Chapter 3: Chapter 3: EarthquakesEarthquakes

Damaged farm structures in the Edgecumbe, New Zealand earthquake, 1987.

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Earthquake Destruction Earthquakes 101 Why do Earthquakes Happen?

What is an What is an earthquake?earthquake?

An earthquake is a trembling or shaking of An earthquake is a trembling or shaking of the ground caused by the sudden release of the ground caused by the sudden release of energy stored in the rocks beneath the energy stored in the rocks beneath the surface of the Earth. In other words, the surface of the Earth. In other words, the motion of the plates causes enormous motion of the plates causes enormous stress on the rock. Like bending a stick, stress on the rock. Like bending a stick, rock can only bend so far before it breaks. rock can only bend so far before it breaks. When the rock breaks, waves of energy are When the rock breaks, waves of energy are released through the Earth as an released through the Earth as an earthquake.earthquake.

What are seismic waves?What are seismic waves?Seismic waves are waves of energySeismic waves are waves of energy

produced by an earthquake. There are produced by an earthquake. There are several different types of seismic several different types of seismic waves.waves.

What is a fault?What is a fault?A fault is a break between two rock A fault is a break between two rock

masses. Faults can be new/active masses. Faults can be new/active (such as the San Andreas Fault) or (such as the San Andreas Fault) or old/inactive (such as the Aspy Fault in old/inactive (such as the Aspy Fault in Northern Cape Breton)Northern Cape Breton)

What causes an What causes an earthquake?earthquake?The classic explanation of what causes an The classic explanation of what causes an

earthquake is called the earthquake is called the elastic rebound elastic rebound theorytheory.. Over many years, tectonic forces Over many years, tectonic forces act on the rock, causing it to bend. act on the rock, causing it to bend. Gradually, the bending becomes more and Gradually, the bending becomes more and more severe. Eventually, the strain exceeds more severe. Eventually, the strain exceeds the breaking strength of the rock, and the the breaking strength of the rock, and the rock breaks suddenly causing an earthquake. rock breaks suddenly causing an earthquake. The two masses of rock move past each The two masses of rock move past each other along a other along a faultfault. The rock may move . The rock may move horizontally, vertically or both. horizontally, vertically or both.

Recently, however, some scientists believe Recently, however, some scientists believe that faults are weak, and that only small that faults are weak, and that only small forces are needed to cause an earthquake.forces are needed to cause an earthquake.

Also, extremely deep earthquakes (100-670 Also, extremely deep earthquakes (100-670 km below the surface) may be caused by km below the surface) may be caused by minerals changing from one form to anotherminerals changing from one form to another

Elastic Rebound TheoryElastic Rebound Theory

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Elastic Rebound Theory (USGS) Breaking Stick

In general, where do earthquakes In general, where do earthquakes occur?occur?

Most earthquakes are associated with Most earthquakes are associated with movement along obvious fault lines movement along obvious fault lines (ex. San Andreas Fault). But many (ex. San Andreas Fault). But many earthquakes occur along very old earthquakes occur along very old fault lines (ex. Ontario & Quebec; fault lines (ex. Ontario & Quebec; offshore of CB) or buried fault lines offshore of CB) or buried fault lines (ex. Northridge quake outside of L.A. (ex. Northridge quake outside of L.A. in 1994). Earthquakes are also in 1994). Earthquakes are also associated with volcanic eruptions associated with volcanic eruptions and plate boundaries (ex. Mid-and plate boundaries (ex. Mid-Atlantic Ridge).Atlantic Ridge).

What is the difference between a focus What is the difference between a focus and an epicentre?and an epicentre?

The The focusfocus is the point is the point ININ the Earth the Earth where seismic waves originate. This where seismic waves originate. This is the centre of the quake and this is is the centre of the quake and this is where the first breakage occurs. The where the first breakage occurs. The point point ONON the Earth’s surface directly the Earth’s surface directly above the focus is the above the focus is the epicentreepicentre..

