Unlike the magnitude scale, which measures the size of an earthquake at its epicentre, the modified Mercalli scale measures the intensity of shaking felt by people on the ground. From observations gathered after an earthquake, it is possible to draw lines of intensity that radiate outwards from the epicentre. These lines, known as isoseismals, are represented by numbers from 1 to 12.
This map shows isoseismal lines drawn from observations of the 1942 Wairarapa earthquake. The greatest effects (measuring 8) were felt at the epicentre, reducing to 4 at the outer margins. The places where people reported the different intensities are also numbered 1–9. The earthquake did extensive damage to buildings in Wairarapa townships. In Wellington windows were shattered, masonry fell from buildings, and 20,000 chimneys were damaged.
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Source: G. L. Downes, Atlas of isoseismal maps of New Zealand earthquakes. Lower Hutt: Institute of Geological and Nuclear Sciences, 1995.