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Chapter 7 Earthquakes

Preview

Section 1 What Are Earthquakes?

Section 2 Earthquake Measurement

Section 3 Earthquakes and Society

Concept Map


Section 1 What Are Earthquakes?Chapter 7

Bellringer

Describe what happens during an earthquake. What do you think causes earthquakes?

Write your answers in your Science Journal.



What You Will Learn

• Earthquakes are ground motions that result from the release of energy when blocks of rock move.

• Most earthquakes occur along tectonic plate boundaries because the movement of tectonic plates causes stress in Earth’s crust.

• Earthquake energy travels through rock as seismic waves.



Where Earthquakes Happen

• Earthquakes are movements or shaking of the ground that happen when blocks of rock move suddenly and release energy. The transfer of this energy causes the ground to shake.

• Most earthquakes take place near the boundaries of tectonic plates. However, some earthquakes have occurred far from tectonic plate boundaries.



Where Earthquakes Happen, continued



Where Earthquakes Happen, continued

• Tectonic plates move in different directions and at different speeds.

• Two plates can push toward or pull away from each other, or slip past each other horizontally.

• These movements break Earth’s crust into a series of faults.



Faults at Tectonic Plate Boundaries

• A fault is a break in Earth’s crust along which blocks of rock slide relative to each other.

• Specific types of plate motion take place at different tectonic boundaries.

• Each type of motion creates a particular kind of fault.



Earthquakes at Divergent Boundaries

• At divergent tectonic plate boundaries, two tectonic plates pull away from each other.

• Tension causes the lithosphere to break into a series of fault blocks.

• Some of these block drop down and form a series of normal faults.



Earthquakes at Divergent Boundaries, continued• A mid-ocean ridge is a

divergent boundary.

• Ocean lithosphere is thin and weak.

• Therefore, earthquakes that happen along normal faults are shallow.



Earthquakes at Convergent Boundaries

• At convergent tectonic plate boundaries, two tectonic plates collide with one another.

• When plates collide, two things may happen: • Both plates can crumple up to form

mountains.• Or one plate can move underneath the other

and sink into the mantle.



Earthquakes at Convergent Boundaries, continued• The process of one plate moving under another is

called subduction.

• When plates collide, the two plates are compressed.

• Compression causes the lithosphere to break into a series of fault blocks.



• Fault blocks are thrust over one another as the plate moves.

• The blocks form a series of reverse faults. Earthquakes happen along reverse faults.

• Two types of earthquakes occur at subduction zones.

Earthquakes at Convergent Boundaries, continued



• Earthquakes at depths of less than 50 km occur between the two plates.

• Earthquakes at depths up to 700 km occur inside the down-going plate.

Earthquakes at Convergent Boundaries, continued



Earthquakes at Transform Boundaries

• At transform boundaries, two plates move past one another horizontally.

• The rocks on both sides of the fault are sheared or broken as they grind past each other in opposite directions.

• The shear stress causes the rock to break into blocks that form a series of strike-slip faults.



Earthquakes at Transform Boundaries, continued• Most transform boundaries exist between plates

made of oceanic lithosphere.

• Some transform boundaries occur between plates made of continental lithosphere.



• Rocks deep below Earth’s surface tend to react to shear stress by folding, not breaking.

• As a result, earthquakes along strike-slip faults occur at depths of less than 50 km.

Earthquakes at Transform Boundaries, continued


Earthquakes

Plate Motion and Earthquake Characteristics

Chapter 7



Fault Zones

• Places along plate boundaries where large numbers of interconnected faults are located are called fault zones.

• Faults in fault zones can occur at different depths, have different lengths, and cut through the lithosphere in different directions.



Fault Zones, continued

• The San Andreas fault zone is located along a transform boundary.

• The fault zone is primarily a strike-slip, or transform, fault system.



Why Earthquakes Happen

• As tectonic plates move, stress on rocks causes the rocks to deform.

• Rocks can deform in a plastic manner, like clay being molded.

• Folded rocks are a result of plastic deformation.



Why Earthquakes Happen, continued

• Rocks can deform in an elastic manner, like a rubber band being stretched.

• Elastic deformation leads to earthquakes.

• Just like a rubber band breaking, rock that deforms in an elastic manner slips and releases energy.




• The sudden return of elastically deformed rock to its original shape is called elastic rebound.

• Elastic rebound happens when stress on rock along a fault becomes so great that the rock breaks.

• Rocks on either side of the fault jerk past each other and release energy.




• Energy travels through rock as seismic waves.

• Seismic waves cause the ground to move.

• The strength of an earthquake is related to the amount of energy that is released during elastic rebound.



Earthquake Waves

• Earthquake waves are the physical result of the movement of energy through Earth as seismic waves.

