Earthquake Hazards

Owain Thomas

Credit: U.S. Geological Survey

An earthquake occurs when rocks break.

The rocks break,this is called afault.

The energy stored in the rocks is released as

earthquake waves.

When the waves reach the surface, they cause a great dealof damage. The area above the fault is called the epicentre.

Plate movements put the rocks under pressure.

The earthquakewaves lose energy

as they pass through the surrounding rocks.

The point wherethe earthquake startsis called the focus.

If the focus is near the surface, the damage will be greater.

Credit: U.S. Geological Survey

Exercise 1Write a short paragraph to explain what causes earthquakes.

fault

plate movement

break stored energy

waves

focus

epicentre

rocks pressure

Credit: U.S. Geological Survey

California is one of the most seismically activeparts of the planet - this is the story of one of themost famous earthquakes ever:

Credit: U.S. Geological Survey

C aliforn ia

Pa

cific Oce

an

S an F ranc isco

Los A ngeles

California has many powerful earthquakes because of its location.This part of the USA is sitting on a plate boundary, Marked by a huge system of faults, including the San Andreas Fault.

The North Americanplate is moving one way and the Pacific plate is moving theopposite way.

Pacific plate

North AmericanPlate

Credit: U.S. Geological Survey

This a picture of the City Hall in San Francisco after the massive 1906 earthquake.

The 1906 San Francisco earthquake

On the 18th of April 1906 a massive earthquake hit San Francisco.It was caused by movement along part of the San Andreas Fault. The section of the fault that moved was about 430 km (267 miles) long.

Credit: U.S. Geological Survey

Credit: U.S. Geological Survey

The earthquake was very powerful. Well constructedbuildings built on solid rock were not badly damagedbut many of the houses were built on soft, weak sand.

During an earthquake, the shaking makes sand grains move aroundknocking into each other, almost like particles in a liquid. Thesand changes into “quicksand” and buildings simply sink andcollapse.

The earthquake cracked gas pipes and very soon there were firesall over the city. Many of the buildings that did not collapse inthe earthquake were destroyed by the fire afterwards.

28,000 buildings destroyed 350,000 people homeless

2500 (estimated) people dead

Credit: U.S. Geological Survey

The power of the earthquake depends on:

• How much the fault moved.• The length of the fault.• How much energy built up in the rocks before it moved.

Its effects also depend on:

• The depth of the earthquake focus.• The types and quality of buildings on the surface.• The behaviour of people.

Exercise 2Copy this information into your book.

Credit: U.S. Geological Survey

Predicting earthquakes is very difficult because there are so many different factors involved.

One way is to monitor the behaviour of the ground.

The scientists measure the distance between two points fixed into theground.

As the rocks are put under pressure, this distance changes.

Eventually, the rocks break causing an earthquake.

Credit: U.S. Geological Survey

Radon gas given off.

Electromagnetic phenomena - earthquakelights.

Minor earthquakes- foreshocks.

Water level in wellschanges suddenly.

Predicting earthquakes

Credit: U.S. Geological Survey

M O O

M O O

M O O

W O O F

Sometimes, animals behave strangely before an earthquake.

(This is a joke, cows do not say “woof” even before an earthquake!)

Credit: U.S. Geological Survey

Exercise 3Make a list of the possible ways of predicting earthquakes.

ground movement

radonanimals

water levels earthquake lights

foreshocks

Credit: U.S. Geological Survey

The EndCredit: U.S. Geological Survey