Where do earthquakes occur?

Earthquakes occur along plate boundaries where two chunks of lithosphere scrape together. The focus is the location where the energy is released and the epicenter is location on the earth’s surface directly above the focus. Earthquakes that occur underwater may trigger a tsunami, a large, fast-moving ocean wave.

Seismic Waves

During an earthquake, several types of waves are generated. The vibrations felt are actually called seismic waves that are traveling through the Earth.


  • Primary Wave (Compression Wave)
  • Travels “phastest” so it arrives at a seismic stations “phirst”
  • Push-pull wave, vibrates forward and backward in the same direction that the wave travels
  • Pass through solids, liquids (magma), and gasses

  • Secondary Wave (Shear Wave)
  • Slow wave, not as fast as the P-wave, arrives at a seismic station second
  • Shake wave, vibrates side-to-side
  • Solids wave, only travels through solid layers


A seismograph is an instrument that detects and records seismic waves. By studying a seismogram, we can determine a seismic wave’s distance and size

Seismic Waves Traveling Throughout the Earth

  • P-Waves travel through solids, liquids, and gasses
  • S-Waves travel only through solids
  • Seismic waves travel faster through denser material.
  • Because of this, the path traveled by a seismic wave is bent towards the surface.
  • Properties of the material (such as density and pressure) that the waves pass through can be inferred by the speed and angle that the waves travel.
  • The layers of the earth are determined by the jumps in velocity and “echoes” of seismic waves.
  • The MOHO is a boundary between the crust and the upper mantle where the velocity of waves jumps up sharply. This sharp increase in velocity is called a discontinuity.
  • A shadow zone occurs on the opposite side of the earth from an earthquake because of the liquid outer core.
  • S-Waves are stopped all together as they are absorbed by the liquid outer core. The P-Waves are refracted (bent) as they change velocities in different density materials. This !creates a zone in which no waves are picked up at all.