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CHAPTER 19 - EARTHQUAKES

FA U LT S F O R M W H E N T H E F O R C E S A C T I N G O N R O C K E X C E E D T H E R O C K ’ S S T R E N G T H .

19.1 – FORCES WITHIN EARTH

STRESS AND STRAIN

• Along the boundaries between two tectonic plates, rocks in the crust often resist movement.

• Over time, stress builds up. • Stress is the total force acting on crustal rocks per

unit of area.

• Three kinds of stress that act on Earth’s rocks: • Compression

• Tension

• Shear

STRESS AND STRAIN

• Compression causes a material to shorten. • Tension causes a material to lengthen. • Shear causes distortion of a material. • The deformation of materials in response to

stress is called strain.

STRESS AND STRAIN

• Even though rocks can be twisted, squeezed, and stretched, they fracture when stress and strain reach a critical point. • At these breaks, rocks can move, releasing the energy built up as a result of stress. • Earthquakes are the result of this movement and release of energy.

ELASTIC DEFORMATION

• Elastic deformation is caused under conditions of low stress when a material is compressed, bent, or stretched. • When the stress is

removed, material returns to its original shape.

PLASTIC DEFORMATION

• When stress builds up past a certain point, called the elastic limit, rocks undergo plastic deformation.• This type of strain produces permanent deformation.

• Most materials exhibit both elastic and plastic behavior. • As pressure increases, rocks require greater

stress to reach the elastic limit. • At high enough temperatures, solid rock can also

deform, causing it to flow in a fluid-like manner. • Reduces stress.

FAULTS

•Crustal rocks fail when stresses exceed the strength of the rocks.

•The resulting movement occurs along a weak region in the crustal rock called a fault, which is any fracture or system of fractures along which Earth moves.

REVERSE FAULT

• Reverse faults form as a result of horizontal and vertical compression that squeezes rock and creates a shortening of the crust. This causes rock on one side of a reverse fault to be pushed up relative to the other side.

NORMAL FAULT

• Movement along a normal fault is partly horizontal and partly vertical. • The horizontal movement pulls rock apart and

stretches the crust. • Vertical movement occurs as the stretching causes

rock on one side of the fault to move down relative to the other side.

STRIKE-SLIP FAULTS

• Strike-slip faults are caused by horizontal shear. • The movement at a strike-slip fault is mainly

horizontal and in opposite directions, similar to the way cars move in opposite directions on either side of a freeway.

EARTHQUAKE WAVES

• Irregular surfaces in rocks can snag and lock along faults when movement occurs. • As stress continues to build in these rocks, they undergo elastic deformation. • Beyond the elastic limit, they bend or stretch. • Before that limit, an earthquake occurs when they slip or crumble.

TYPES OF SEISMIC WAVES

• The vibrations of the ground produced during an earthquake are called seismic waves.

• Every earthquake generates three types of seismic waves: • Primary waves

• Secondary waves

• Surface waves

• Also referred to as P-waves, primary waves squeeze and push rocks in the direction along which the waves are traveling.

TYPES OF SEISMIC WAVES

• Secondary waves, called S-waves, are named with respect to their arrival times.

• They are slower than P-waves, so they are the second set of waves to be felt.

• S-waves have a motion that causes rocks to move perpendicular to the direction of the waves.

TYPES OF SEISMIC WAVES

• The third and slowest type of waves are surface waves, which travel only along Earth’s surface.

• Surface waves can cause the ground to move sideways and up and down like ocean waves.

• Also known as Rayleigh Waves

GENERATION OF SEISMIC WAVES

• The focus of an earthquake is the point of initial fault rupture, which is usually several kilometers below Earth’s surface.

• The point on Earth’s surface directly above the focus is the epicenter.

S E I S M I C W A V E S C A N B E U S E D T O M A K E I M A G E S O F T H E I N T E R N A L S T R U C T U R E O F E A R T H .

19.2 – SEISMIC WAVES & EARTH’S INTERIOR

SEISMOMETERS & SEISMOGRAMS

•Most of the vibrations caused by seismic waves cannot be felt at great distances from an earthquake’s epicenter, but they can be detected by sensitive instruments called seismometers, which measure horizontal or vertical motion during an earthquake.

