Earthquakes

transcript

EarthquakesChapter 8 & Then Some

Quiz• 1. Most Earthquakes occur along ________________.• 2. The first seismic waves to arrive are______________.• 3. The second seismic waves to arrive are

_____________.• 4. The last seismic waves to arrive are_______________.• 5. Which seismic waves travel the fastest?___________• 6. Which type of seismic wave can move through a

solid, liquid or a gas?________________• 7. Which seismic wave cannot travel through material

that is completely liquid?______________• 8. Which seismic waves are the slowest and the most

destructive?_______________

Where do Earthquakes occur?

• Most earthquakes take place near the edges of tectonic plate.

• Earthquakes occur along faults, where blocks of crust move.

Earthquake Zones• Earthquakes can occur near the Earth’s

surface or far below the surface.

• Most earthquakes occur at plate boundaries (in earthquake zones), but some happen at faults located in the middle of tectonic plates.

Earthquake Locations Around the World

Earthquakes in Alabama?!

Surely, you’re kidding? How? Why?

The New Madrid Fault• If there is no plate boundary in the middle

of the United States, why do these earthquakes take place?

• Geologists are beginning to understand the answer. The New Madrid Fault Zone is part of an ancient plate boundary.

• In this area, the North American Plate tried to form a divergent plate boundary about 500 million years ago. The splitting stopped before new plates could form.

The New Madrid Fault• The faults in the New Madrid Zone are remnants of

this old event. Earthquakes occur because the North American Plate is still "settling down".

• The faults in the New Madrid Zone do not reach the Earth’s surface.

• They are buried beneath thousands of feet of rock and sediment deposited by the Mississippi River.

• Geologists have located them by looking at the patterns of earthquakes in the zone.

New Madrid Fault

Several of the largest Several of the largest earthquakes ever earthquakes ever recorded in the United recorded in the United States occurred in the States occurred in the Midwest, far from any Midwest, far from any plate boundary. These plate boundary. These earthquakes took place earthquakes took place in an area called the in an area called the New Madrid Fault Zone, New Madrid Fault Zone, named after the town of named after the town of New Madrid, Missouri.New Madrid, Missouri.

Alabama’s Earthquake Zone

Earthquakes in Alabama• The last earthquake to occur in Alabama

was on July 27, 2007.– It measured a 2.6 on the Richter scale.

(A little stronger than the one crazed LSU fans caused in October 9,1988, when they defeated Auburn 7-6 for the SEC title. 80,000 people jumping up & down created surface waves, triggering an earthquake. Imagine that many people jumping up & down on a trampoline.)

• This was not a very strong earthquake, but it was recorded by a seismograph, an instrument used to measure earthquakes.

What Causes Earthquakes?

• Caused by movement along faults (breaks in the Earth’s surface)

• Rock deforms (changes) due to stress

• Two types of deformation:– Plastic deformation: bending of rock;

no earthquakes created– Elastic deformation: bending of rock

until it breaks; causes earthquakes

What is Elastic Rebound?

• occurs when rock is suddenly returned to its undeformed (original) shape– Example: rubberband snapping back after being

stretched

• The rock will bend until enough force causes the rocks on either side of a fault to move and release energy.

• Think of it as a rubber band being stretched to its limit and released—it snaps back.

Elastic Rebound

3 Types of Faults Associated with Earthquakes

PLATE MOTION

FAULT TYPE

Transform Strike –Slip Fault

Convergent Reverse FaultDivergent Normal Fault

Strike-Slip Fault occurs at a Transform Boundary

Reverse Fault occurs at a Convergent Boundary

Normal Faults occur at Divergent Boundaries

Earthquakes and Faults Organizer: Draw the boundary on the plate motion side & the fault moving on the fault

type side (label Hanging Wall & Foot Wall)

PLATE MOTION FAULT TYPETransform Boundary: Plates move past each other

Strike –Slip Fault: Fault blocks move past each other

Convergent: Plates push toward each other

Reverse Fault: Fault blocks move toward each other

Divergent Boundary: Plates move apart

Normal Fault: Fault blocks move apart

Earthquake Energy: Just How Strong Was

It Anyway?Earthquake Energy=Seismic Energy

How do Earthquake Waves Travel?

