The Geology Behind Earth’s Features

The Geology Behind Earth’s The Geology Behind Earth’s FeaturesFeatures

Natural DisastersNatural Disasters

Essential QuestionsEssential Questions

What is an earthquake?What is an earthquake? What is the way to detect an earthquake?What is the way to detect an earthquake? What is a Tsunami?What is a Tsunami? What are the ways to prepare for a natural What are the ways to prepare for a natural

disaster?disaster?

What is an earthquake?What is an earthquake?

Bill Nye - EarthquakesBill Nye - Earthquakes

http://http://learning.aliant.net/Player/ALC_Player.asp?Plearning.aliant.net/Player/ALC_Player.asp?ProgIDrogID=DEP_BN027=DEP_BN027

Complete the Question sheet provided.Complete the Question sheet provided. 24mins24mins

Earthquake:Earthquake:– Occur at the boundaries between tectonic Occur at the boundaries between tectonic

platesplates– Occur as a result of the forces of stress, strain, Occur as a result of the forces of stress, strain,

and strengthand strength

StressStress– Is the local force per unit area that causes rocks to Is the local force per unit area that causes rocks to

deformdeform StrainStrain

– Is the relative amount of deformation, expressed as the Is the relative amount of deformation, expressed as the percentage of distortion (eg. compression of a rock by percentage of distortion (eg. compression of a rock by 1% of its length)1% of its length)

StrengthStrength– Rocks fail that is they lose cohesion and break into two Rocks fail that is they lose cohesion and break into two

or more parts when they ae stressed beyond a critical or more parts when they ae stressed beyond a critical valuevalue

Seismic WavesSeismic Waves

There are three types of seismic wavesThere are three types of seismic waves– P-wavesP-waves– S-wavesS-waves– Surface wavesSurface waves

P-wavesP-waves Primary wavesPrimary waves, also called , also called P wavesP waves or or compressional wavescompressional waves P waves arrive first at any surface locationP waves arrive first at any surface location can travel through solid, liquid and gascan travel through solid, liquid and gas Can travel through rock at 6km/sCan travel through rock at 6km/s are waves that have the same direction of vibration along their are waves that have the same direction of vibration along their

direction of travel, which means that the vibration of the medium direction of travel, which means that the vibration of the medium (particle) is in the same direction or opposite direction as the motion of (particle) is in the same direction or opposite direction as the motion of the wave the wave

As they travel through rock, the waves move tiny rock particles back As they travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and then back together and forth -- pushing them apart and then back together

http://www.geo.mtu.edu/UPSeis/images/P-wave_animation.gifhttp://www.geo.mtu.edu/UPSeis/images/P-wave_animation.gif

S-wavesS-waves Secondary wavesSecondary waves, also called , also called S wavesS waves or or shear shear

waveswaves As these waves move, they displace rock particles As these waves move, they displace rock particles

outwardoutward S waves don't move straight through the earthS waves don't move straight through the earth only travel through solid material only travel through solid material the ground is displaced perpendicularly to the moves the ground is displaced perpendicularly to the moves

alternately to one side and then the other alternately to one side and then the other http://www.geo.mtu.edu/UPSeis/images/S-wave_animatihttp://www.geo.mtu.edu/UPSeis/images/S-wave_animati

on.gifon.gif

Surface WavesSurface Waves

sometimes called long waves, or simply L wavesometimes called long waves, or simply L wave are responsible for most of the damage associated are responsible for most of the damage associated

with earthquakes, because they cause the most with earthquakes, because they cause the most intense vibrationsintense vibrations

stem from body waves that reach the surfacestem from body waves that reach the surface are something like the waves in a body of water -- are something like the waves in a body of water --

they move the surface of the earth up and downthey move the surface of the earth up and down L waves are the slowest moving of all waves L waves are the slowest moving of all waves

Both P and S waves refract or reflect at points Both P and S waves refract or reflect at points where layers of differing physical properties meet. where layers of differing physical properties meet. They also reduce speed when moving through They also reduce speed when moving through hotter material. These changes in direction and hotter material. These changes in direction and velocity are the means of locating discontinuities. velocity are the means of locating discontinuities.

Seismic discontinuities (aSeismic discontinuities (a surface at which surface at which velocities of seismic waves change abruptly) velocities of seismic waves change abruptly) aid in aid in distinguishing divisions of the Earth into inner core, distinguishing divisions of the Earth into inner core, outer core, D", lower mantle, transition region, outer core, D", lower mantle, transition region, upper mantle, and crust (oceanic and continental).upper mantle, and crust (oceanic and continental).

EarthquakesEarthquakes

Did you know…Did you know…– According to the United States Geological According to the United States Geological

Survey, more than three million earthquakes Survey, more than three million earthquakes occur every year. That's about 8,000 a day, or occur every year. That's about 8,000 a day, or one every 11 seconds! one every 11 seconds!

How does an Earthquake occur?How does an Earthquake occur?

