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I am developing an android application (system) which measures the building vibration and calculates its natural frequency. I was planning to map these natural frequencies and during a disaster, this application can be used for risk assessment. But since only magnitude of an earthquake is known it seems impossible to compare the earthquake frequency and building natural frequency. I referred many articles, but I was unable to find what can be done if only the natural frequency of the building is known. If anybody have some idea about this matter, please share me.
I will be grateful for any help you all can provide.

Exact frequency might be a difficult task to handle. The thing you can do is start with an approximate value. This will help in disaster management even on slight less or more frequency as well.