "fault lines" in the happen along "fault lines" in the earth’s crust. Earthquakes can be earth’s crust. Earthquakes can be felt over large areas although felt over large areas although they usually last less than one they usually last less than one minute. minute.

Earthquakes cannot be Earthquakes cannot be

predicted -- although predicted -- although

scientists are working on scientists are working on it! it!

Most of the time, you will notice an Most of the time, you will notice an earthquake by the gentle shaking of the earthquake by the gentle shaking of the ground. You may notice hanging plants ground. You may notice hanging plants swaying or objects wobbling on shelves. swaying or objects wobbling on shelves. Sometimes you may hear a low rumbling noise Sometimes you may hear a low rumbling noise or feel a sharp jolt. A survivor of the 1906 or feel a sharp jolt. A survivor of the 1906 earthquake in San Francisco said the sensation earthquake in San Francisco said the sensation was like riding a bicycle down a long flight of was like riding a bicycle down a long flight of stairsstairs

The intensity of an earthquake can be The intensity of an earthquake can be measured. One measurement is called the measured. One measurement is called the Richter scale. Earthquakes below 4.0 on the Richter scale. Earthquakes below 4.0 on the Richter scale usually do not cause damage, Richter scale usually do not cause damage, and earthquakes below 2.0 usually can’t be and earthquakes below 2.0 usually can’t be felt. Earthquakes felt. Earthquakes

over 5.0 on the scale can cause over 5.0 on the scale can cause damage. A magnitude 6.0 damage. A magnitude 6.0 earthquake is considered strong earthquake is considered strong and a magnitude 7.0 is a major and a magnitude 7.0 is a major earthquake. The Northridge earthquake. The Northridge Earthquake, which hit Southern Earthquake, which hit Southern California in 1994, was magnitude 6.7. California in 1994, was magnitude 6.7.

Earthquakes often occur when tectonic plate Earthquakes often occur when tectonic plate collidecollide

What happens when plates collide? It depends how What happens when plates collide? It depends how the plates are moving when they meet:the plates are moving when they meet:

When two plates collide head-on, they push each When two plates collide head-on, they push each other up and form mountains. That's how the other up and form mountains. That's how the Himalayas and other great mountain ranges Himalayas and other great mountain ranges (including the Rockies, long ago) were created.(including the Rockies, long ago) were created.

When one plate dives below another plate, it When one plate dives below another plate, it creates a subduction zone as the diving plate is creates a subduction zone as the diving plate is crushed and melted. This process often creates crushed and melted. This process often creates volcanoes as the magma (molten rock) rises up volcanoes as the magma (molten rock) rises up to the to the

surface. surface.

When two When two plates plates slide past slide past each each other, they other, they create a create a transform transform fault, like fault, like the San the San Andreas Andreas fault.fault.

Earthquakes can happen in any of these Earthquakes can happen in any of these situations. Despite the powerful forces situations. Despite the powerful forces driving plate movement, the plates driving plate movement, the plates themselves spend much of the time themselves spend much of the time locked in place by the friction of the locked in place by the friction of the plates rubbing against each other. plates rubbing against each other. Eventually, however, they build up so Eventually, however, they build up so much much

pressure that the plates pressure that the plates

abruptly snap forward. abruptly snap forward.

Then the ground can shift Then the ground can shift

a few feet—or a few dozen! a few feet—or a few dozen!

Shock waves from that Shock waves from that

sudden motion shoot out sudden motion shoot out

in all directions, creating an earthquake.in all directions, creating an earthquake.

Two great plates, the Pacific and the North Two great plates, the Pacific and the North American, meet in California. The Pacific Plate American, meet in California. The Pacific Plate is moving north, creating a transform fault (the is moving north, creating a transform fault (the San Andreas and related faults) Over the last San Andreas and related faults) Over the last 20 million years the 20 million years the

Pacific Plate has slid about Pacific Plate has slid about

200 miles north. If it keeps 200 miles north. If it keeps

moving as predicted, San moving as predicted, San

Francisco will become Francisco will become

neighbors with Seattle in neighbors with Seattle in

20 million years!20 million years!

Because the San Andreas Because the San Andreas fault curves around Los fault curves around Los Angeles, and then again Angeles, and then again into the Pacific in northern into the Pacific in northern California, the two plates California, the two plates cannot slide smoothly cannot slide smoothly against each other. against each other. Instead, the complex Instead, the complex stresses of plate stresses of plate movement have fractured movement have fractured the land and created the land and created dozens of smaller fault dozens of smaller fault lines.lines.

Seismologists have been Seismologists have been studying California's studying California's faults for decades. They faults for decades. They now say that the San now say that the San Francisco Bay Area has a Francisco Bay Area has a 70% chance of a major 70% chance of a major earthquake before 2030. earthquake before 2030. This forecast is based on This forecast is based on years of study of the many years of study of the many faults in the area. The faults in the area. The map shows the probability map shows the probability of a quake from each of of a quake from each of these faults. these faults.

