Le Japon vient d’être frappé par un séisme de 8.9 sur l’échelle de Richter. Le Japon a des règles de construction parasismiques très strictes, ce qui a permis de limiter la catastrophe.
Un Tsunami parcourt en ce moment le pacifique.
L’échelle de Richter
L’échelle de Richter permet de comparer entre elles les énergies libérées dans les différents séismes. Elle est dite objective, parce que basé sur les mesures faites par les sismographes et exprime la magnitude d’un séisme .
La magnitude correspond au logarithme de l’amplitude des vibrations enregistrées par un sismographe étalonné en fonction de la distance épicentrale .
Elle a été crée en 1935 par Charles Francis Richter et Beno Gutenberg, deux membres du California Institute of Technology. C’est une échelle logarithmique : les ondes sismiques d’un séisme de magnitude 6 ont une amplitude dix fois plus grande que celles d’un séisme de magnitude 5 et le séisme de magnitude 6 libère environ trente et une fois plus d’énergie. La magnitude de Richter mesure l’énergie émise sous forme d’ondes élastiques. Un séisme de magnitude 5.0 correspond à peu près à l’énergie dégagée par la bombe nucléaire qui détruisit Hiroshima. La relation qui existe entre la magnitude et l’énergie sismique libérée montre qu’un séisme de magnitude 7 libère à lui seul autant d’énergie qu’une trentaine de séisme de magnitude 6.
On ne doit pas confondre magnitude et intensité : A l’inverse de la magnitude qui se calcule, l’intensité d’un séisme ne peut donner lieu qu’à une estimation.
- # La magnitude est une valeur associée uniquement au séisme. L’intensité est associée au lieu d’observation.
- # Il n’existe pas de véritable relation entre magnitude et intensité. Ainsi deux séismes de même magnitude peuvent donner en surface des intensités différentes. Inversement deux séismes de même intensité en un lieu peuvent avoir des magnitudes différentes.
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