• Seismic waves that travel through Earth’s interior are called body waves.

• Seismic waves that travel along Earth’s surface are called surface waves.



Earthquake Waves, continued

• There are two types of body waves: P waves and S waves.

• P waves, or pressure waves, are the fastest seismic waves.

• P waves are also called primary waves, because they are always detected first after an earthquake.




• P waves can travel through solids, liquids, and gases.

• P waves squeeze and stretch rock as they travel forward.

• S waves, or shear waves, are the second-fastest seismic waves.




• S waves are also known as secondary waves, because they arrive later than P waves after an earthquake.

• S waves cannot travel through parts of Earth that are completely liquid.

• S waves shear rock horizontally from side to side.


Earthquakes

Body Waves: S Waves and P Waves

Chapter 7




• Surface waves travel more slowly than body waves. They produce motion only near the top of Earth’s crust.

• Because their energy focuses on the surface, surface waves tend to cause the most damage.

• Surface waves can travel in an up-and-down or back-and-forth motion.


Section 2 Earthquake MeasurementChapter 7

Bellringer

How do major earthquakes and minor earthquakes affect the area in which they happen?

What causes the difference in their effects?




What You Will Learn

• To find an earthquake’s epicenter, you must triangulate by using data from three or more seismometers.

• Magnitude is a measure of an earthquake’s strength.

• The intensity of an earthquake depends on four major factors.



Studying Earthquakes

• Scientists use instruments called seismometers, or seismographs, to record seismic waves.

• Seismometers record the vibrations of P waves, S waves, and surface waves.

• Seismometers also record the time it takes for waves to arrive at a seismometer station.



Studying Earthquakes, continued

• Seismometers create a tracing of earthquake motion called a seismogram.

• Seismograms help to locate the earthquakes epicenter, the point on Earth’s surface directly above the earthquake’s starting point.

• The earthquake’s starting point inside Earth is called the focus.




• The earthquake’s starting point inside Earth is called the focus.

• The epicenter is directly above the focus.




• The lag time between the arrival of P waves and S waves tells scientists how far the waves have traveled.

• Scientists draw a circle around a seismometer station that has a radius equal to the distance the waves have traveled.

• Scientists draw circles around three seismometer stations and find the point of intersection.




• The point at which all circles intersect is the epicenter.

• This process of locating the epicenter is called triangulation.


EarthquakesChapter 7

S-P Time Method: Finding an Epicenter



Earthquake Magnitude

• Magnitude is the measure on an earthquake’s strength.

• The greater the magnitude, the stronger the earthquake.

• In the past, the Richter scale was used to describe earthquake strength. Now, scientists use the magnitude moment scale.



Earthquake Magnitude, continued

• The Richter Scale measures ground motion from an earthquake and adjusts for distance to find an earthquake’s magnitude.

• Richter-scale values range from 0-9.

• Each increase of one number represents a tenfold increase in strength.



Earthquake Magnitude, continued

• The magnitude moment scale is a more accurate measure of earthquake strength.

• Magnitude moment (Mw) represents the:• size of the area of the fault that moves• average distance moved by fault blocks, and• rigidity of rocks in the fault zone.



Earthquake Intensity

• An earthquake’s intensity is the effect of the earthquake on people.

• The Modified Mercalli scale describes earthquake intensity.

• Intensity ranges from barely noticeable to total destruction of an area.



Earthquake Intensity, continued



Earthquake Intensity, continued

• Earthquake intensity maps show the level of intensity expected in different areas that experience the same earthquake.

• Data from past earthquakes are used to create earthquake intensity maps.



The Effects of Earthquakes

• Effects of earthquakes can vary over a wide area.

• Effects depend on the size of the earthquake.

• Effects also depend on three other factors: distance from the epicenter, local geology, and type of construction in the area.



The Effects of Earthquakes, continued

Distance from the Epicenter

• The total energy in a seismic wave stays relatively constant as the wave travels.

• Seismic waves grow increasingly larger as they move away from the epicenter.




• As seismic waves grow larger, the amount of energy at any one point decreases.

• Therefore, an earthquake is less destructive to areas that are farther away from the epicenter.




Local Geology

• The amount of damage caused by an earthquake depends on the material through which seismic waves travel.

• Seismic waves are particularly dangerous when they travel through water-saturated soil or sediment.




• When seismic waves shake water-saturated sediment or soils, sediment grains lose contact with each other and are surrounded by water.

• This process is called liquefaction.




• Liquefaction can intensify ground shaking.

• Liquefaction can also cause the ground to settle, which can cause structures to tilt or collapse.




Earthquake-Resistant Construction

• Brick and concrete structures are easily damaged by earthquakes.

• Wood and steel structures are more flexible and less likely to be damaged.

