Earthquakes happen when rocks in Earth’s crust suddenly break or slip after stress has built up along a fault. Tectonic plates are always moving slowly, but friction can lock their edges together for years, decades, or longer. When the stored elastic energy becomes too large, the rocks snap into a new position and release energy as seismic waves.
Understanding this process helps people identify hazards, design safer buildings, and respond more effectively after shaking begins.
The point underground where slipping first starts is called the focus, and the point directly above it on the surface is the epicenter. Energy spreads outward from the focus in waves that shake the ground, including fast P waves, slower S waves, and surface waves that often cause the most damage. Different fault types form depending on how rocks are pushed, pulled, or sheared by plate motion.
Seismographs record the waves, allowing scientists to locate earthquakes and estimate their magnitude.
Key Facts
- Earthquakes occur when stored elastic strain energy is released by sudden motion along a fault.
- Stress = force / area, or σ = F / A.
- The focus is the underground starting point of rupture, while the epicenter is the surface point directly above it.
- P waves travel fastest and move rock by compression and expansion.
- S waves travel slower than P waves and move rock side to side or up and down.
- Average speed relation: v = d / t, where v is wave speed, d is distance, and t is travel time.
Vocabulary
- Fault
- A fault is a fracture in Earth’s crust where blocks of rock have moved past each other.
- Tectonic plate
- A tectonic plate is a large, moving section of Earth’s lithosphere made of crust and uppermost mantle.
- Focus
- The focus is the location inside Earth where an earthquake rupture begins.
- Epicenter
- The epicenter is the point on Earth’s surface directly above an earthquake’s focus.
- Seismic wave
- A seismic wave is an energy wave produced by an earthquake that travels through Earth or along its surface.
Common Mistakes to Avoid
- Confusing the focus with the epicenter is wrong because the focus is underground and the epicenter is on the surface above it.
- Thinking earthquakes happen only during volcanic eruptions is wrong because most earthquakes are caused by sudden slip along faults due to tectonic stress.
- Assuming tectonic plates move quickly right before every earthquake is wrong because plates usually move slowly while stress builds over long periods.
- Treating magnitude and damage as the same thing is wrong because damage also depends on depth, distance, ground type, building design, and population density.
Practice Questions
- 1 A P wave travels 120 km in 20 s. What is its average speed in km/s?
- 2 An earthquake focus is 15 km below the surface, and the epicenter is directly above it. If a seismic station is 36 km horizontally from the epicenter, what is the straight-line distance from the station to the focus?
- 3 A fault has been locked for many years while the plates on either side continue to move. Explain why this can lead to a sudden earthquake instead of smooth motion.