Earthquakes and Seismic Waves

Wave Demonstration

Earthquakes

• Shaking & trembling from the movement of rock
• Cause: movement of Earth's plates
• Plate movements cause stress in the crust
• Stress increases at fault lines until rock breaks
• Earthquake releases the energy stored in rocks

Earthquakes

• begin in the lithosphere
• start underground at the focus
• epicenter is on the surface above the focus

Types of Seismic Waves

• carries energy away from the focus
• travels through the Earth's interior
• travels across the Earth's surface
• there are 3 types of waves

P Waves

• first waves to arrive after an earthquake
• compress and expand like an accordian
• P = primary waves
• can travel through liquids and solids

S Waves

• vibrate from side to side and up & down
• shake the ground back and forth
• travel through solids only
• S = secondary waves

Surface Waves

• when P & S waves reach the surface
• move slower than other waves
• produce severe ground movements
• make ground roll like ocean waves

Wave Diagrams

Measuring Earthquakes

• Magnitude = size of quake
• There are 3 commonly used methods
• Mercalli, Richter and Moment Magnitude

Mercalli Scale

• rates quakes by level of damage

Richter Scale

• rates quakes based on seismic waves
• also rates the fault movement that occurs
• waves are measured by a seismograph

Moment Magnitude Scale

• estimates the total energy released
• uses seismographs
• measure how strong the waves are the types
• used for quakes of all sizes and all in all places
• more accurate than Richter scale

Comparing Magnitudes

• 1 point increase is 32 times more energy
• Below 5 causes little damage
• 6 and above can cause great damage
• Chile 1960 & Alaska 1964 mag.over 9

Locating the Epicenter

• use seismic waves 
• travel at different speeds
• measure the difference between arrival times
• need measurements from 3 locations
• 3 circles are drawn and the overlap is the epicenter

Earthquake Safety

Earthquake Risk

• determine risk by locating faults
• also looks at where past quakes happened
• what areas are at highest risk?

How Earthquakes Damage

• Shaking
• Liquefaction
• Aftershocks
• Tsunamis

Shaking

• can trigger avalanches and landslides
• damages buildings
• loose soil shakes more than solid rock

Liquefaction

• shaking turns loose, soft soil into liquid mud
• happens where soil is full of moisture
• the ground gives way

Aftershocks

• earthquake that occurs after a larger quake
• can happen hours, days, or months later

Tsunamis

• caused by plate movement on ocean floor
• ocean floor rises slightly and displaces water
• large wave is formed
• height wave is low until it reaches shallow water

Steps to Earthquake Safety

• drop, cover and hold (protect your head)
• avoid windows/mirrors/furniture that could fall
• outdoors - move to an open area and sit down
• avoid power lines
• prepare an earthquake kit