• Shorter buildings, on strong, anchored foundations are also less likely to be damaged.


Section 3 Earthquakes and SocietyChapter 7

Bellringer

If you have experienced an earthquake, write a short paragraph that describes how you felt and what you did to protect yourself.

If you have not experienced an earthquake, write a paragraph that describes what you think you would do and how you think you would feel during a moderate earthquake.




What You Will Learn

• The magnitude of earthquakes may be related to how frequently earthquakes happen.

• Earthquakes and tsunamis can affect human societies.

• Homes, buildings, and bridges can be strengthened to decrease earthquake damage.



Earthquake Hazard

• Earthquake hazard is a measure of how likely an area is to have damaging earthquakes in the future.

• Earthquake-hazard level is determined by past seismic activity.

• California has a very high earthquake-hazard level.



Earthquake Hazard, continued



Earthquake Forecasting

• Forecasting earthquakes is difficult.

• Scientists study earthquakes to discover patterns in earthquake strength and frequency.

• The relationship between earthquake strength and frequency is based on the amount of energy released during earthquakes.



Earthquake Forecasting, continued

• Stronger earthquakes are much rarer than weaker earthquakes.

• Millions of small earthquakes release the same amount of energy as one large earthquake does.




• The gap hypothesis is a way to forecast earthquake location, strength, and frequency.

• The gap hypothesis states that sections of active faults that have had relatively few recent earthquakes are likely to be the sites of strong earthquakes in the future.




• The areas along an active fault where relatively few earthquakes have happened are called seismic gaps.

• Stress has a long time to build at seismic gaps.

• When a fault breaks at a seismic gap, the sudden release of stress can cause a large-magnitude earthquake.




• In 1988, scientists predicted a > 6.5 magnitude earthquake would happen within 30 years in a seismic gap near Santa Cruz.

• The 1989 Loma Prieta earthquake of magnitude 6.9 happened in the gap.



Gap Hypothesis and Seismic Gaps



Reducing Earthquake Damage

• Much of the loss of human life during earthquakes is caused by buildings that collapse.

• Retrofitting can make older buildings more earthquake-resistant.

• Common ways to retrofit include strengthening structures with steel and fastening the building to its foundation.



Reducing Earthquake Damage, continued



Are You Prepared for an Earthquake?

• You can plan ahead to protect yourself and your property from earthquake damage.

• Safeguard your home by putting heavy objects on low shelves.

• Ask your parents about having your home strengthened.



Are You Prepared for an Earthquake?, continued• Find places that are safe within each room of your

home and outside your home.

• Make a plan to meet with others in a safe place after the earthquake.

• Store water, nonperishable food, a fire extinguisher, flashlight with batteries, radio, medicines, and a first aid kit in a safe place.



• If an earthquake happens when you are indoors, stay indoors until the earthquake stops.

• Crouch or lie face down under a table or desk in the center of the room.

Are You Prepared for an Earthquake?, continued



• If you are outside, stay outside.

• Lie down away from buildings, power lines, or trees and cover your head with your hands.

• If you are in a car, stop the car and remain inside.




• After the earthquake, stay calm and get your bearings as quickly as possible.

• Identify immediate hazards, such as downed power lines, broken glass, or fire hazards.

• Stay out of damaged buildings.




• Return home only when someone in authority says it is safe.

• Remember that there may be aftershocks, which may cause more damage.




Tsunamis

• Earthquakes on the ocean floor can generate tsunamis.

• A tsunami is an extremely long wave that can travel across the ocean at speeds of up to 800 km/h.



Tsunamis, continued

• Tsunamis most often form when an earthquake causes a vertical movement of the sea floor, which displaces an enormous volume of water.



Tsunamis, continued

• In the open ocean, tsunami waves can seem very small.

• As tsunami waves enter shallow water along a coastline, the energy of the waves is compressed.

• The waves get rapidly taller. By the time they reach shore, waves can be taller than 30 m.



Tsunamis, continued

• Tsunamis can cause damage and loss of life by smashing into and washing away anything in their path.

• Almost 150 tsunamis happened worldwide during the 20th century.

• In 2004, an undersea earthquake of magnitude 9.3 caused a tsunami that killed more than 280,000 people and left 1.25 million people homeless.



Tsunamis, continued

• Tsunamis are monitored by most of the nations that border the Pacific Ocean.

• These nations provide seismic and tide data to the Pacific Tsunami Warning Center (PTWC) in Hawaii.

• If a tsunami has been generated by an undersea earthquake, the center sends a bulletin to warn officials in threatened areas.


Chapter 7 Earthquakes

Use the terms below to complete the concept map on the next slide.

seismometer

earthquakes

body waves

seismic waves

surface waves

S waves

Concept Map



Concept Map

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