Earthquakes: Faults, Focus & Seismic Waves

• Earthquakes and volcanoes are caused by geological activity.
• Lesson one addresses earthquakes.
• Lesson two addresses volcanoes.

Earthquakes

• Earthquakes are vibrations in the Earth's ground.
• Earthquakes are due to the movement of plates at fault lines.
• Earthquakes mostly occur along plate boundaries

Faults

• Faults represent the break in the Earth's lithosphere
• Faults are where one block of rock moves away from or passes another block or rock.
• Strike slip faults occur at transform plate boundaries.
• Normal faults occur at divergent plate boundaries.
• Reverse faults occur at convergent plate boundaries.

Earthquakes, Focus and Epicenter

• Seismic waves are energy that travels as vibration in the earth.
• The focus is a point inside the earth where the earthquake first starts.
• The epicenter is the location on the earth's surface directly above the focus.

Types Of Seismic Waves

• Primary waves travel in a push-pull motion.
• Primary waves are the fastest-moving seismic waves.
• Primary waves travel through solids and liquids.
• Secondary waves are slower than primary waves but faster than surface waves.
• Secondary waves only travel through solids.
• Secondary wave particles move in an up-and-down motion.
• Surface waves move in a rolling motion, the slowest seismic wave.
• Surface waves cause the most damage to the Earth's surface.

Earth's Interior

• S-waves cannot travel through liquids.
• P-waves can travel through both solid and liquid.
• Scientists discovered the Earth's outer core is liquid because the s-wave cannot travel through it but the p-wave can.

Finding an Epicenter

• First find the difference between the arrival time of the p-wave and the arrival time of the s-wave (lag time).
• Next use an earthquake distance graph to determine the distance from the epicenter.
• Then draw a circle with the correct distance around the station and repeat for two more stations.
• The epicenter is where the circles intersect.

How to Measure Earthquakes

• The Richter scale measures the amount of ground motion at a given distance.
• The Moment magnitude scale measures the amount of energy released by an earthquake.
• The Modified Mercalli scale measures the intensity of an earthquake based on the amount of damage.

Earthquake Risks

• Seismologists use five indicators to assess earthquake risk.
• The five indicators are past earthquakes, probability, population density, geology around a fault and building designs.

Volcanoes

• A volcano is a vent in the Earth's crust through which molten rock forms.
• Volcanoes form where plates collide which causes one plate to subduct under another plate.
• Volcanoes form where two plates separate and magma comes out, such as at a mid-ocean ridge and rift valley.
• Volcanoes associated with play boundaries form chains of islands, like Hawaii.

Three Types of Volcanoes

• Shield volcano
• Composite volcano
• Cinder cone volcano

Shield Volcano

• Shield volcanoes are large, shield-shaped volcanoes with gentle slopes and eruptions.

Composite Volcano

• Composite volcanoes are large, steep-sided volcanoes that result from explosive eruptions.

Cinder Cone Volcano

• Cinder cone volcanoes are small, steep-sided volcanoes that erupt gas-rich, basaltic lava with moderately explosive eruptions.

Caldera

• A caldera is a large volcanic depression created when the summit of the volcano collapsed during a violent eruption.

Two Types of Eruptions

• One type is Violent eruptions, which has lava with a high viscosity and gas content.
• Another type is Quiet eruptions, which has lava with a low viscosity and gas content.

Four Effects of Volcanic Eruptions

• Lava flows slowly destroy towns.
• Ash fall causes breathing problems and can disrupt air traffic by cooling the Earth's atmosphere.
• Mudflows are caused by snow and ice melt that mixes with mud ash, causing the mudflows.
• Pyroclastic flows can be deadly when violent eruptions throw gas and rock into the air.

Predicting Volcanoes

• Prediction is done by observing ground deformation, increases in earthquakes or volcanic gases.
• Prediction is also done by observing if water near volcanoes is becoming more acidic.

Climate

• Volcanic eruptions affect the climate.
• Volcanic ash blocks the sun.
• Volcanic eruption causes a decrease in global temperatures which can also cause rain.

This lesson explores the causes and characteristics of earthquakes. It covers faults, the focus and epicenter of earthquakes, and different types of seismic waves. It is ideal for earth science students.