Earthquakes: What are they and what causes them to happen?

What is an Earthquake? An earthquake is a shaking of the

ground caused by a sudden release of energy in the Earth’s crust. More than a million of them occur each year! However, most are too small to be felt or to cause damage.

What are What are Earth’s Layers?Earth’s Layers?The Earth’s crust is the outer

layer and is made of rock. It is very thin compared to the other layers.

The mantle is the very hot layer of rock beneath the crust. No one has ever been to the mantle, but rock from the mantle sometimes reaches the Earth’s surface through volcanoes.

The outer core is made of liquid, or molten iron.

The inner core is made of solid iron. Even though the core is the hottest layer, great pressure at the center of the Earth keeps the inner core solid.

Inner Core

Outer core

Mantle Crust

What are Plate Tectonics? Earth’s surface is made

up of many plates that float on the soft rock of the mantle. As the mantle moves, the plates also move.

Plates move only a few centimeters each year.

When one plate moves, it affects the other plates.

As plates move around, they cause great changes in the Earth’s surface, such as mountains, valleys, volcanoes, and earthquakes!

What Happens When Plates Move?

Plate Tectonics

Convergent Plate Boundaries

Divergent Plate Boundaries

Transform Boundaries

plates crunch together and release

energy

mountains develop, volcanoes erupt, and

earthquakes can happen

plates pull apart andnew crust is formed

valleys and volcanoesdevelop, earthquakes

can occur

plates slide past each other

Lots of earthquakes!

Types of Boundaries

What is a Fault?

Many earthquakes occur along faults in the Earth’s crust. A fault is a place where pieces of the plates move.

A normal fault is where tension weakens the crust until the rock fractures, and one rock moves downward from another. This occurs when two plates are pulling apart, as in a divergent boundary.

A strike-slip fault is where two blocks of rock are moving past each other horizontally, as in a transform boundary. The famous San Andreas Fault is a strike-slip fault.

A reverse fault is where a fault block is forced upward, usually during a collision with

another block, as in a convergent boundary.

What Happens When an What Happens When an Earthquake Starts?Earthquake Starts?

The sudden release of energy from an earthquake sends out several different shaking movements, or seismic waves.Earthquakes generate two types of waves: Primary (P) waves and Secondary (S) waves.

What Happens When an What Happens When an Earthquake Starts?Earthquake Starts? Surface waves are ripples of energy

that spread outward when rocks slip past each other along a fault, just like throwing a stone into a calm pond.

Body waves are seismic waves that travel through material rather than over its surface. There are two types of body waves: P-waves and S-waves.

The P-wave is also known as the sound wave. It travels through the interior of the Earth rather than over the surface as a series of squeezes and stretches. P-waves reach everywhere around the Earth after about 20 minutes.

The S-wave, or shear wave, produces a shaking motion, like if you tied a rope to a poll and shook the other end side to side.

S-waves can only travel in solid material and travel slower than P- waves.

How Do Scientists Know Where an Earthquake Has Happened?

• The location directly above the hypocenter on the Earth’s surface is called the epicenter.

It take three seismographs to locate an earthquake. Scientists draw a circle on a map around the three seismographs where the radius of each is the distance from the station to the earthquake, the intersection of those three circles is the epicenter.

The focus is the area beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake.

How are Earthquakes How are Earthquakes Measured?Measured?

Earthquakes are measured by instruments called seismographs. It has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the heavy weight does not. The spring that it is hanging from absorbs all the movement. The seismograph records the difference in position between the shaking part and the motionless part.

The recording is called a seismogram. It is used to determine how large the earthquake was. A short wiggly line that wiggles very little means a small earthquake. A long wiggly line that wiggles a lot means a large earthquake.

The Richter Magnitude Scale

DescriptionDescriptionRichter Richter

MagnitudesMagnitudes Earthquake EffectsEarthquake Effects

MicroMicro Less than 2.0Less than 2.0 Microearthquakes, not felt.Microearthquakes, not felt.

Very MinorVery Minor 2.0-2.92.0-2.9 Usually not felt, but recorded.Usually not felt, but recorded.

MinorMinor 3.0-3.93.0-3.9 Often felt, but rarely causes Often felt, but rarely causes damage.damage.

LightLight 4.0-4.94.0-4.9 Noticeable shaking of indoor items, Noticeable shaking of indoor items, rattling noises. Significant damage rattling noises. Significant damage unlikely. unlikely.

ModerateModerate

5.0-5.95.0-5.9 Can cause major damage to poorly Can cause major damage to poorly constructed buildings over small constructed buildings over small regions. Slight damage to well-regions. Slight damage to well-designed buildings. designed buildings.

StrongStrong 6.0-6.96.0-6.9 Can be destructive in areas up to Can be destructive in areas up to about 100 miles across in populated about 100 miles across in populated areas. areas.

MajorMajor 7.0-7.97.0-7.9 Can cause serious damage over Can cause serious damage over larger areas.larger areas.

GreatGreat 8.0-8.98.0-8.9 Can cause serious damage in areas Can cause serious damage in areas several hundred miles across. several hundred miles across.

Rare GreatRare Great 9.0 or greater9.0 or greater Devastating in areas several Devastating in areas several thousand miles across. thousand miles across.

How Long Does an Earthquake Last?

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake.

The largest, main earthquake is called the mainshock.

Mainshocks always have

aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, or even years after the mainshock!

Famous Earthquakes Around the Famous Earthquakes Around the

WorldWorld San Francisco, California: April 18, 1906

(Magnitude: About 8)

Tokyo, Japan: September 1, 1923 (Magnitude: about 8.25)

Chile: May 22, 1960 (Magnitude: About 9)

Anchorage, Alaska: March 27, 1964 (Magnitude: About 8.5)

How Many Earthquakes How Many Earthquakes Happen Each Year?Happen Each Year?

1

10

100

1,000

10,000

100,000

1,000,000

1 2 3 4 5 6 7 8

Magnitude of Earthquakes

Num

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of

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thqu

akes

Where do Earthquakes OccurWhere do Earthquakes Occur Most Often in the United States? Most Often in the United States?

Earthquake Damage