Assessing the effect of input motion duration on seismic site responses of layered soil deposits using spectrally equivalent records

The characteristics of input motions at bedrock have a significant impact on the dynamic behavior of soils at various sites. This study aims to statistically analyze the conditions under which the long-duration (LD) seismic responses of layered soil deposits differ from those common short-duration (SD) responses. Furthermore, this study provides thresholds for the amplification prediction of sites where the differences caused by the duration effect are large enough to be practically significant. To investigate the duration effect of input motions, a series of LD and SD record pairs are assembled from earthquake databases, specifically focusing on records with VS30 > 500 m/s. Each pair of records is carefully selected to ensure that the amplitude and frequency content are equivalent. Subsequently, numerous site response simulations are conducted using the time-domain nonlinear method for a range of soil profiles under all selected records. The relative differences in spectral accelerations at the surface between LD and SD motions are subsequently evaluated. The results indicate that the occurrence of LD/SD response differences strongly depends on the site condition, selected frequency (or period), and intensity level of input motions. For smaller peak ground acceleration levels (PGAinput), both LD and SD input motions with equivalent spectral shapes yield consistent site responses. However, for larger PGAinput levels, significant differences emerge for frequencies >0.5 Hz, especially at softer sites. Finally, frequency-dependent thresholds of PGAinput with 20 % differences are proposed as the conditions in which the duration effect of input motions cannot be ignored in site response analysis.