Development of a Vertical Design Spectrum for Iranian Plateau Using V/H Spectral Ratio

In last two decades, some earthquakes like Kobe (1995), Chi-chi (1999) and Bam (2003) have shown the importance of vertical component of ground motion in inflicting damage on a variety of structural systems. Moreover, it has been shown that the effects of vertical component of earthquake in structural responses are more pronounced in the near fault regions. Therefore, It seems that, both horizontal and vertical design spectra are required in structural design procedure to reduce the vulnerability of structural systems to seismic loads. However, some of the existing design codes or guidelines are providing the designers only with the horizontal spectrum. In some others, the vertical spectrum is defined using a unified ratio of 2/3 with respect to the horizontal one. A new trend in design codes approaches is to introduce vertical design spectrum for the ground motion. Typically, there are two approaches in obtaining the vertical component of response spectrum using Ground Motion Prediction Equations (GMPEs). The differences between these approaches are based on the method of using GMPEs in development of vertical spectrum. The first approach is a direct application of GMPEs for vertical component of earthquake and in the second one; the attenuation model is in the form of vertical to horizontal spectral ratio function (V/H). The attenuation model in this case is used to scale the horizontal spectrum to the vertical one. While V/H ratio usually scales down the horizontal spectrum, it may scale up the spectrum in near distances particularly for the short period range of the response spectrum.
GMPEs have a key role in seismic hazard evaluation for site-specific spectra. To propose a GMPE for any specific region the magnitude, source-to-site distance and peak ground characteristics of earthquakes in that region are required. In addition, some other parameters such as site class, faulting mechanism and so on might be considered necessary in development of GMPEs for a particular region. Although, various GMPEs have been developed for horizontal component of earthquake, there are no reliable GMPEs for vertical component of earthquake in Iranian plateau.
In this study, after selecting the required GMPEs (GMPEs for horizontal and vertical component as well as GMPE for vertical to horizontal spectral ratio), the integrity of the results for development of vertical spectrum is evaluated. The sensitivity analyses for the V/H model show the relative independancy of this ratio to the magnitude and faulting mechanism of earthquakes (as well as site classes). Therefore, the source-to-site distance parameter is chosen as the sole contributor in defining the V/H ratio. Later, a simplified model for V/H ratio in terms of distance (source to site) is proposed in this study. Seismic hazard analysis for vertical component of earthquake is performed using V/H at a desired site and compared with the results of uniform hazard spectrum (UHS) analysis for the same component of earthquake in the region. Later, a vertical design spectrum for the Iranian plateau based on V/H ratio is proposed. At the end, using a calibration technique that can convert the horizontal uniform hazard spectrum to the design code horizontal spectrum is used to find the disgn response spectrum for vertical component of earthquake for Iranian plateau.