|Résumé||(disponible en anglais seulement)|
Earthquake intensity describes the severity and effects of ground shaking, for a given earthquake at a specific location, on humans, man-made structures and the
natural environment. As examples, the intensity describes how the vibrations were felt by people (eg, not felt, light, strong, intense), and the impact on contents and components of buildings.
Worldwide, the principal earthquake intensity scales
are: the Modified Mercalli (Wood and Neumann, 1931), Japan Meteorological Agency (Japan), and European Macroseismic (Grunthal, 1998) scales. All intensity scales are 'bounded' with a set range - for example, the Modified Mercalli (MMI) scale uses
Roman numerals from I to XII (see Figure 1). As examples, MMI II indicates: 'Felt only by a few people'; MMI IV indicates 'Felt indoors by many, outdoors by few, frightened no one', MMI VI indicates 'Strong shaking, Felt by all, indoors and
outdoors, a few instances of fallen plaster', MMI VIII indicates 'Severe shaking, fall of chimneys, walls, 'heavy furniture overturned'. Traditionally, seismologists relied on newspaper accounts or returned mail questionnaires to estimate the impact
of earthquakes at different localities. Since the early-mid-2000's, citizens are encouraged to answer questions online (e.g., 'USGS Did You Feel it; Wald et al., 1999). In these questionnaires, individuals choose the description that best
corresponds to the felt effects and observed impacts of the shaking where they were at the time of the earthquake. Based on these answers, a computer program rates the reported earthquake impact on the MMI scale. By plotting each MMI report on what
is called an isoseismal map, the area where the earthquake was felt is obtained (called the 'felt area'), as well as the areal extent of damage at a certain intensity level. Because a correspondence exist between the MMIs and the levels of ground
motions (velocity, acceleration), it is possible to use recorded vibrations to infer intensities, and vice-versa.
Because the intensity scale does not rely on instruments, the effects and sizes of earthquakes that may have occurred hundreds (or
even thousands) of years ago can be estimated. From either the felt area, the damaged area or the maximum intensity, the magnitude and locations of pre-instrumental earthquakes can be estimated. Much care must be taken in converting one into the
other: intensities depend on a number of factors, some due to the earthquake itself, such as the magnitude, focal depth, directivity of ground motions, while some are due to conditions where the observer was at the time of the earthquake, such as
distance from the epicenter, local geology, topography, building type and condition and on the individual itself (e.g., at rest, moving, in a car). Earthquake intensity scales as described above) have applications for evaluating geological hazards.
For example, empirical relationships have been developed between MMI and landslide potential as a function of distance (e.g., see Keefer, 2002). There are also important engineering geology applications from 'intensity measures' that are based on
instrumental recordings. As one example, Arias intensity and other 'intensity scales' (see Kramer and Mitchell, 2006 and references therein) that utilise the strength and duration of ground shaking to evaluate liquefaction potential.