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1.7.1Electricity, Energy and Waves

Properties of Seismic Waves

5 min read 6 parts Lesson 39 of 54 Unit quiz

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Section 1 of 6Earthquakes and Seismic Waves

1.7.1 Properties of Seismic Waves

Seismic waves carry the energy of an earthquake through and over the Earth.

Cause
  • Tectonic plates can lock together, stress builds up, and the rock suddenly fractures, releasing energy as shaking.
Focus
  • The focus is the point below ground where the earthquake starts.
Epicentre
  • The epicentre is the point on the surface directly above the focus.
Waves
  • The released energy travels outwards as seismic waves, which can shake the ground for seconds to minutes.

Section 2 of 6Types of Seismic Wave

Types of Seismic Wave
P-waves
  • Primary waves are longitudinal, travel fastest, pass through solids and liquids, and arrive first.
S-waves
  • Secondary waves are transverse, slower than P-waves, travel only through solids and arrive second.
Surface waves
  • These travel along the surface, are the slowest, arrive last and cause most damage.
WaveTypeSpeedTravels through
PLongitudinalFastestSolids and liquids
STransverseSlower than PSolids only
SurfaceLongitudinal and transverseSlowestSurface only

Section 3 of 6Detecting Earthquakes

Detecting Earthquakes
Seismometer
  • A large mass attached to a sensor lags behind the ground's motion, and the relative movement generates a small current.
Seismogram
  • The recorded signal is displayed with time on the horizontal axis and the size of the vibrations on the vertical axis.
Order of arrival
  • On a seismogram, P-waves appear first, then S-waves, then surface waves.
Time lag
  • The time between arrival of P- and S-waves is the time lag, which increases with distance from the epicentre.

Section 4 of 6Locating an Epicentre

1
Each recording station measures the time lag between P- and S-waves.
2
A larger time lag means the earthquake was further away.
3
The lag is converted into a distance using a calibration graph or scale.
4
A circle of that radius is drawn around each station on a map.
5
Three stations are needed; the epicentre is where the three circles meet, which is called triangulation.

Section 5 of 6Worked Example

Question

  • A P-wave travels at 6.0 km/s and an S-wave at 3.0 km/s.
  • A seismometer is 900 km from an earthquake.
  • Calculate the time lag between the arrival of the two waves.

  1. Time for P-wave = distance ÷ speed = 900 ÷ 6.0 = 150 s.
  2. Time for S-wave = 900 ÷ 3.0 = 300 s.
  3. Time lag = 300 − 150 = 150 s.

Answer Time lag = 150 s (2.5 minutes)

Section 6 of 6Exam Checklist & Tips

1.7.1 – Key Points for the Exam

  • Focus The point inside the Earth where an earthquake begins.
  • Epicentre The point on the surface directly above the focus.
  • Seismometer An instrument that detects ground vibrations.
  • Triangulation Finding a position using distances from three known points.
  • Time lag The delay between the arrival of P-waves and S-waves.

Common Mistakes

  • Mixing up focus and epicentre; the focus is underground and the epicentre is on the surface.
  • Saying S-waves are faster than P-waves.
  • Using only one station to find the epicentre rather than three.
  • Describing P-waves as transverse; they are longitudinal.
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