Focus vs. EpicentreFocus vs. Epicentre

What are the two types of seismic What are the two types of seismic waves?waves?

1.1. Body Waves:Body Waves: These travel through These travel through the the BODYBODY of the Earth (or, through of the Earth (or, through the Earth’s interior), spreading out the Earth’s interior), spreading out in all directions. This happens just in all directions. This happens just like the exploding Death Star in Star like the exploding Death Star in Star Wars Episode I.Wars Episode I.

2.2. Surface Waves:Surface Waves: Surface waves Surface waves travel on the Earth’s surface away travel on the Earth’s surface away from the epicentre, like water from the epicentre, like water waves when a pebble is thrown into waves when a pebble is thrown into a pond.a pond.

There are two types of body waves:There are two types of body waves:1.1. Primary wave (or P wave)Primary wave (or P wave) Compression wave or “pushing” wave in which Compression wave or “pushing” wave in which

the rock vibrates in the same direction.the rock vibrates in the same direction. Just like a sudden push on the end of a Just like a sudden push on the end of a

stretched slinkystretched slinky Travels very fast (4-7 km/sec) Travels very fast (4-7 km/sec) INSIDE THE INSIDE THE

EARTHEARTH Primary waves are the first (or Primary waves are the first (or primaryprimary) wave ) wave

after a quakeafter a quake

2.2. Secondary Wave (or S-wave)Secondary Wave (or S-wave) Back & forth (or shearing) kind of wave in which Back & forth (or shearing) kind of wave in which

the rock vibrates perpendicular to the direction the rock vibrates perpendicular to the direction of the wave of the wave

Essentially, the rock vibrates back and forth just Essentially, the rock vibrates back and forth just like shaking a ropelike shaking a rope

Secondary waves are slower (only 2-5 km/sec) Secondary waves are slower (only 2-5 km/sec) and is the and is the SECONDSECOND type of body wave that is type of body wave that is formedformed

Surface WavesSurface Waves

Surface waves are the slowest waves Surface waves are the slowest waves that are set off by an earthquakethat are set off by an earthquake

Surface waves cause the most damage Surface waves cause the most damage because they take longer to passbecause they take longer to pass

There are two types of surface waves:There are two types of surface waves:

1.1. Love WavesLove Waves

2.2. Rayleigh WaveRayleigh Wave

Love Waves Vs. Rayleigh Waves

1.1. Love Waves:Love Waves: The ground moves side-to-side The ground moves side-to-side

horizontallyhorizontally Tend to knock buildings off their Tend to knock buildings off their

foundations and destroy highway foundations and destroy highway overpassesoverpasses

2.2. Rayleigh Waves:Rayleigh Waves: Act just like ocean waves, moving Act just like ocean waves, moving

up and downup and down Very destructive because they Very destructive because they

produce more ground movement produce more ground movement and take longer to passand take longer to pass

How do we determine the How do we determine the location of an earthquake?location of an earthquake?

P-waves are faster and always arrive at a P-waves are faster and always arrive at a seismograph seismograph beforebefore the slower S-waves. the slower S-waves. The farther the distance the seismograph is The farther the distance the seismograph is located from the epicentre, the bigger the located from the epicentre, the bigger the delay between the first and second wave. delay between the first and second wave. This time interval can be used to calculate This time interval can be used to calculate the distance to a quake. If three different the distance to a quake. If three different seismograph stations are used, the exact seismograph stations are used, the exact location of the quake can be determined. location of the quake can be determined. Also, most quakes are shallow focus quakes, Also, most quakes are shallow focus quakes, in which the focus of the quake is 0-70 km in which the focus of the quake is 0-70 km deep (85% of quakes are shallow).deep (85% of quakes are shallow).

