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Les quatre cartes délimitant les zones sismiques en fonction de l'accélération spectrale montrent l'intensité des secousses sismiques à des périodes de 0,2, 0,5, 1,0 et 2,0 secondes (qui équivalent à des fréquences de 5, 2, 1, et 0,5 Hz respectivement).
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The four spectral acceleration seismic hazard maps show levels of ground shaking at periods of 0.2, 0.5, 1.0 and 2.0 seconds (equivalent to frequencies of 5, 2, 1, and 0.5 Hertz). This is important because different buildings are susceptible to different frequencies of earth motion, and damage is frequently associated with a resonance between earthquake ground motion and the building's own natural frequency. A high-rise of ten stories or more may sway with a natural period of 1 or 2 seconds, whereas in response to the same earthquake a brick bungalow across the street may vibrate at nearly 10 Hertz.
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