The rate of large earthquakes in the San Francisco Bay region The rate of large earthquakes in the San Francisco Bay region abruptly dropped after the Great 1906 Earthquake. The San abruptly dropped after the Great 1906 Earthquake. The San Andreas Fault slipped so much over such a great length in that Andreas Fault slipped so much over such a great length in that quake that the strain was reduced on most faults throughout quake that the strain was reduced on most faults throughout the region. Strain has been slowly building up again.the region. Strain has been slowly building up again.

Earthquakes can also occur within plates, Earthquakes can also occur within plates, although plate-boundary earthquakes are although plate-boundary earthquakes are much more common. Less than 10 percent of much more common. Less than 10 percent of all earthquakes occur within plate interiors. all earthquakes occur within plate interiors. The New Madrid earthquakes of 1811-1812 and The New Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake occurred the 1886 Charleston earthquake occurred within the North American plate.within the North American plate.

Earthquakes: Facts and Fiction Earthquakes: Facts and Fiction Fiction: Earthquakes usually happen in the Fiction: Earthquakes usually happen in the

morning. morning. Fact: Earthquakes happen in both the day and Fact: Earthquakes happen in both the day and the night. There is no pattern. the night. There is no pattern.

Fiction: There is such a thing as "earthquake Fiction: There is such a thing as "earthquake weather." weather." Fact: There is no connection between Fact: There is no connection between earthquakes and weather. Remember, earthquakes and weather. Remember, earthquakes happen deep in the earth, far away earthquakes happen deep in the earth, far away from the weather! from the weather!

Fiction: Earthquakes are on the increase. Fiction: Earthquakes are on the increase. Fact: It may seem like we’re having more Fact: It may seem like we’re having more earthquakes because there are more reporting earthquakes because there are more reporting stations, but the truth is we’re not. stations, but the truth is we’re not.

Fiction: We can prevent earthquakes from Fiction: We can prevent earthquakes from happening. happening. Fact: No. You can protect yourself by doing Fact: No. You can protect yourself by doing things to secure buildings, like your home, but things to secure buildings, like your home, but earthquakes can’t be prevented -- or predicted. earthquakes can’t be prevented -- or predicted.

The point beneath the Earth's The point beneath the Earth's surface where the rocks break surface where the rocks break and move is called the and move is called the focusfocus of of the earthquake. The focus is the the earthquake. The focus is the underground point of origin of underground point of origin of an earthquake. an earthquake.

Directly above the Directly above the

focus, on the Earth's focus, on the Earth's

surface, is the surface, is the

epicenter.epicenter.

Earthquake waves are known as Earthquake waves are known as seismic waves. There are three seismic waves. There are three main types of seismic waves. main types of seismic waves.

Each type of wave Each type of wave

has a characteristic has a characteristic

speed and manner speed and manner

of travel.of travel.

Primary waves.Primary waves. Seismic waves that travel the Seismic waves that travel the

fastest are called primary fastest are called primary waves, or P waves. P waves waves, or P waves. P waves arrive at a given point before arrive at a given point before any other type of seismic wave. any other type of seismic wave. P waves travel through solids, P waves travel through solids, liquids and gases.liquids and gases.

P waves are P waves are push-pull wavespush-pull waves. As P waves . As P waves travel, they push rock particles into the travel, they push rock particles into the particles ahead of them, thus compressing the particles ahead of them, thus compressing the particles. The rock particles then bounce particles. The rock particles then bounce back. They hit the particles behind them that back. They hit the particles behind them that are being pushed forward. The particles move are being pushed forward. The particles move back and forth in the direction the waves are back and forth in the direction the waves are moving.moving.

Secondary WavesSecondary Waves Seismic waves that do not travel through the Seismic waves that do not travel through the

Earth as fast as P waves do are secondary waves, Earth as fast as P waves do are secondary waves, or S waves. S waves arrive at a given point after or S waves. S waves arrive at a given point after P waves do. S waves travel through solids but P waves do. S waves travel through solids but not through liquids and gases.not through liquids and gases.

Surface WavesSurface Waves The slowest-moving seismic waves are The slowest-moving seismic waves are

called surface waves, or L waves. L waves called surface waves, or L waves. L waves arrive at a given point after primary and arrive at a given point after primary and secondary waves do. L waves originate at secondary waves do. L waves originate at the epicenter. Surface waves travel along the epicenter. Surface waves travel along the surface of the earth, rather than down the surface of the earth, rather than down into the earth. Although they are the into the earth. Although they are the slowest of all the earthquake waves, L slowest of all the earthquake waves, L waves usually cause more damage than P waves usually cause more damage than P or S waves.or S waves.

This is an image of a seismograph, an This is an image of a seismograph, an instrument used to record the energy instrument used to record the energy released by an earthquake. When the released by an earthquake. When the needle is moved by the motion of the needle is moved by the motion of the earth, it leaves a wavy line.earth, it leaves a wavy line.

Blue primary waves followed by red Blue primary waves followed by red secondary waves move outward in secondary waves move outward in concentric circles from the epicenter of concentric circles from the epicenter of an earthquake off British Columbia and an earthquake off British Columbia and Washington State.Washington State.

Thank you