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Picking Arrivals in jAmaSeis

Background

Earthquakes are caused by the sudden movement of blocks of the Earth’s crust. Such movement involves the fracture of brittle rocks and the movement of rock along the fractures. A fracture in a rock along which movement occurs is called a fault.

When an earthquake occurs, it releases energy in the form of waves that radiate from the earthquake source in all directions. The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities.

  • The fastest waves, and therefore the first to arrive at a given location, are P waves (Primary waves) or compressional waves. Like sound waves, they move by alternately compressing and expanding material in the direction of motion.

  • The next significant waves to arrive are the S waves(Secondary waves). They travel through the earth moving material up and down and side to side perpendicular to the direction of motion.

  • The last to arrive are Surface waves, which travel slower than P and S waves and move the ground like a rolling ocean wave (Rayleigh) or side to side like S waves (Love). These waves are constrained to travel within the upper layers of the Earth.

  • Tips for Picking P, S, and Surface Waves
  • The P wave is the first arrival

  • The S wave is generally the next large arrival
  • If the event is large, distant, and not too deep you will record Surface waves