What are earthquakes and how

do they occur?

Chapter 5: Lesson 3:

What is an earthquake?It is the sudden movement of earth’s crust.

When layers of rock that are

stuck together suddenly slip an

earthquake occurs.

When two plates slide past each

other, they create a transform fault,

like the San Andreas fault.

Slipping along a fault begins under the earth’s surface.

(80 km – 644 km deep)

The place where the slipping begins is called the focus.

Waves of energy ripple outwards from the focus.

When the waves reach the surface they form the epicenter.

The epicenter is the point from where the waves of the earthquake

spread.(People start feeling the

earthquake)

Earthquakes occur at plate boundaries

(like volcanoes)

Seismometers

Instruments used to measure the waves produces by an earthquake.

There are 3 types of seismic waves

P waves(Primary waves)

S waves(Secondary

waves)

Lg waves(Surface waves)

P waves- Move back and forth- Fastest- Travels through solid and liquid

layers of earth.

S waves- Moves up and down.- Travel half as fast as P waves.- Travels through solid layers of

earth only.

Lg waves- Travels as ripples across earth’s

surface.- Slowest.- Causes most damage.

Lg waves

P waves

S waves

Locating earthquakes

Seismometer station 1

Earthquake epicenter can

happen any where along

circumference

Seismometer station 1

Seismometer station 2

Seismometer station 3

The point where the 3 circles meet is the

epicenter

MagnitudeThe measure of the amount of

energy released by an earthquake.

Richter scale Mercalli scale

Richter scale

Measures the magnitude at the epicenter

(From weakest to strongest)

The Richter scale starts from 1 and each whole number has 30 times more energy than the number before it.

Mercalli scale

Measures what people felt and what happens to things

in a place

On the mercalli scale earthquakes are rated from I – XII(XII is the most severe)

Mercalli scale

The earthquake in Pakistan on October 8, 2005 was rated XII

the mercalli scale.

Aftershock

Earthquakes with lesser magnitude.

(Felt after the main earthquake)

Earthquakes with larger magnitudes have many aftershocks with large magnitudes.

When earthquakes occur on the ocean floor waves of

water are pushed upwards in all directions.Deep in the sea the

energy is spread through the water so the waves are

not tall.

In shallow waters the waves are very

tall causing tsunamis.

When the waves reach shallow water the energy is spread through less water.