Warm-Up (10/26)  What factors play into the severity of an Earthquake at any given location? Explain in complete sentences.

Earthquakes An in-depth look

What are they?  Earthquake – Natural vibrations of the ground caused by movement along gigantic fractures in Earth’s crust, or sometimes, by volcanic eruptions.

Are they dangerous/destructive?  Yes, but it depends on the size of the Earthquake. More violent shaking will generally cause more destruction. (The larger the Earthquake the more destruction). Some earthquakes are barely felt and cause little damage to a particular area.

Why do they occur?  Caused by the breaking of a rock deep within the Earth (Generally). These breaks are called fractures or faults.  When the stress, forces acting on an area, exceeds the strength of the rock it can break.  Three types:  Compression – Squeezes the rock together causing faults  Tension – Pulls the rock apart causing faults  Shear – Twisting causes the rock to fault  The material then changes shape, which is called strain.

Faults – What are they and why are they important.  When the stress on a rock is applied too quickly or in too large of amounts, causing the rock to fail, the resulting fractures are called faults.  Faults are fractures along which unintended movement can occur.  Move along the fault plane.

Types of Faults  Reverse Fault – fractures formed by horizontal compression.  Causes a shortening of the crust.  Normal Fault – fractures cause formed by horizontal tension.  Causes a lengthening of the crust.  Strike-slip Fault – fractures caused by horizontal shear.  “twists” the earth’s crust.  Ex. San Andreas Fault in California.

How does it cause vibrations?  As the pieces of Earth rub against each other it causes friction. That friction sends off vibrations through the Earth.

Seismic Waves  Any Vibrations sent through the ground during an earthquake.  Three types:  Primary Waves – P-waves – Squeeze and pull rocks in the same direction as the movement of waves. Pass through the Earth’s Interior.  Secondary Waves – S-waves – cause rocks to move at right angles (perpendicular) in relation to the direction of the waves. Pass through the Earth’s Interior  Surface Waves – move in two ways through the rock (up and down and side to side) as the wave travels through it. Pass along the surface of the Earth.

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Where seismic waves come from  Focus - The point inside the crust of the Earth that the failure of the rock occurred due to stress. This is where an Earthquake originated from.  Epicenter – the point on the Earth surface directly above the focus

Warm-up 10/27  Why is

Recap from 10/26  Two main categories of waves:  Body Waves: Waves that move through the interior of the Earth.  Two Types:  Primary Waves: (p-waves) First to arrive, move compressionally, move through solid, liquid and gas.  Secondary Waves: (s-waves) Second to arrive, move perpendicular to direction traveling, Move through solid and semi-solid but not liquid or gas.  Surface Waves: Move along the surface of the Earth.  Surface waves: last to arrive, move in two directions perpendicular to the direction wave is traveling.

Body Waves vs. Surface Wave Movement Surface Wave Body Waves

Complete the Graphic Organizer Seismic Waves Body WavesSurface Waves Move and

Seismology  Seismology is the study of Earthquakes.  Seismographs – also called seismometers – a sensitive instrument that picks up the vibrations/seismic waves sent out by an earthquake.  Seismogram – record of an earthquake captured by a seismograph.

Seismorgraph  Consists of a base, frame, spring or wire, mass, pen, and rotating drum (with paper)  As the earth shakes the base and the frame move with the earth. The spring being held by the weight tries to stay in place which marks the movement of the base and frame on the paper contained in the rotating drum.  It is measuring the mass remaining at rest due to inertia.

Seismogram  Data collected on the seismogram shows the p-waves and s- waves of earthquakes.  Data has been collected year after year by scientist at the seismograph locations.

Travel-Time Curves  P – Waves and S-waves always travel at the same speed for each earthquake.  Due to this fact we know that p-waves travel faster and s- waves travel slower. This data has been collected over years from countless earthquakes.  Data is used to create a Travel Time Curve.  We can use the difference between the arrival of p-waves and s-waves to determine the distance the earthquake traveled.

Travel-Time Curve

How this gives us a location.  One seismograph cannot determine the epicenter of an earthquake.  We use three seismographs to triangulate the epicenter. It is found by drawing radii around the city of the seismograph.  This process is called triangulation. The Epicenter is where all three radii intersect with each other.

Triangulation

Questions for Earthquakes  1. Explain how a seismometer works.  2. What is a seismogram?  3. What is a seismic travel-time curve, and how is it used to study earthquakes?  4. Even though surface waves arrive at a location last why do the cause so much damage?  5. What is stress and strain?  6. Compare and contrast the three types of faults.  7. What causes most earthquakes?