Chapter 5: Earthquakes 5.1 Forces in Earth’s Crust 5.2 Earthquakes and Seismic Waves 5.3 Monitoring Earthquakes.

Chapter 5: Earthquakes

5.1 Forces in Earth’s Crust5.2 Earthquakes and Seismic Waves

5.3 Monitoring Earthquakes

Why do earthquakes occur more often in some places than in others?

• BRAINSTORM. – Then check your answer once we reach the end of

the chapter!

5.1 Forces in Earth’s CrustHow Does Stress Change Earth’s Crust?

• Think back to the movement of the plates. The pieces of crust can be strong or weak, causing bends and fold.

• Stress is a force that acts on rock to change its shape or volume. This is expressed in unit per area.

• As force increases, stress increases. Force is an increase in energy

Kinds of Stress

• Types of stress act on rocks over millions of years to change the shape and volume of rock.– Tension: plates pull apart, crust becomes thinner

in the middle– Compression: plates come together, plates

squeeze– Shearing: plates slip past each other, grind past

each other

Earthquake Risk Around the World

• Most earthquake activity occurs near the Ring of Fire in the Pacific Ocean– West coast of Central America– West coast of South America

• India, China and Pakistan due to the Indo-Australian Plate colliding with the Eurasian Plate.

• Common where the Eurasian Plate meets the Arabian and African Plates

How Do Faults Form?

• When enough stress (force) builds up, the rock will break and a fault will form.

• A hanging wall is a piece of rock that sits over a fault.

• A footwall is a piece of rock that lies under the fault.

Types of Faults• Normal Fault: faults cut at an angle so one block of rock sits over the fault and one sits under. Occur where two plates diverge, or pull apart. The hanging wall slips down.

• Reverse Fault: same structure as a normal fault. Hanging wall moves up.

• Strike-Slip Fault: rocks on either side of the fault slip past each other. Think of a transform boundary!

How Does Plate Movement Create New Landforms?

• Over millions of years, the forces of plate movement can change a flat plain into features such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus.

Folding Earth’s Crust

• How Folds Form– Compression shortens and thickens Earth’s crust

• How Anticlines and Synclines Form– Compression forces fold crust upward into an arch

(anticline) and downward into a v-shape (syncline)• How Folded Mountains Form– Collision of two plates can cause compression and

folding of the crust over a wide area

Stretching Earth’s Crust

• Mountains that are separated by broad valleys or basins have been formed from tension in Earth’s crust that cause faulting.

• Footwalls move outward and hanging walls slip downward.

Uplifting Earth’s Crust

• The forces that raise mountains can also raise plateaus.

• A plateau is made of many flat layers and is wider than it is tall.

• Famous Plateau: Four Corners in Arizona, Utah, Colorado and New Mexico.

5.2 Earthquakes and Seismic Waves

• When you focus on something you are looking at a specific point. The focus of an earthquake is the point at which a rock beings to move, causing an earthquake.

• An earthquake is the shaking and trembling that results form movement of rock beneath Earth’s surface. – Some can be unnoticed, some can cause the earth to

break open, shift mountains and cause great damage.– What are Earthquakes?

Cause of Earthquakes• Forces of plate movement cause earthquakes.

Remember forces are transfers of energy. When the energy builds so high, an earthquake occurs.

• Seismic waves are vibrations that are similar to sound waves. They travel through Earth carrying energy released by an earthquake.

Types of Seismic Waves• Waves race out from the focus of an earthquake.• The point on the surface directly above the focus

is the epicenter.• Three types of seismic waves:– P Waves: 1st waves to arrive, compress and expand

ground like an accordion, travel through liquids and solids; damage buildings

– S Waves: 2nd waves to arrive, vibrate, only travel through liquids, move left and right (think of swaying)

– Surface Waves: move more slowly but produce ground movements that roll like ocean waves.

How Are Earthquakes Measured?

• The amount of earthquake damage or shaking that is felt is rated using the Modified Mercalli Scale. The magnitude, or size, of an earthquake is measured on a seismograph using the Richter scale or moment magnitude scale.

• A seismograph is an instrument that records and measures an earthquake’s seismic waves.

The Modified Mercalli Scale

• Rates the amount of shaking form an earthquake.

• Rated by observations, not instrumentsRank Description

I-III People notice vibrations like those form a passing truck. Unstable objects disturbed.

IV-VI Some windows break. Plaster may fall.

VII-IX Moderate to heavy damage. Buildings jolted off foundations.

X-XII Great destruction. Cracks appear in ground. Waves seen on surface.

The Richter Scale

• The magnitude is a single number that geologists assign to an earthquake based on the earthquake’s size.

The Moment Magnitude Scale• Used to rate the total energy an earthquake releases.• Use seismographs

Comparing Magnitudes• Each one-point increase in magnitude

represents the release of about 32 times more energy.

• Magnitudes above 6 can cause great damage.• Magnitudes of 8 or above are rare and the

most powerful.

How is an Epicenter Located?• Geologists use seismic waves to locate an

earthquake’s epicenter.– Use seismographs from all over the world.– Observe the arrival of the P and S waves.– Circles are drawn around three seismograph

stations to locate the epicenter of an earthquake. The radius of each circle is the distance form that seismograph to the epicenter. The point where the three circles intersect is the location of the epicenter.

5.3 Monitoring Earthquakes• Seismic waves cause a simple seismograph’s drum

to vibrate, which in turn cause the pen to record the drum’s vibrations.

Measuring Seismic Waves

• The pen of the seismograph moves and the paper stays stationary. Since the pen is suspended, the movement of the earthquake causes the seismograph to move because it is anchored to the ground and vibrates when the seismic wave arrives.

• How Seismographs Work

Reading a Seismogram

• When an earthquakes seismic waves reach a seismograph, the vibrations are recorded. – P waves arrive first and the fastest– S waves arrive shortly after the P waves– Surface Waves produce the largest disturbances

on the seismogram.– An AFTERSHOCK is a smaller earthquake that

occurs after a larger earthquake.

What Patterns Do Seismographic Data Reveal?

• From past seismographic data, geologists have created maps of where earthquakes occur around the world. The maps show that earthquakes often occur along plate boundaries.

• Earthquake risk largely depends on how close a given location is to a plate boundary.– At risk: California, Washington, Alaska

So, why do earthquakes occur more often in some places than in others?

• Earthquakes often occur along plate boundaries, where stress caused by plate movement stores energy in rock that makes up the crust.