Earthquakes and Seismic Waves

6th Grade: Section 2 Chapter 2

Review: Forces in the Earth’s Crust

• Shearing is the movement that happens to the crust along a Transform Boundary

• Tension is the movement that happens to the crust along a Divergent Boundary

• Compression is the movement that happens to the crust along a Convergent Boundary

What are the 3 types of faults we have studied?

• Normal Fault:– occurs when plates diverge– results from tension – The hanging wall slips below the footwall

• Reverse Fault:– Occurs when plates converge– Results from compression – The hanging wall moves up above the footwall

• Strike-slip Fault:– Occurs when plates slip past each other – Results from shearing – Sideways motion, not up and down

Where do Earthquakes come from?

• PLATE TECTONICS!

• When plates move and create faults, the force exerted on the crust (compression, tension, or shearing) creates Earthquakes

What is the definition of an Earthquake?

• The shaking and trembling that results from tectonic plates shifting, causing rocks to break along fault lines and releasing energy throughout the earth’s interior.

What are the components of an Earthquake?

• Focus: the area within the Earth’s lithosphere where the rock breaks, triggering the Earthquake

• Epicenter: the point on the Earth’s surface directly above the focus

Earthquake components: Seismic Waves

• Seismic Waves: the vibrations traveling through the Earth that carry the energy released during an Earthquake

– 3 types of Seismic Waves: • P waves (primary)• S waves (secondary)• Surface waves

Seismic Waves

• P waves: seismic waves that compress and expand like an accordion

• S waves: seismic waves that vibrate from side to side as well as up and down

• Surface waves: when p and s waves reach the epicenter, and move across the surface of the Earth in the motion of waves like the ocean

Waves in the Earth’s Interior:Slinky Demonstration:http://www.youtube.com/watch?v=KZaI4MEWdc4

• P-wave motion: – They move the fastest and can move through

solids and liquids– Can damage buildings when they reach the

surface

• S-wave motion: – Move more slowly than P waves – Cannot move through liquids

Common Questions after Earthquakes:

• “How big was the quake?”

• “Where was the earthquake centered?”

Measuring Earthquakes

• 1) Mercalli scale

• 2) Richter scale

• 3) Moment Magnitude Scale

Measuring Earthquakes

• Mercalli Scale: 12 step rating based on how much damage the earthquake causes on the surface of the Earth– The same earthquake can have different ratings

at different locations

--uses Roman numerals: I, II, III, IV, V, VI, VII, VIII, IX, X, XI, XII

Mercalli Scale:

• I-III: People notice vibrations like those from a passing truck

• IV-VI: Slight damage. People run outdoors. • VII-IX: Moderate to heavy damage. Buildings

jolted off foundations or destroyed• X-XII: Great destruction. Cracks appear in the

ground. Waves seen on the surface.

Measuring Earthquakes

• The Richter Scale: a measurement of an earthquake’s magnitude (the size of the waves)

• Scientists assign a magnitude by measuring an earthquake’s seismic waves

• Scientists use a SEISMOGRAPH to measure the seismic waves on a Richter Scale

• Provides accurate measurements for small, nearby earthquakes. But does not work well for large or distant earthquakes

Measuring Earthquakes

• Moment Magnitude Scale: – Estimates the total energy released by an

Earthquake

– Measure the magnitude of the seismic waves, the strength of the rocks that broke, the distance that the fault slipped

Canterbury, Sept.4, 2010 magnitude= 7.1

Myanmar, March 25th 2011magnitude 6.8

Virginia August 23 2011magnitude 5.8

Great Chilean Earthquake: 1960, magnitude= 9.5

Magnitude: tells how much energy was released by the earthquake

• Each one point increase in magnitude represents a release of 32 times as much energy

• Earthquake with a magnitude of 5 releases 32 times as much energy as an earthquake with a magnitude of 4