Unit 3: Earthquakes and Seismic Waves
How rock breaks and releases energy as waves you can track across the planet, including the shadow zone that revealed a liquid core nobody had seen.
7 topics · 23 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
What Causes Earthquakes: Elastic Rebound
- How it worksHow does rock bend and store elastic energy along a locked fault before it finally breaks?
- How it worksWalk me through the elastic rebound of rock on either side of a fault the instant it slips.
- Common mistakeWhy doesn't a fault release all of its stored energy in one single, final earthquake?
Your turn1 of 2
A fault accumulates strain at a rate of 22 millimeters per year. The fault ruptures once enough strain has built up to produce 2122 millimeters of sudden slip. About how many years pass between ruptures?P Waves and S Waves
- How it worksHow does a P wave compress and stretch rock as it travels, like a slinky pushed end to end?
- How it worksWalk me through how an S wave shakes rock side to side, perpendicular to the direction it travels.
- Common mistakeWhy can't an S wave travel through the liquid outer core the way a P wave can?
- Going furtherWhat happens to a P wave's speed the moment it crosses from solid mantle into liquid outer core?
Your turn1 of 2
P waves travel through Earth's crust at roughly 7 kilometers per second, while S waves from the same earthquake travel at roughly 4 kilometers per second. How many kilometers per second faster does the P wave travel?Surface Waves and Ground Shaking
- How it worksHow do Love waves and Rayleigh waves roll and twist the ground once P and S waves reach the surface?
- Going furtherWhat happens to surface wave damage when an earthquake's waves pass through soft, wet soil instead of bedrock?
- Ties togetherHow do surface waves arrive later than P and S waves despite causing the most damage?
Your turn1 of 2
Surface waves travel at roughly 3 kilometers per second, slower than S waves at roughly 4.15 kilometers per second, for the same earthquake. Over a distance of 380 km, how many more seconds does the surface wave take to arrive than the S wave?Locating an Earthquake's Epicenter
- How it worksHow does the growing time gap between a P wave and an S wave arriving reveal how far away a quake was?
- Worked exampleFind the distance to an earthquake if its P wave and S wave arrive 4 minutes apart at one station.
- How it worksWalk me through drawing three distance circles on a map to triangulate a single epicenter.
Your turn1 of 2
At a seismic station, the P wave and S wave from an earthquake arrive 4 minutes apart. Using the rule that each minute of gap corresponds to roughly 126 km of distance to the earthquake, about how far away was the earthquake?Measuring Earthquake Size: Magnitude and Intensity
- How it worksHow does a magnitude scale use the height of a wiggle on a seismogram to measure an earthquake's size?
- Worked exampleFind how many times more energy a magnitude 8 earthquake releases than a magnitude 6 earthquake.
- Common mistakeWhy isn't magnitude the same thing as the shaking intensity felt in a particular town?
Your turn1 of 2
Each whole step up the magnitude scale corresponds to about 31.6 times more released energy. A magnitude 8 earthquake is 2 whole magnitude steps larger than a magnitude 6 earthquake. About how many times more energy does it release?The S-Wave Shadow Zone and Earth's Liquid Core
- How it worksHow does a ring-shaped shadow zone where S waves never arrive reveal that Earth's core must be liquid?
- How it worksWalk me through tracing S-wave paths outward from an earthquake until they hit the shadow zone's edge.
- Common mistakeWhy doesn't the P-wave shadow zone appear in the same place as the S-wave shadow zone?
- Ties togetherHow does the S-wave shadow zone connect to the boundary between the liquid outer core and solid inner core?
Your turn1 of 2
Seismic stations located roughly opposite an earthquake on the far side of the planet detect no S waves at all, forming a ring-shaped shadow zone. How does this ring-shaped shadow zone reveal that Earth's core must be liquid?Earthquake Hazards and Faults
- How it worksHow do a normal fault, a reverse fault, and a strike-slip fault each move rock in a different way?
- How it worksWalk me through how a sudden vertical shift on an underwater fault generates a tsunami.
- Common mistakeWhy doesn't a normal fault get its name from being the most common type of fault?
Your turn1 of 2
A tsunami's wave speed in the open ocean can be estimated as the square root of gravity, 9.8 meters per second squared, times the water depth. If the ocean depth is 3786 meters, about how many meters per second does the tsunami travel?