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📘 What causes earthquakes?

Look down at this road after California’s 2019 Ridgecrest earthquake. Painted edges that once met are displaced sideways, recording several feet of right-lateral motion across the rupture. No wandering plate crossed that distance during the

4
lessons
~20 min
to learn
Adults
level
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What you’ll learn

  1. A fault stores the slow motionExplain how slow plate motion, frictional locking and elastic strain lead to sudden slip on a pre-existing fault.Plate tectonics loads rough fault zones slowly; locked patches make surrounding rock deform elastically until resistance is overcome and sudden slip releases stored energy.
  2. Rupture runs through a faultDistinguish fault types, hypocenter and epicenter, then trace how a rupture propagates and redistributes stress into an aftershock sequence.A rupture nucleates on a fault, spreads patch by patch according to geometry and stress, slips over a finite area, and leaves a changed stress field that produces aftershocks.
  3. A break becomes shakingDescribe how P, S and surface waves carry energy, how seismograms locate earthquakes, and how source magnitude differs from local shaking intensity.Sudden slip launches several wave types whose different motions and speeds appear on networked seismograms; those records reveal location and source size while local intensity still varies.
  4. Plate boundaries set the patternsConnect global earthquake patterns to plate boundaries, compare shallow megathrust and deep-slab events, and explain how human activity can trigger a stressed fault.Plate boundaries concentrate deformation, subduction creates both giant interface events and deep slab earthquakes, and human-caused pressure or stress changes can sometimes provide the final push on existing faults.

Questions this course answers

What is the immediate cause of most tectonic earthquakes?

Plate motion loads the crust slowly; an earthquake begins when part of a fault slips suddenly and releases stored strain energy.

Why can a locked fault be hazardous even when it has been quiet?

Friction can hold the fault while continuing plate motion bends nearby rock, so quiet time can be loading time.

How does an earthquake hypocenter differ from its epicenter?

The focus or hypocenter is a subsurface starting point, while the epicenter is its vertical map projection.

Why does the time gap between P and S waves increase at distant stations?

The two wave types leave the source together but have different speeds, so their arrival separation grows with distance.

Why can some wastewater injection induce earthquakes?

Induced seismicity requires a particular combination of pressure pathways, a capable fault and a favourable stress state; most wells do not cause felt events.

Grounded in trusted sources

  • What is an earthquake and what causes them to happen? — U.S. Geological Survey — https://www.usgs.gov/faqs/what-earthquake-and-what-causes-them-happen
  • The Science of Earthquakes — U.S. Geological Survey Earthquake Hazards Program — https://www.usgs.gov/programs/earthquake-hazards/science-earthquakes
  • Elastic Rebound — U.S. Geological Survey Earthquake Hazards Program — https://www.usgs.gov/media/videos/elastic-rebound
  • Understanding Plate Motions — U.S. Geological Survey — https://pubs.usgs.gov/gip/dynamic/understanding.html
  • Seismographs—Keeping Track of Earthquakes — U.S. Geological Survey Earthquake Hazards Program — https://www.usgs.gov/programs/earthquake-hazards/seismographs-keeping-track-earthquakes
  • What are the Effects of Earthquakes? — U.S. Geological Survey Earthquake Hazards Program — https://www.usgs.gov/programs/earthquake-hazards/what-are-effects-earthquakes
  • At what depth do earthquakes occur? What is the significance of the depth? — U.S. Geological Survey — https://www.usgs.gov/faqs/what-depth-do-earthquakes-occur-what-significance-depth
  • Collection of 3D Geometries of Global Subduction Zones — U.S. Geological Survey Earthquake Hazards Program — https://www.usgs.gov/programs/earthquake-hazards/science/collection-3d-geometries-global-subduction-zones

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

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