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years old. Since the recurrence period of strong earthquakes (including design earthquakes) is
significantly longer than 100 years, it is imperative to make some use of the historical record
of earthquakes in seismic hazard analysis. For some regions of the world (the best example
being China) historical records go back to more than a thousand years, but their completeness
and quality vary greatly. The critical information that can be found in such records regards the
effects of past earthquakes on humans and on structures.
The (macroseismic) intensity of an earthquake refers to the way an earthquake is felt at a
specific site (i.e. its effects on humans, structures and the ground). Therefore, the intensity is a
measure of the severity of ground shaking on the basis of observed effects in a certain area
(rather than a measure of the energy release or the seismic moment). The major advantage of
intensity is that it can be estimated from the historical records, therefore it is essentially the
only viable tool in historical seismicity, and it can be estimated in all affected areas, including
those where no instrumental records exist; hence it is also useful today as a complement to
instrumental measurements. The major disadvantages of intensity are that it varies
significantly within the area affected by an earthquake (note that an earthquake has one
magnitude but several intensities), and its estimation involves substantial subjective
judgement.
A major problem in estimating intensity is that similar structures respond differently to the
same earthquake, due to several reasons whose discussion falls beyond the scope of this book.
Hence the need for appropriately classifying the effects of damage (with at least some rough
allowance for its statistics) and also for appropriately defining the extent of the areas for
which a uniform intensity should be assumed. Typically these areas should correspond to a
village or a relatively small town, or parts of a large city, but strict rules are difficult to set
(European Seismological Commission, 1998).
Starting from the late 1800s, several intensity scales have been suggested. The ones most
commonly used today are the Modified Mercalli intensity (I MM), employed in the Americas,
and the Medvedev-Sponheur-Karnik (MSK) intensity (I MSK), widely used in Europe. Both
scales have 12 degrees, and are generally equivalent (there is a small discrepancy at the lower
end of the scales only). It used to be common to denote the degrees with Roman numerals (I–
XII), primarily to discourage arith-metical manipulation, but the need for computer processing
of intensity data has made it common nowadays to use normal (Arabic) numerals. Since 1992
the European Seismological Commission (ESC) has been developing an updated version of
the MSK scale, called the ‘European Macroseismic Scale’ (ESC Working Group on
Macroseismic Scales, 1998), which might be used extensively in the future.
All the aforementioned intensity scales share several common features, the most important
one being that they are descriptive, in the sense that each degree on the scale is characterized
by a set of ‘diagnostics’ referring to specific effects of an earthquake on humans, buildings,
objects and the nature in general. As an example, a diagnostic referring to humans is ‘many
people find it difficult to stand, even