SEISMOMETERS & SEISMOGRAMS

• The frame of a seismometer is anchored to the ground. •When an earthquake occurs, the frame moves but the hanging mass and attached pen do not. • The mass and pen record the relative movement as the recording device moves under them.

SEISMOMETERS & SEISMOGRAMS

•The record produced by a seismometer that can provide individual tracking of each type of seismic wave is a seismogram.

TRAVEL-TIME CURVES

• Travel-time curves provide the average time it takes for P- and S-waves to reach seismic stations located at different distances from an earthquake’s epicenter.

DISTANCE FROM THE EPICENTER

• Waves recorded on seismograms from more distant stations are farther apart than waves recorded on seismograms at stations closer to the epicenter.• The separation of seismic waves on

seismograms can be used to determine the distance from the epicenter of an earthquake to the seismic station that recorded the seismogram.• 3 Stations worth of information is needed to

find the epicenter.

EARTH’S INTERNAL STRUCTURE

• Seismic waves change speed and direction when at the boundaries between different materials.

• As P-waves and S-waves initially travel through the mantle, they follow fairly direct paths.

• When P-waves strike the core, they are refracted, which means they bend.

EARTH’S INTERNAL STRUCTURE

• To find out what happens to S-waves generated by an earthquake, seismologists first determined that the back-and-forth motion of S-waves does not travel through liquid. • They then noticed that S-waves do not travel

through Earth’s center.

• The data collected for the paths and travel times of the waves inside Earth led to the current understanding that Earth has an outer core that is liquid and an inner core that is solid.

EARTH’S COMPOSITION

•Seismic waves change their speed as they encounter boundaries between zones of different materials. •Shadow zones do not receive seismic waves because the waves are bent or stopped by materials of different density.

VISUALIZING SEISMIC WAVES

•The travel times and behavior of seismic waves provide a detailed picture of Earth’s internal structure. •These waves also provide clues about the composition of the various parts of Earth.

IMAGINING EARTH’S INTERIOR

•Because the speed of seismic waves depends on temperature and composition, it is possible to use seismic waves to create images of structures such as slabs and plumes. • In general, the speed of seismic waves decreases as temperature increases.

IMAGING EARTH’S INTERIOR

• Images like this one from Japan are generated by capturing the path of seismic waves through Earth’s interior.

S C I E N T I S T S M E A S U R E T H E S T R E N G T H A N D C H A R T T H E L O C A T I O N O F E A R T H Q U A K E S U S I N G S E I S M I C W A V E S .

19.3 – MEASURING & LOCATING EARTHQUAKES

RICHTER SCALE

• The Richter scale, devised by a geologist named Charles Richter, is a numerical rating system that measures magnitude of an earthquake.

• Magnitude is the measure of the energy released during an earthquake.

• The numbers in the Richter scale are determined by the height, called the amplitude, of the largest seismic wave. • Scale goes from 1-10• Each move magnitude number is a 10 fold increase

in energy release.• Example) A earthquake with a magnitude of 3 releases 10

times more energy than a magnitude 2 earthquake.

MOMENT MAGNITUDE SCALE

• The moment magnitude scale is a rating scale of the energy released by an earthquake, taking into account the size of the fault rupture, the amount of movement along the fault, and the rocks’ stiffness.

MODIFIED MERCALLI SCALE

• The modified Mercalli scale is used to measure earthquake intensity on a scale from I to XII. • The higher the number, the greater the damage the

earthquake has caused.

• The intensity of an earthquake depends primarily on the amplitude of the surface waves generated. • Maximum intensity values are observed in the

region near the epicenter; Mercalli values decrease to I at distances far from the epicenter.

DEPTH OF FOCUS

• Earthquakes are classified as shallow, intermediate, or deep, depending on the location of the focus. • Shallow-focus earthquakes are the most damaging.

LOCATING AN EARTHQUAKE

•The location of an earthquake’s epicenter and the time of the earthquake’s occurrence are usually not known at first. •However, the epicenter’s location, as well as the time of occurrence, can be determined using seismograms and travel-time curves.

