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📘 The ground is moving, even when it feels still

Look at a world map of tectonic plates: it appears still, but its boundaries are active seams. Each plate is a huge slab of rigid rock moving slowly over hotter, weaker material below. At some boundaries, plates pull apart; at others, they

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. Where the strain beginsExplain how tectonic plate motion, friction, fault locking, and elastic rebound create an earthquake.Plates move slowly, but rough fault surfaces can lock. Strain accumulates around the stuck section until friction is overcome, the fault slips, and the crust rebounds. Surface rupture may reveal that movement, although much of the process happens underground.
  2. How the shaking travelsDistinguish the focus from the epicenter and describe how seismic waves and seismographs reveal an earthquake.Rupture begins at the focus and projects to the epicenter at the surface. Faster compressional P waves arrive before shear S waves, while surface waves follow. Seismographs record those arrivals so networks can infer an earthquake's location and size.
  3. Why the effects varyRelate magnitude, intensity, local ground, buildings, and undersea displacement to the varied effects of earthquakes.Magnitude describes the source; intensity describes shaking at a place. Distance, depth, wave path, soil, and structures change the outcome. Undersea vertical seafloor movement can also transfer energy to the ocean and generate a tsunami.

Questions this course answers

What happens when a locked fault finally overcomes friction?

A stuck fault can store strain while plate motion continues. Sudden slip releases that stored energy, sending seismic waves through the crust.

Put the earthquake sequence in order, from slow preparation to the first shaking.

The earthquake is the rapid release at the end of a longer loading process: motion, locking, strain, then slip.

Match each earthquake term with what it identifies.

These terms distinguish source location, surface location, and two different types of wave motion and speed.

In your own words, why can one earthquake produce different levels of shaking in different places?

Magnitude describes the source, while intensity describes what happens at a particular place. Distance, geology, wave direction, and building structure can change the local result.

Grounded in trusted sources

  • U.S. Geological Survey, The Science of Earthquakes, https://www.usgs.gov/programs/earthquake-hazards/science-earthquakes
  • U.S. Geological Survey, Reid's Elastic Rebound Theory, https://earthquake.usgs.gov/earthquakes/events/1906calif/18april/reid.php
  • U.S. Geological Survey, What are the Effects of Earthquakes?, https://www.usgs.gov/programs/earthquake-hazards/what-are-effects-earthquakes
  • Incorporated Research Institutions for Seismology, Seismic Wave Motions—4 waves animated, https://www.iris.edu/hq/inclass/animation/seismic_wave_motions4_waves_animated
  • NOAA National Centers for Environmental Information, Introduction to Global Historical Tsunami Data, https://www.ncei.noaa.gov/products/natural-hazards/tsunamis-earthquakes-volcanoes/tsunamis/global-historical-data

Every Wunder lesson is built from real, reputable sources — never invented.

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