12. Time–Frequency Analysis of Strong Ground Motions from the 1994 Northridge Earthquake

Accelerograms recorded in an earthquake are time-domain signals and do not convey information on the frequency content of the wave. The frequency domain representation of the accelerogram shows the distribution of the frequency content but hides information on the time characteristics. Since both the amplitude and frequency of the seismic wave contribute to structural damage, a comprehensive understanding of the possible structural damage can be understood using the three-dimensional approach of continuous wavelet transform (CWTs). Due to damage in an earthquake, the stiffness of a structure degrades, and the natural period elongates. The vulnerability of the structure thereby also depends on the sequence of arrival of damaging frequencies. In this paper, the strong ground motions, recorded within a fault distance of 20 km, in the devastating 1994 Northridge earthquake (Mw 6.7) are characterized in both the time domain and the frequency domain. The CWTs for these ground motions are generated to understand the time sequence of the arrival of high amplitude frequencies. The information obtained from the CWTs is analyzed and correlated with the observed structural damage, in a variety of buildings having a varied range of natural periods, due to the earthquake.