Earthquakes & Volcanoes

12.2

Earthquakes• Most Earthquakes (95%) occur along PLATE

BOUNDARIES

Why do they occur?

Ring of Fire• 80% of the world’s earthquakes occur in a ring

that borders the pacific ocean

• This ring is called “the Ring of Fire”

...Not...

IN BC...

How they begin...

• An Earthquake starts at a location inside the Earth called a “focus” (foci)

Release of energy starts here...

EPICENTRE• The “epicentre” is the point on the EARTH’s

SURFACE directly above the focus.

• It is not the center where the Earthquake starts!

DEPTHS of EARTHQUAKES...CLASSIFICATION DEPTH OF FOCUS

Shallow Focus 0 – 70 km

Intermediate Focus 70 – 300 km

Deep Focus > 300 km

Most Earthquakes have focus less than 100 km deep

About 3% of quakes

SEISMIC WAVES• Seismic waves are the vibrations (waves) of

energy released by a quake!

THERE ARE THREE TYPES OF WAVESBODY

WAVESSURFACE WAVES

P- waves

S- waves

L- waves

BODY WAVES (P and S)• P WAVES (P= Primary)

- Fastest (6 km/s)

- Causes ground to squeeze and stretch like a spring!

- Can travel though solids, liquids, and gases

BODY WAVES (P and S)• S WAVES (S= Secondary)

- Slower (3.5 km/s)

- Causes ground to SHAKE

- Can travel though solids (not liquids)

These waves are usually larger...do the most damage!

Travelling Through Layers

P and S Waves

P

S

S No S Waves (liq)

SURFACE WAVES (L)• L WAVES (Surface waves)

- Slowest!

- Travel along Earth’s surface (like pond ripples)

- Causes ground to move in “rolling motion”

Measuring Earthquakes

• “Seismometers”Devices that measure the amount of ground motion

Produces a “seismogram”

Seismogram

10 sec

Tell us... How long it lasted, how fast waves travelled, amount of ground shaking, magnitude...

Ancient Chinese Seismometers

MAGNITUDE•Magnitude = a NUMBER that

rates the strength (energy)

With every magnitude increase of 1 on the scale.... the seismic wave is 10 x higher!

Magnitude Earthquake Effects Estimated NumberEach Year

2.5 or lessUsually not felt, but can be recorded by seismograph.

900,000

2.5 to 5.4Often felt, but only causes minor damage.

30,000

5.5 to 6.0Slight damage to buildings and other structures.

500

6.1 to 6.9May cause a lot of damage in very populated areas.

100

7.0 to 7.9Major earthquake. Serious damage.

20

8.0 or greaterGreat earthquake. Can totally destroy communities near the epicenter.

One every 5 to 10 years

Earthquake Magnitude Scale

7.8 magnitude (China)

VOLCANOES• We will look at 3 types of volcanoes

(1) Composite Volcanoes(2) Shield Volcanoes(3) Rift Eruptions

(1) Composite Volcanoes• Cone-shaped

volcanoes (layers build up)

• Usually found near “subduction” zones (converging plates)

• Ex: Mt St. Helen’s

Mt St Helen’s, Washington

(2) Shield Volcanoes• These are the LARGEST

volcanoes• DO NOT occur at plate

boundaries...• They form over HOT SPOTS• Usually occur in ocean basins

(weak spot in lithosphere...magma breaks through)

Shaped like a “soldier’s shield” lying down

(3) Rift Eruptions• Occur at “rifts” and “ridges”

(divergent plates)• “Fountains of lava” • Not very destructive or violent

Common along the Mid-Atlantic Ridge

Assignment