Measuring the Size of an Measuring the Size of an EarthquakeEarthquake

The size of quakes is measured in two ways. One The size of quakes is measured in two ways. One way is to measure the way is to measure the intensityintensity of the damage of the damage (what kind of damage and how much damage). (what kind of damage and how much damage). The scale that is used to measure intensity is The scale that is used to measure intensity is called the called the Mercalli Instensity Scale Mercalli Instensity Scale (pg. 82).(pg. 82).

The second method measures the amount of energy The second method measures the amount of energy released by the quake. The larger the quake, the released by the quake. The larger the quake, the more the ground vibrates and the larger the more the ground vibrates and the larger the wiggle on a seismogram. Scientists can then wiggle on a seismogram. Scientists can then assign a number called a assign a number called a magnitudemagnitude. The scale . The scale that is used to measure an earthquake magnitude that is used to measure an earthquake magnitude is called the is called the Richter ScaleRichter Scale..

Richter ScaleRichter Scale

Facts About the Richter Facts About the Richter ScaleScale

A higher number on the Richter Scale indicates a more A higher number on the Richter Scale indicates a more severe earthquakesevere earthquake

Largest recorded quake was 8.6 on the Richter ScaleLargest recorded quake was 8.6 on the Richter Scale The Richter Scale is a logarithmic scale. This means The Richter Scale is a logarithmic scale. This means

that for each increase in the scale, there is actually a that for each increase in the scale, there is actually a 10X increase in the amount of movement.10X increase in the amount of movement.

Ex. If an Richter Scale 4 (or R4) quake Ex. If an Richter Scale 4 (or R4) quake moves the ground 1cm, an R5 quake would moves the ground 1cm, an R5 quake would move the ground 10X more (or 10 cm). And, an R6 move the ground 10X more (or 10 cm). And, an R6 quake would move the ground 10X10 cm or 100cm; quake would move the ground 10X10 cm or 100cm;

R7 would move the ground 10X10X10 or 1000cm; R7 would move the ground 10X10X10 or 1000cm; R8=10000cm; etc. R8=10000cm; etc.

So, and R8 quake does So, and R8 quake does NOTNOT move the ground twice as move the ground twice as much as an R4 quake. Because the Richter Scale is much as an R4 quake. Because the Richter Scale is logarithmic, an R8 quake actually moves the ground logarithmic, an R8 quake actually moves the ground 10,000 more than an R4 quake.10,000 more than an R4 quake.

Energy from EarthquakesEnergy from Earthquakes

In terms of energy, In terms of energy, a 10X increase in a 10X increase in ground movement ground movement causes a 32X causes a 32X increase in the increase in the amount of energy amount of energy that is released that is released from a quake. from a quake. Look at this Look at this example:example:

MagnitudMagnitudee

Energy ReleasedEnergy Released

R4R4 11

R5R5 1x32=321x32=32

R6R6 32x32=100032x32=1000

R7R7 32x32x32=3200032x32x32=32000

R8R8 32x32x32x32=1,000,32x32x32x32=1,000,000000

Where do Earthquakes Occur Where do Earthquakes Occur on a Global Scale?on a Global Scale?

1.1. Pacific Rim (80% of the world’s Pacific Rim (80% of the world’s shallow quakes)shallow quakes)

2.2. Mediterranean-Himalayan BeltMediterranean-Himalayan Belt

3.3. Mid-Ocean RidgesMid-Ocean Ridges

4.4. Some volcanoes (ex. Hawaii; Mount Some volcanoes (ex. Hawaii; Mount St. Helens)St. Helens)

Active Earthquake Zones on a Global Scale

What is a Benoiff Zone?What is a Benoiff Zone?

When one ocean plate slides under the When one ocean plate slides under the continental plate forming a trench, continental plate forming a trench, many earthquakes often occur. This many earthquakes often occur. This zone of increased seismic activity is zone of increased seismic activity is called the called the Benoiff ZoneBenoiff Zone..

