Determining the spectral ratio of the vertical and horizontal components of near-field earthquakes

Based on studies conducted in recent decades, it has been determined that the characteristics of earthquakes in the near-field zone are different from those of earthquakes far from faults[1], Among these differences, we can mention the effect of the vertical component of the earthquake.Earthquakes near faults have unique features that neglecting them can increase the damages and human and environmental disasters caused by these types of earthquakes, and their effects on the environment and existing structures can be several times greater. Most of these damages occur in the short distance from the epicenter, especially in the horizontal components of long-span bridges, structures, and buildings. The effects of the vertical component of the earthquake in the past were considered negligible, so in past researches and the old versions of some regulations, the effect of the vertical component of the earthquake was ignored or it was considered as a coefficient (two-thirds) of the horizontal component. Therefore, one of the simple methods to determine the design spectrum of the vertical earthquake component is to use empirical relationships such as the V/H spectral ratio (vertical to horizontal component).