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Earthquakes are vibrations triggered by sudden rock movements

deep underground, which cause the Earth’s surface to shake.

Major earthquakes can shatter whole cities, killing people

and bringing buildings and bridges crashing down.

WHAT CAUSES EARTHQUAKES?

Earthquakes are caused by the movements of the huge tectonic plates that make

up the Earth’s outer crust. Driven by currents in the semi-molten layer below the

crust, the plates slowly drift over the Earth’s surface and collide, grind together,

or pull apart. Most earthquakes occur on fault lines – cracks in the Earth’s crust

where two plates meet and grind together.

WHERE DO MOST EARTHQUAKES STRIKE?

Most earthquakes and also volcanic eruptions occur

on or near to the edges of the Earth’s tectonic plates.

They are most common on the “Ring of Fire”, the

name given to the edge of the vast Pacific Plate that

lies beneath the Pacific Ocean. Japan, the Philippines,

New Zealand, and the western coastline of North and

South America all lie in this major fault zone.

WHAT HAPPENS DURING AN EARTHQUAKE?

As tectonic plates grind together at a fault line, the

rocks on either side stretch to absorb a certain amount

of pressure. If the pressure becomes too great, the

rocks shatter, releasing shock waves that shake

the surface. Buildings then sway and topple, and fires

may start as gas and electricity lines are ripped apart.

HOW ARE EARTHQUAKES MEASURED?

The study of earthquakes is called seismology.

Scientists measure and record earthquakes using

devices called seismometers. The size of an

earthquake is measured according to its magnitude

(the size of the shock waves and the energy

produced) or its effects.

1 SEISMIC WAVES
Most earthquakes begin deep underground at a point called the focus.
As the rocks shatter at the focus, shock waves called seismic waves
radiate outwards in all directions. The point on the Earth’s surface
directly above the focus is called the epicentre. This is where most
damage occurs.

Earthquakes

1 EARTHQUAKE DAMAGE

Most earthquakes last only a few seconds, but the destruction they cause

can take years to clear up. The shaking can cause certain types of soil to

liquefy (turn to mud), making buildings sink or fall. An earthquake in

Kobe, Japan, in 1995 (above), damaged many of the city’s roads.

4 THE SAN ANDREAS FAULT

The San Andreas region on the

western coast of North America lies

on the “Ring of Fire.” Rocks along a

fault line where two plates meet are

cracked and buckled.

Shock waves grow fainter

farther from the epicentre, and

destruction on the Earth’s

surface is less severe

Shifting tectonic plates

send vibrations or shock waves

through the ground

Shock waves

travel outwards

from the focus

Epicentre

Focus

FIND OUT MORE. Continents 39 • Earth Sciences 38 • Planet Earth 36–37 • Volcanoes 44

earthquakes