4 The size of earthquakes

4.1 Earthquake intensity

Why are some earthquakes more destructive than others? There are three main reasons: location (an earthquake with a focus under an ocean at a large distance from land, for example, is not usually destructive, unless it generates a tsunami), depth (shallower earthquakes can be more devastating), but, most importantly, the size of the earthquake.

A straightforward way to measure the size of an earthquake is to look at the damage it caused. This is a measure of the strength of the ground shaking caused by an earthquake, and is called the earthquake intensity. To estimate the intensity of an earthquake at a single place, descriptions of what happened to people and structures during the earthquake are collected, and the description compared to a scale of earthquake effects, such as ‘objects fall off shelves’ or ‘most chimney pots fall off’. The descriptions are given numbers on a 12-point intensity scale, with intensity 1 being not felt, 8 including ‘most chimney pots fall off’ and 12 being total devastation. The intensity scale is a qualitative scale because it is not based on specific measurements, even though it has numbers.

It is important to realise that intensity refers to the damage caused at a particular place by an earthquake. An earthquake will have different intensities in different places. It is therefore wrong to refer to the intensity of an earthquake; it is necessary to state the intensity in, say, Los Angeles, or ‘the maximum intensity’. The intensity at the epicentre of the 1989 Loma Prieta earthquake was intensity 8, but the maximum intensity (and damage) was in parts of San Francisco and Oakland, about 100 km to the northwest of the epicentre, where intensity 9 was recorded. This was mainly due to the presence of a loose sand layer at the ground surface in the San Francisco Bay area which did not support structures as well as the solid rock nearer to the epicentre.

Intensity scales have two major disadvantages; they are no use for earthquakes under the oceans and no use for uninhabited areas. Even in inhabited areas, building standards vary in different areas, so earthquake effects are different. To overcome these problems in measuring the size of earthquakes, a different way of measuring earthquake size, using instruments, is often used instead of intensity.