|Résumé||(disponible en anglais seulement)|
Definition: Earthquake magnitude (M) describes the energy release (or size) of an earthquake.
There are many types of magnitude scales, with the vast
majority based on recorded seismic waveforms. Magnitude scales are logarithmic, correct for distance from the earthquake, and are unbounded (the smallest earthquakes are less than zero, and the largest recorded event to date is the 1960 M9.5, Chile
earthquake). Some magnitude scales are based on measurements of shorter-period body waves (primary (P) or secondary (S)-waves) and some are based on longer-period surface waves. The original (and most famous) magnitude scale, the 'Richter scale' was
developed in the 1930's for California earthquakes (Richter, 1935). The modern, and most commonly-used magnitude scale is moment magnitude, Mw (Hanks and Kanamori, 1979). It is based on seismic moment (Mo) release, which directly relates to the
fault rupture area and amount of slip. It is important to note that each unit increase in magnitude represents a 10-fold increase in amplitude of shaking, and a 33-fold increase in energy release. For example, a M7 earthquake releases 1000 times as
much energy as a M5 earthquake, and has shaking that is 100 times stronger. Earthquakes can generally be felt starting at M 2-3. Earthquakes may cause minor damage starting at M ~4-5, and earthquakes of M7 or larger are considered major and can be
felt (and have the potential to cause damage) up to 100s of km away. Small earthquakes are much more frequent than large earthquakes ' for example, there are, on average, about 1.3 million M2-2.9 earthquakes around the world each year, compared to
15-20 M 7-7.9 events. Scientists are limited to the instrumental recording period (since the late 1800s) for the accurate estimation of earthquake magnitude. Prior to that time, they rely on written and oral reports that describe the earthquake's
'intensity'. Intensity describes the effects of an earthquake on humans or the environment, and requires no instrumental records. Earthquake magnitude has important applications for engineering geology, including numerous empirical relationships
developed between magnitude and potential for triggering of landslides and liquefaction. Specifically, a historical (and global) review of earthquake-triggered landslides (including rockfalls, delayed-initiation landslides, lateral spreads and flows)
as a function of earthquake magnitude is provided by Keefer (2002). For example, a M 7 earthquake can be expected to generate lateral flows to distances of ~80 km and landslides to distances of ~170 km. A relationship between areas of potential
liquefaction and earthquake magnitude is provided by Wang et al. (2006). Based on their work (and references therein) a M 7 earthquake may cause liquefaction to hypocentral distances of ~160 km.