EARTHQUAKESToolbox

What we will be studying today: The Earth’s structure What are Tectonic plates and what do

they have to do with an Earthquake? The different types of plate boundaries. What is an Earthquake? How does it

occur? How do we measure an Earthquake? Can we predict Earthquakes? What are the impacts of an Earthquake?

The Earths Structure

The earth consists of several layers. The three main layers are the core, the mantle and the crust. The core is the inner part of the earth, the crust is the outer part and between them is the mantle.

Cross section of the Earth

Cross section of the Earth

Tectonic Plates

Before we can understand Tectonic plates, lets take a look at Continental drifts:

In 1912, a German scientist called Alfred Wegener proposed that South America and Africa were once joined together and had subsequently moved apart. 

He believed that all the continents were once joined together as one big land mass called Pangaea and this was intact until about 200 million years ago.

The idea that continents are slowly shifting their positions is called continental drift.

Continental drift

Tectonic Plates

What are Tectonic plates?

the two sub-layers of the earth's crust (lithosphere) that move, float, and sometimes fracture and whose interaction causes continental drift, earthquakes, volcanoes, mountains, and oceanic trenches.

the two sub-layers of the earth's crust (lithosphere) that move, float, and sometimes fracture and whose interaction causes continental drift, earthquakes, volcanoes, mountains, and oceanic trenches

The ocean floors are continually moving, spreading from the centre and sinking at the edges.

At the edges of these plates (plate boundaries) earthquakes and volcanoes occur.

Convection currents in the mantle move the plates. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.

Two Types of Plates

Oceanic plates, as the name goes, are crusts which 'carry' the oceans above it. It is made of lighter but denser materials than the continental crust.

Continental plates are crust that have continents (Large land masses) and some areas of ocean on them. They are usually named after the continents they 'carry'. Because they are made of lighter and less dense substances than an oceanic plate

Oceanic Plates Continental Plates

Plate names

African

Indo-Australian Plate

North American

South American

Eurasian

Pacific

Nazca

Antarctic

Pacific

Earthquakes

Earthquakes are vibrations caused by earth movements at plate boundaries and at major fault lines (cracks in the earth’s surface).

When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again.

What are they? How do they Occur?

What are the different Tectonic Plate Boundaries that cause Earthquakes?

Divergent Boundaries: At divergent boundaries new crust is created as two or more plates pull away from each other. Oceans are born and grow wider where plates diverge or pull apart. As seen below, when a diverging boundary occurs on land a 'rift', or separation will arise and over time that mass of land will break apart into distinct land masses and the surrounding water will fill the space between them. The pressure from the break and heat from the rising lava can cause an earthquake

Why do the plates move?

The plates that form the crust of the earth are floating on the molten interior of the planet, and they are constantly moving and pushing each other because of the heat escaping.

What are the different Tectonic Plate Boundaries that cause Earthquakes?

In plate tectonics, a convergent boundary, also known as a destructive plate boundary is an actively changing region where two (or more) tectonic plates move toward one another and collide. The oceanic plate descends under the continental plate because it is denser. As the plate descends it starts to melt due to the friction caused by the movement between the plates.

As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near convergent boundaries. When two plates move towards one another, they form either a subduction zone or a continental collision.

What are the different Tectonic Plate Boundaries that can cause Earthquakes? Conservative plate margins

At a conservative margin two plates try to slide past each other slowly. Quite often, the two plates stick and pressure builds up; the release of this pressure creates a severe earthquake.

Conservative plate boundary

How is an Earthquake Measured?

This measures the magnitude of a tremor (how powerful it is) using an instrument called a seismograph. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. Although the Richter Scale has no upper limit, the largest earthquake ever recorded was in 1960 in Chile. It measured 9.5 on the Richter Scale.

It is a logarithmic scale which means that a size ‘6’ on the Richter Scale is 10 times larger than a size ‘5’ and 100 times larger than a size ‘4’.

The Haiti Earthquake measured 7.0 on the Richter Scale!

The Christchurch Earthquake measured 6.3

The earthquake in Japan measured a whopping an 8.9

The Richter Scale

A seismograph

Mercalli Scale

The Mercalli scale measures how much damage is caused by the earthquake based on observations.

It is measured on a scale between 1 and 12.

Mercalli Scale

I Felt by almost no one.II Felt by very few people.III Tremor noticed by many, but they often do not realise it is an earthquake.IV Felt indoors by many. Feels like a truck has struck the building.V Felt by everyone; many people are awakened. Swaying trees and poles may be observed.VI Felt by all; many people run outdoors. Furniture is moved.VII Everyone runs outdoors. Poorly built structures considerably damaged. Slight damage elsewhere.VIII Specially designed structures damaged slightly, others collapse.IX All buildings considerably damaged, many shift off foundations. Noticeable cracks in the ground.X Many structures destroyed. Ground badly cracked.XI Almost all structures fall. Bridges wrecked. XII Total destruction. Waves seen on ground surfaces.

Can we Predict an Earthquake?

YES AND NO!

Seismologists study the activity of plate boundaries and the earths crust in order to gain a better understanding of when Earthquakes will occur. Currently they are able to predict an earthquake within a decade or so, however this is not specific to year, month, day or time.

Impacts of an Earthquake?:

DeathsInjuriesDestruction of housesCollapsed buildings like: schools, hospitals, aged care, universities, shops, skyscrapers, hotels, fire stations, police stationsRoads are crackedWater, sewage and storm pipes are damagedElectricity and Reception poles/cables are damagesNatural habitat

Impact on economyGovernment funding Families lose everything and have to rebuild their livesLoss of loved onesFood supply is affectedNo electricityWater supply can become contaminatedSevere injuries can impact on an individuals entire lifeDisease outbreakNot enough resources to treat the sick and injured

Start to think about these impacts! We will look at this more in the Socio-Economic Natural Disaster Toolbox!