DISTANCE TO AN EARTHQUAKE

• Seismologists determine the distance to an earthquake’s epicenter by measuring the separation on any seismogram and identifying that same separation time on the travel-time graph.

DISTANCE TO AN EARTHQUAKE

• To locate the epicenter of an earthquake, scientists identify the seismic stations on a map, and draw a circle with the radius of distance to the epicenter from each station. The point where all the circles intersect is the epicenter.

TIME OF AN EARTHQUAKE

•Seismologists can use a seismogram to gain information about the exact time that an earthquake occurred at the focus.•The time can be determined by using a table similar to a travel-time graph.

SEISMIC BELTS

• The majority of the world’s earthquakes occur along narrow seismic belts that separate large regions with little or no seismic activity.

• The locations of most earthquakes correspond closely with tectonic plate boundaries.

T H E P R O B A B I L I T Y O F A N E A R T H Q U A K E ’ S O C C U R R E N C E I S D E T E R M I N E D F R O M T H E H I S T O R Y O F E A R T H Q U A K E S A N D

K N O W I N G W H E R E A N D H O W Q U I C K LY S T R A I N A C C U M U L A T E S .

19.4 – EARTHQUAKES & SOCIETY

EARTHQUAKE HAZARDS

• Earthquake hazards are factors that determine the severity of damage produced by an earthquake.

• Identifying earthquake hazards in an area can sometimes help to prevent some of the damage and loss of life.

STRUCTURAL FAILURE

• One type of damage caused by earthquakes is called pancaking; shaking causes a building’s supporting walls to collapse and the upper floors to fall one on top of the other like a stack of pancakes.

STRUCTURAL FAILURE

• If the shaking caused by an earthquake has the same frequency of vibration as the natural sway of buildings of certain heights, those buildings will sway the most during the earthquake. •Shorter and taller buildings, however, are less likely to be affected.

LAND AND SOIL FAILURE

• In sloping areas, earthquakes can trigger massive landslides. • In areas with sand that is nearly saturated with water,

seismic vibrations can cause the ground to behave like a liquid in a phenomenon called soil liquefaction. http://www.youtube.com/watch?v=YlSuosOTAV0

LAND AND SOIL FAILURE

• Soil liquefaction can cause trees and houses to fall over or to sink into the ground and underground pipes and tanks to rise to the surface. • The type of ground material can affect the severity of an earthquake in an area. • Ground motion is amplified in some soft materials, such as unconsolidated sediments. • It is muted in more resistant materials, such as granite.

TSUNAMI

• Another type of earthquake hazard is a tsunami—a large ocean wave generated by vertical motions of the seafloor during an earthquake.• Caused when a sudden movement of the ocean floor pushes against the water.• Can travel thousands of kilometers in all directions.• http://www.bbc.co.uk/news/world-asia-pacific-12725646

EARTHQUAKE FORECASTING

• There is currently no completely reliable way to forecast the exact time and location of the next earthquake. • Instead, earthquake forecasting is based on calculating the probability of an earthquake.• The probability of an earthquake’s occurrence is based on two factors: • the history of earthquakes in an area • the rate at which strain builds up in the rocks

WHAT’S BEING DONE TO PREVENT CATASTROPHE

•Buildings can be constructed to withstand vibrations.•Flexible, circular moorings under buildings; made of alternating layers of rubber and steel.•Securing gas appliances•Careful placement of heavy objects

SEISMIC RISK

•The probability of earthquakes in seismic belts is much greater than elsewhere on Earth.•The history of an area’s seismic activity can be used to generate seismic-risk maps.

RECURRENCE RATES

•Earthquake-recurrence rates along a fault can indicate whether the fault ruptures at regular intervals to generate similar earthquakes.

SEISMIC GAPS

• Seismic gaps are sections located along faults that are known to be active, but which have not experienced significant earthquakes for a long period of time.• Earthquakes in 1912 and 1999 happened on either side of Istanbul, Turkey. • The earthquakes around the city leave a seismic gap that indicates that an earthquake is likely to occur in that area.

STRESS ACCUMULATION

• The stress accumulated in a particular part of a fault, together with the amount of stress released during the last earthquake in a particular part of the fault, can be used to develop a stress-accumulation map.