How do Earthquakes Occur?How do Earthquakes Occur? When brittle rocks being stressed suddenly fail When brittle rocks being stressed suddenly fail

along a geologic faultalong a geologic fault Most large earthquakes are caused by ruptures of Most large earthquakes are caused by ruptures of

pre-existing faults, where past earthquakes have pre-existing faults, where past earthquakes have already weakened the rocks on the fault surfacealready weakened the rocks on the fault surface

The two blocks of rock on either side of the fault The two blocks of rock on either side of the fault slip suddenly, releasing energy in the form of slip suddenly, releasing energy in the form of seismic wavesseismic waves– When the fault slips, the stress is reduced, dropping to a When the fault slips, the stress is reduced, dropping to a

level below he rock strengthlevel below he rock strength

A fault rupture does not happen all at onceA fault rupture does not happen all at once– It begins at the focus and expands outward It begins at the focus and expands outward

along the fault surface ~2-3km/salong the fault surface ~2-3km/s– It stops where the stresses become insufficient It stops where the stresses become insufficient

to continue breaking the fault or where the to continue breaking the fault or where the rupture enters ductile material in which it can no rupture enters ductile material in which it can no longer propagate as a fracturelonger propagate as a fracture

Fault ruptures in the largest earthquakes can Fault ruptures in the largest earthquakes can extend for more than 1000km and the fault extend for more than 1000km and the fault slip can be as large as 20mslip can be as large as 20m

Earthquake TerminologyEarthquake Terminology FocusFocus

– The point at which fault slipping beginsThe point at which fault slipping begins EpicenterEpicenter

– The geographic point on Earth’s surface directly above The geographic point on Earth’s surface directly above the focusthe focus

Focal depthFocal depth– In continental crust is ~2-20kmIn continental crust is ~2-20km– Below 20km is rare because under the high Below 20km is rare because under the high

temperatures and pressures the crust behaves as a temperatures and pressures the crust behaves as a ductile materialductile material However in subduction zones where cold oceanic crust plunges However in subduction zones where cold oceanic crust plunges

into the mantle earthquakes can originate at depths as great as into the mantle earthquakes can originate at depths as great as 690km690km

Earthquake Terminology continuedEarthquake Terminology continued

ForeshockForeshock– A small earthquake that occurs near, but before, a A small earthquake that occurs near, but before, a

mainshockmainshock AftershockAftershock

– Large earthquakes trigger smaller earthquakesLarge earthquakes trigger smaller earthquakes– Follow the mainshockFollow the mainshock

Their foci are distributed in and around the rupture plane of the Their foci are distributed in and around the rupture plane of the mainshockmainshock

– Happen where that stress exceeds he rock strengthHappen where that stress exceeds he rock strength– The number and sizes depend on the magnitude of the The number and sizes depend on the magnitude of the

mainshockmainshock– P 348 fig 13.6P 348 fig 13.6

The Elastic Rebound TheoryThe Elastic Rebound Theory

Proposed by Henry Fielding Reid of John Proposed by Henry Fielding Reid of John Hopkins University in 1910Hopkins University in 1910

Explains why earthquakes recur on active Explains why earthquakes recur on active faultsfaults

P345 Fig 13.1P345 Fig 13.1

P 347 fig 13.5P 347 fig 13.5

What is the way to detect an What is the way to detect an earthquake?earthquake?

DetectionDetection

Detecting an earthquake is much easier than Detecting an earthquake is much easier than predicting one. predicting one. – a powerful earthquake can be felt by people in the area, a powerful earthquake can be felt by people in the area,

and the damage it causes can be seen. and the damage it causes can be seen. SeismographSeismograph

– An instrument that records the seismic waves generated An instrument that records the seismic waves generated by earthquakesby earthquakes

SeismogramSeismogram– A record, graphed or digital, of the seismic activities of A record, graphed or digital, of the seismic activities of

an areaan area

Ideal SeismographsIdeal Seismographs

It would be a device affixed to a stationary It would be a device affixed to a stationary frame not attached to Earth when the frame not attached to Earth when the ground shook, the seismograph would ground shook, the seismograph would measure the changing distance between the measure the changing distance between the frame which did not move and the vibrating frame which did not move and the vibrating groundground

Current SeismographsCurrent Seismographs Attach a dense mass, such as a piece of steel to Earth so Attach a dense mass, such as a piece of steel to Earth so

loosely that the ground can vibrate up and down or side to loosely that the ground can vibrate up and down or side to side without causing much motion of the massside without causing much motion of the mass

Attachment is usually a spring (for vertical movement) or Attachment is usually a spring (for vertical movement) or hinge (for horizontal movement)hinge (for horizontal movement)

When seismic waves move the ground, the mass tends to When seismic waves move the ground, the mass tends to remain stationary because of its inertia, but the mass and remain stationary because of its inertia, but the mass and the ground move relative to each other because the spring the ground move relative to each other because the spring compresses or stretches or the hinge swings left and rightcompresses or stretches or the hinge swings left and right

Record bthe movements automaticallyRecord bthe movements automatically

Reading a SeismogramReading a Seismogram

When you look at a seismogram, there will be When you look at a seismogram, there will be wiggly lines all across it. These are all the seismic wiggly lines all across it. These are all the seismic waves that the seismograph has recorded. waves that the seismograph has recorded.