Earthquakes & Plate Earthquakes & Plate BoundariesBoundaries

Earthquakes at Divergent BoundariesEarthquakes at Divergent Boundaries:: Shallow quakes (not very deep)Shallow quakes (not very deep) Lower magnitude (not as severe)Lower magnitude (not as severe) Small area affectedSmall area affected Form rift valleys (like on Mid-Atlantic Form rift valleys (like on Mid-Atlantic

Ridge)Ridge) Example: Quakes along the Mid-Atl. Example: Quakes along the Mid-Atl.

RidgeRidge

Earthquakes at Transform BoundariesEarthquakes at Transform Boundaries:: Often shallow earthquakesOften shallow earthquakes Can be single or many fault linesCan be single or many fault lines Example: San Andreas Fault in CaliforniaExample: San Andreas Fault in California

Earthquakes at Convergent BoundariesEarthquakes at Convergent Boundaries:: Wide zones of shallow earthquakesWide zones of shallow earthquakes Plates may buckle (during mountain Plates may buckle (during mountain

building) or subduct (when trenches form), building) or subduct (when trenches form), both of which create earthquakesboth of which create earthquakes

Example: Marianas Trench near the Example: Marianas Trench near the Philippines; Himalayas MountainsPhilippines; Himalayas Mountains

Earthquakes Away from Plate BoundariesEarthquakes Away from Plate Boundaries:: Very rare; represents only about 5% of quake Very rare; represents only about 5% of quake

energy released in a yearenergy released in a year Also called intraplate earthquakesAlso called intraplate earthquakes Triggered by a build-up of stress between the crust Triggered by a build-up of stress between the crust

and the upper mantleand the upper mantle Occur in Ottawa region, St. Lawrence River Valley Occur in Ottawa region, St. Lawrence River Valley

and around the Grand Banks (off the coast of and around the Grand Banks (off the coast of Louisbourg)Louisbourg)

Earthquake probability in Southern Ontario in the Earthquake probability in Southern Ontario in the next 50 years:next 50 years:

M5M557% (occurred in 2010 & 2011)57% (occurred in 2010 & 2011)M6M66%6%M7M71%1%

NoteNote: Miriamichi, NB experienced : Miriamichi, NB experienced two large two large quakes (M5.7 & M5.1) in 1982quakes (M5.7 & M5.1) in 1982

Can we predict where earthquakes occur?Can we predict where earthquakes occur?

There are many ways to detect slight changes in There are many ways to detect slight changes in the rock and in the Earth’s surface:the rock and in the Earth’s surface:

Small cracks open up causing small tremors to Small cracks open up causing small tremors to increaseincrease

Changes in rock magnetism & other propertiesChanges in rock magnetism & other properties Changing water levels in wellsChanging water levels in wells Increase in radon gas emissions in wellsIncrease in radon gas emissions in wells Changes in Old Faithful and other geysersChanges in Old Faithful and other geysers Surface of Earth tilts & changes in elevationSurface of Earth tilts & changes in elevation Animal behaviour (dogs howling)Animal behaviour (dogs howling) Analyzing patterns of earthquakes (where they Analyzing patterns of earthquakes (where they

occur & how frequently)occur & how frequently) Studying places along fault lines where quakes Studying places along fault lines where quakes

have not occurred for a long timehave not occurred for a long time

TsunamisTsunamis

Tsunami is Japanese for “big wave in Tsunami is Japanese for “big wave in harbour”harbour”

A tsunami is a large wave created by A tsunami is a large wave created by an earthquake, landslide, or an earthquake, landslide, or meteorite impact.meteorite impact.

Usually, tsunamis are caused by Usually, tsunamis are caused by earthquakes greater than 6.5 on the earthquakes greater than 6.5 on the Richter ScaleRichter Scale

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Tsunami FormationTsunami Formation Tsunami AnimationTsunami Animation Alaskan Mega TsunamiAlaskan Mega Tsunami Japanese Tsunami 2011Japanese Tsunami 2011 East Coast TsunamiEast Coast Tsunami