The 'size' of earthquakes

Richter magnitude      Moment magnitude      Mercally Intensity Scale Peak ground acceleration      Related links

Whenever a large destructive earthquake occurs in the world the press inmediately wants to know where the eartquake occurred and how big it was. However, it is not easy to describe in a unique way the 'size' of an earthquake, since it depends in many things, from the parameter we choose to quantify (energy released at the focus? amplitude of the seismic waves in a particular place? the damage done to a city?), to our subjective idea 'big' and 'small'.

An earthquake's magnitude is a measure of its size. Magnitude was first defined by Charles F.Richter, a Californian seismologist, in 1935, as a useful way of comparing one earthquake with another. The Richter magnitude is calculated from the maximum amount of ground motion at a known distance from the epicenter. It works like that: for each increase in 1 in the Richter Magnitude, there is a tenfold increase in amplitude of the wave. Notice that it is incorrect to say that 'each increase of 1 in Richter Magnitude represents a tenfold increase in the size of the Earthquake' (as is commonly incorrectly stated by the Press). This scale give us a rough idea of the amount of energy released in the earthquake, but doesn't accurately tell us the full force of the earthquake, since, of course, its value depends on where we measure it and it will be different in different points on the Earth's surface. (Click here for more details about the Richter magnitude)

The Richter magnitude scale has recently begun to be replaced in favour of the more accurate moment magnitudescale. Instead of being based on the amplitude of a particular seismic wave at a particular distance, the moment magnitude is based on the forces involved in the earthquake rupture process. It can be calculated from an analysis of seismograms, or from a direct measurement of the area of rupture and the amount of slip in an earthquake.

Magnitude is a single statistic (like shoe size), a handy label to put on an earthquake, both to give us a general idea of how strong it was, and for the purposes of comparison with other earthquakes. But it does not tell the full story. From a human perspective, it is not only the energy released in an earthquake which matters but also the earthquake depth and position. A large magnitude eartquake does not necessarily do a lot of damage or cause a tragedy, but even a low magnitude earthquake can cause intense shaking and subsequent damage if it occurs at shallow depth close to a town or city. Shallow earthquakes affect the small area above them badly, but the total area over which they are felt is small. Deeper earthquakes send shock waves further, giving less severe shaking close to the epicenter but moderate shaking over a much larger area.

A more detailed scale to describe the size of an earthquake in terms of the damage is given by the intensity scale. There are different versions for it, but the most used one is the Modified Mercally Intensity Scale (MM scale).

Intensity is defined as the earthquake's actual effect on people, buildings and the environment. It gives us an idea of how much the ground actually shook in different areas. The intensity of any given earthquake varies with area, depending on factors such as the distance from the epicenter, and the structure of the ground in the local environment. Click here for more details about the Modified Mercally Intensity Scale.

Engineers, however, are not really happy with either the MM or the various magnitude scales. What they really want to know, in order to design buildings that can withstand eartquakes, is how much the ground is likely to accelerate in an earthquake, what the frequency of these accelerations will be, and how long the motion will last. However, such things are difficult to place into a simple scale such as those of magnitude or intensity. Instead, scientists talk about Peak Ground Acceleration (PGA) which is expressed as a fraction of gravity. For example, if you experience a vertical acceleration of 1 g you have been pushed hard enought to leave the ground. Since buildings are already designed to withstand gravity, vertical accelerations from earthquakes are not as damaging as horizontal ones. Acceleration can be very high near the source of even quite small earthquakes. That's why earthquakes with small magnitudes can be more damaging than bigger ones.

• Mercally Intensity Scale