Shaking Intensity Information

The Modified Mercalli Intensity scale is used to classify an earthquake by examining its effects on people and the environment. The answers to your felt reports help us to determine what category of shaking you have experienced.

Through the study of New Zealand's historic earthquakes, models have been developed that relate instrumentally-measured shaking with the Modified Mercalli Intensity scale. Due to the simplicity of these relationships, it is not possible for the derived intensities to be accurate in all cases. These intensities may be displayed in addition to the publicly reported intensities to give a wider picture of the effects of an earthquake.

The Modified Mercalli Intensity scale is used to classify an earthquake by examining its effects on people and the environment. The answers to your felt reports help us to determine what category of shaking you have experienced.

Instrument shaking

The peak acceleration at each seismic recording station is measured in units of percent-g (%g), where g is the acceleration due to the force of gravity, 9.80655 m/s². For moderate to large earthquakes (magnitude 5 and above), PGA gives good agreement with reported shaking. However, the pattern of peak ground acceleration is typically quite complicated, showing variability even over distances of a few kilometres. This can be explained by the differing soil types and topography near the stations that can significantly change the characteristics of the seismic waves.

The peak velocity at each seismic recording station is measured in units of centimetres per second (cm/s). Once again, for moderate to large earthquakes (magnitude 5 and above), PGV gives good agreement with reported shaking. Furthermore, the pattern of peak ground velocity is usually simpler than the pattern of peak ground acceleration.

The maximum Wood-Anderson amplitude at each seismic recording station is measured in units of millimetres (mm). The Wood-Anderson torsion seismometer was the instrument that formed the basis of Charles Richter's magnitude scale developed in 1935. In order to use that scale faithfully, seismic waveforms are synthesised to have the same characteristics as if they had been recorded on a Wood-Anderson instrument. This also allows us to conveniently compare amplitudes recorded on different seismometers. For small to moderate earthquakes (less than magnitude 5), W-A gives good agreement with reported shaking.

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