1 agosto 2016

Measuring the force of an earthquake: intensity and magnitude

The damage generated by an earthquake is linked to the presence of man and his buildings, without which the tremors would cause no damage. Observation of effects was the first method used to classify the force of an earthquake.

The Mercalli scale

In 1902 the Italian seismologist Giuseppe Mercalli elaborated the idea that others before him had already had of classifying the effects that an earthquake has on man, buildings and the environment, dividing them into 12 degrees of intensity: the Mercalli scale, which then formed the basis for later macroseismic scales.
The macroseismic intensity classifies macroscopic effects, more evident in an earthquake and it reaches its peak at the epicentre, decreasing the further the distance from the epicentre. It does not however decrease evenly, because the effects depend not only on the characteristics of the seismic wave but also and above all on those of the ground that the wave meets at the surface and the characteristics of the buildings. Intensity cannot therefore be considered an objective measurement of the size of the earthquake because it is linked to the location in question (urban or rural area) and to how man has occupied the territory and what he has built.

The Richter scale
In 1935 the American seismologist Charles Richter introduced an objective method of measuring an earthquake. Starting with the method used to defined the size of stars, based on observation of their brightness, he defined the magnitude of an earthquake as a value that could be calculated from the recordings of seismic instruments and linked with the energy associated with the event.
A unique value that refers to the hypocentre, regardless of the method of propagation and the characteristics of the territory involved, which has no direct correspondence with what is felt or observed during the shaking.
Magnitude, calculated using a model that links the amplitude on the seismogram with the distance from the epicentre, is a logarithmic quantity, in other words one variation of a unit corresponds to a movement 10 times greater of the pen motor, and therefore of the ground, and is equivalent to an earthquake 30 times bigger.
The energy of an earthquake of magnitude 7.0, similar to that in Reggio Calabria and Messina in 1908, is almost 1000 times greater than that of an earthquake of magnitude 5.0. The maximum magnitude to date attributed to a seismic event is 9.0. (South East Asia, 26 December 2004).