• Energy released from moving plates & faults travels through the Earth as waves.

• Waves travel at different speeds & in different ways, depending on what kind of material they are moving through.

Earthquake EnergyThis energy is called seismic energy

and is measured in seismic waves:

– Two types of seismic waves:

• body waves • surface waves

Body Waves: waves that travel through the Earth’s interior

• There are two types of body waves:– P-waves– S-waves

P and S wave comparison

P-waves: primary waves or pressure waves

• travel through solid, liquids and gases

• fastest waves (travel 5-8 km/h), arrive first & are first ones to be detected

• P-waves cause rock to squeeze and stretch. Imagine a slinky!

P-Waves

S-waves: secondary waves

• second fastest waves; 2nd to arrive; shear waves

• slower than P-waves; travel 3-5 km/h• S-waves move rock from side to side.• S-waves can’t travel through parts of

the Earth that are completely liquid.

S-Waves

Surface Waves• move along the surface of the Earth• can move up, down & around or

side to side• Surface waves move the slowest &

cause the most destruction.– Think about it? Where do most people

live: on the Earth’s crust or in it? Shouldn’t these waves be the most destructive since they occur where there is more stuff to destroy?

Surface Waves

Seismic Wave Arrival

• First- P-waves • Second- S-waves• Third- Surface Waves

Measuring Earthquakes

Seismographs & SeismogramsWait aren’t those the same?

Measuring Earthquakes• Seismographs: instruments used to

sense earthquakes

• Seismogram: tracing of earthquake movement made by seismograph

• Seismologists use seismograms to locate earthquakes.

Picture of a Seismograph

Time and Location of Earthquakes• Seismologists use seismograms to find the

epicenter of earthquakes: – the point on top of the Earth above the true

center of an earthquake which is called the focus—it’s inside the Earth. (Also called the hypocenter.)

Seismograms

Measuring Earthquakes• Earthquakes are defined by their

strength and depth, which is measured at the place the earthquake occurs.

• 2 Scales are used to rank the quakes:– Richter Scale—measures magnitude– Mercalli Scale—measures intensity

Earthquake strength & intensity

• Richter scale: measures magnitude (think strength) of ground motion on a scale of 2.0-8.0 (numbers)

• Mercalli scale: measures degree to which an earthquake is felt by people & amount of damage caused, which is known as intensity– Scale of I-XII (Roman Numerals)

Magnitude Effects

2.0(2-2.9 very minor)

Only felt by seismograph

3.0(3-3.9, minor) Felt at epicenter

4.0(4-4.9 light)

Felt by most people in area

5.0(5-5.9, moderate)

Damage at epicenter

6.0(6-6.9, strong)

Widespread damage

7.0 (7-7.9 major)

Great, widespread damage

8.0 or higher(8 & up, great)

Catastrophic destruction

Reading Richter Scale Maps

Reading Mercalli Scale Maps

Seismograph• A seismograph is an instrument used by scientists to measure earthquakes.• Seismologists who study earthquakes can determine when an earthquake

started by noting the arrival times of P-waves and S-waves.• A seismograph records vibrations in the Earth and determines the strength

and location of an earthquake.

Chinese SeismographAncient Chinese Seismograph. The ball would drop from the dragon to the frog. It told the people from which direction the earthquake came.

Seismograms

Time in Minutes0 1 2 3 4 5 6 7 8

1. How many minutes did it take for the

P-Waves to arrive?

2. How many minutes did it take for the

S-waves to arrive?

3. How long did the surface waves last?

Mercalli Intensity Scale

1. What is the intensity at Monterey?

2. What is the intensity at the epicenter?

3. What is the intensity at San Jose?

4. What is the intensity at Santa Cruz?

5. What is the intensity at Smith?

Earthquakes

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