Seismic Wave Amplification in the Gilan and Qazvin Thick Sedimentary Basins, Northern Iran

Summary

The Gilan and Qazvin sedimentary basins in northern Iran are characterized by a thick Cenozoic sedimentary cover exceeding ~10 km. This study investigates seismic wave attenuation/amplification in these young, thick basins to assess its impact on seismic hazard, particularly amid the rapid growth of multi-story buildings. Using waveforms from 350 local earthquakes recorded by 72 seismic stations, we estimated amplification across the region. Our results reveal significant amplification within the Gilan and Qazvin sedimentary basins over frequency range of ~0.8–5.0 Hz, with peak factors exceeding 5 at frequencies around 1.0 Hz. Amplification is observed in both horizontal and vertical components, though more pronounced on the horizontal component. For longer periods, we applied spectral ratio method to teleseismic shear waves from 189 teleseismic earthquakes, finding persistence of the large amplification for periods between 1-10 s. Additionally, our Horizontal-to-Vertical Spectral Ratio analysis of the recorded local earthquakes and noise data vividly shows that the observed amplification is not related to shallow low-velocity soil layers. A positive correlation between long-period amplification and the sedimentary basin depth suggests that the deep sedimentary layers dominate ground-motion amplification in the Gilan and Qazvin plains. Unlike low-velocity soil layers, the low-velocity young sedimentary rocks would exhibit linear behavior at larger magnitudes, leading to persistent amplification for strong motions. These findings provide the first quantitative assessment of low-frequency amplification in Iran’s thick Cenozoic basins and underscore the need to incorporate basin effects into hazard models, especially given the expanding inventory of vulnerable multi-story structures.