Earthquakes

What is an earthquake?

What is an earthquake?

An earthquake is a trembling or shaking of the earth’s crust.

Most earthquakes occur because of a sudden movement at a fault.

A fault is a break or a crack in the earth’s crust where there is constant pressure or movement.

How does an earthquake happen?

1. The constant movements of the earth’s crust produce great pressure and tension which may then result in a sudden movement at a fault.

2. This sudden movement (which may result in the breaking of rocks) produces tremendous energy.

Where the sudden movement or breaking of rocks occurs is called the focus. The area at ground level, above the focus, is called the epicenter.

3. The energy produced is carried along by seismic waves (vibrations) through the earth.

4. As the waves pass, the earth shakes and trembles.

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What are the different types of seismic waves?

1. Body waves – travel through the earth’s interior

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a. P waves (primary or compressional waves) – fastest; move by compression and expansion; travel through the solid and liquid parts of the earth

b. S waves (secondary or shear waves) – slower than P waves; move perpendicular to the source; travel only through solids

What are the different types of seismic waves?

2. Surface waves – travel through the earth’s surface; slower than body waves; travel through great distances; cause the most damage

a. Love waves

b. Rayleigh waves

How are earthquakes detected?

Seismograph (seismometer) – measures and records vibrations produced by earthquakes

*seismogram – recorded data

How are earthquakes detected?

ELF sensors –magnetometers that detect extremely low frequency (ELF) magnetic fluctuations which are produced days or weeks before an earthquake; used at ground level and on board satellites

QuakeSat

How are earthquakes detected?

Global Positioning System (GPS) – a navigation and precise-positioning tool used in earthquake studies to measure tectonic motions during and in between earthquakes

a GPS satellite

Visit brainpop for a movie about GPS (see technology)!

How is earthquake size categorized?

Intensity – a qualitative measure of the effects of an earthquake at a particular place on humans, structures and the land itself; used as the basis of the Mercalli Scale

Magnitude – a quantitative measure of the strength of an earthquake or energy released by it, as determined by seismographic observations; used as the basis of the Richter Scale

The Modified Mercalli Intensity Scale

I. People do not feel any Earth movement.

II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.

III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.

IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

The Modified Mercalli Intensity Scale

V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.

VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

The Modified Mercalli Intensity Scale

VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture break. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.

VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.

The Modified Mercalli Intensity Scale

IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.

X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.

The Modified Mercalli Intensity Scale

XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.

XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

The Richter Scale

based on a logarithmic value originally defined by Charles Richter (1935)

an increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released

The Richter Scale

DescriptorRichter Magnitudes

Earthquake Effects

Frequency of Occurence

Micro Less than 2.0Microearthquakes, not felt.

About 8,000 per day

Very minor 2.0-2.9Generally not felt, but recorded.

About 1,000 per day

Minor 3.0-3.9Often felt, but rarely causes damage.

49,000 (estimated) per year

DescriptorRichter Magnitudes

Earthquake Effects

Frequency of Occurence

Light 4.0-4.9

Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.

6,200 (estimated) per year

Moderate 5.0-5.9

Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.

800 per year

Strong 6.0-6.9

Can be destructive in areas up to about 100 miles across in populated areas.

120 per year

DescriptorRichter Magnitudes

Earthquake Effects

Frequency of Occurence

Major 7.0-7.9Can cause serious damage over larger areas.

18 per year

Great 8.0-8.9

Can cause serious damage in areas several hundred miles across.

1 per year

Rare Great 9.0 or greater1 per 20 years

The 1990 Baguio City Earthquake

Go to http://www.geocities.com/lingayenbay/earthquake.html

The End