Response spectrum

The resulting plot can then be used to pick off the response of any linear system, given its natural frequency of oscillation.

The result is typically different from that which would be calculated directly from an input, since phase information is lost in the process of generating the response spectrum.

This was observed in the 1985 Mexico City Earthquake[1] where the oscillation of the deep-soil lake bed was similar to the natural frequency of mid-rise concrete buildings, causing significant damage.

In 1941 at Caltech, George W. Housner began to publish calculations of response spectra from accelerographs.

For "regular" low-rise buildings, the structural response to earthquakes is characterized by the fundamental mode (a "waving" back-and-forth), and most building codes permit design forces to be calculated from the design spectrum on the basis of that frequency, but for more complex structures, combination of the results for many modes (calculated through modal analysis) is often required.