Most of these waves were so small that nobody Most of these waves were so small that nobody felt them. felt them.

Reading a Seismogram Reading a Seismogram continuedcontinued

The P wave will be the first wiggle that is bigger than the rest The P wave will be the first wiggle that is bigger than the rest of the little ones (the microseisms). of the little ones (the microseisms). – Because P waves are the fastest seismic waves, they will usually be Because P waves are the fastest seismic waves, they will usually be

the first ones that your seismograph records. the first ones that your seismograph records. The next set of seismic waves on your seismogram will be The next set of seismic waves on your seismogram will be

the S waves. the S waves. – These are usually bigger than the P waves.These are usually bigger than the P waves.– If there aren't any S waves marked on your seismogram, it probably If there aren't any S waves marked on your seismogram, it probably

means the earthquake happened on the other side of the planet. S means the earthquake happened on the other side of the planet. S waves can't travel through the liquid layers of the earth so these waves can't travel through the liquid layers of the earth so these waves never made it to your seismograph. waves never made it to your seismograph.

The surface waves are the other, often larger, waves The surface waves are the other, often larger, waves marked on the seismogram. marked on the seismogram. – They have a lower They have a lower frequencyfrequency. . – Surface waves travel a little slower than S waves. Surface waves travel a little slower than S waves.

Finding the EpicenterFinding the Epicenter Measure the distance between the first Measure the distance between the first

P wave and the first S wave. In this P wave and the first S wave. In this case, the first P and S waves are 24 case, the first P and S waves are 24 seconds apart. seconds apart.

Find the point for 24 seconds on the left Find the point for 24 seconds on the left side of the chart below and mark that side of the chart below and mark that point. According to the chart, this point. According to the chart, this earthquake's epicenter was 215 earthquake's epicenter was 215 kilometers away. kilometers away.

Measure the amplitude of the strongest Measure the amplitude of the strongest wave. The wave. The amplitudeamplitude is the height (on is the height (on paper) of the strongest wave. On this paper) of the strongest wave. On this seismogram, the amplitude is 23 seismogram, the amplitude is 23 millimeters. Find 23 millimeters on the millimeters. Find 23 millimeters on the right side of the chart and mark that right side of the chart and mark that point. point.

Place a ruler (or straight edge) on the Place a ruler (or straight edge) on the chart between the points you marked chart between the points you marked for the distance to the epicenter and the for the distance to the epicenter and the amplitude. The point where your ruler amplitude. The point where your ruler crosses the middle line on the chart crosses the middle line on the chart marks the marks the magnitudemagnitude (strength) of the (strength) of the earthquake. This earthquake had a earthquake. This earthquake had a magnitude of 5.0.magnitude of 5.0.

MagnitudeMagnitude

Is related to the total area of the fault ruptureIs related to the total area of the fault rupture– Most earthquakes are very small and the Most earthquakes are very small and the

rupture never breaks the ground surfacerupture never breaks the ground surface– In large earthquakes surface breaks are In large earthquakes surface breaks are

commonbcommonb

The Richter ScaleThe Richter Scale

The Richter Scale is the best known scale for The Richter Scale is the best known scale for measuring the magnitude of earthquakes. measuring the magnitude of earthquakes.

The energy released by an earthquake The energy released by an earthquake increases by a factor of 30 for every unit increases by a factor of 30 for every unit increase in the Richter scale. increase in the Richter scale.

An earthquake that measures 4.0 on the Richter An earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures scale is 10 times larger than one that measures 3.0 3.0

Richter scale no.Richter scale no. No. of earthquakes per No. of earthquakes per yearyear

Typical effects of this Typical effects of this magnitudemagnitude

< 3.4< 3.4 800 000800 000 Detected only by Detected only by seismometersseismometers

3.5 - 4.23.5 - 4.2 30 00030 000 Just about noticeable Just about noticeable indoorsindoors

4.3 - 4.84.3 - 4.8 4 8004 800 Most people notice them, Most people notice them, windows rattle.windows rattle.

4.9 - 5.44.9 - 5.4 14001400Everyone notices them, Everyone notices them,

dishes may break, dishes may break, open doors swing.open doors swing.

5.5 - 6.15.5 - 6.1 500500Slight damage to buildings, Slight damage to buildings,

plaster cracks, bricks plaster cracks, bricks fall.fall.

6.26.2   6.9 6.9 100100Much damage to buildings: Much damage to buildings:

chimneys fall, houses chimneys fall, houses move on foundations.move on foundations.

7.0 - 7.37.0 - 7.3 1515Serious damage: bridges Serious damage: bridges

twist, walls fracture, twist, walls fracture, buildings may collapse.buildings may collapse.

7.4 - 7.97.4 - 7.9 4 4 Great damage, most Great damage, most buildings collapse.buildings collapse.

> 8.0> 8.0 One every 5 to 10 yearsOne every 5 to 10 yearsTotal damage, surface Total damage, surface

waves seen, objects waves seen, objects thrown in the air.thrown in the air.

What is a Tsunami?What is a Tsunami?

What are the ways to prepare for a What are the ways to prepare for a natural disaster?natural disaster?