Resonance effect on shaking of tall buildings in Kathmandu Valley during the 2015 Gorkha earthquake in Nepal

Kathmandu Valley in Nepal is built on the soft soil deposits of lacustrine origin. The structural damage record in the valley during the past earthquakes indicates that resonance effect between the ground and structures plays an important role in intensifying the damage to the structures. Especially, in soft soil deposits, even a small-scale earthquake generates long-period shaking of the ground for comparatively long time with greater displacement. During the 2015 Gorkha earthquake in Nepal, the Kathmandu Valley ground was heavily shaken for more than 60 s with about 4 s of period, which probably was the main cause of heavy damage to multi-story tall buildings in the Kathmandu urban core. This situation of long-period ground shaking during earthquakes and heavy damage to long-period structures may happen anywhere in earthquake-prone regions of the world. So, in this study, after the 2015 Gorkha earthquake (in Nepal), we recorded microtremor and ambient vibration noise data at 33 tall building (from 7 to 19 stories) locations in the Kathmandu Valley and analyzed the recorded data for estimating predominant natural frequencies and natural periods using Floor-Spectral-Ratio technique for the buildings and Horizontal-to-Vertical Spectral Ratio technique for the ground. The results obtained by analyzing the recorded noise data for the buildings are compared with those of the corresponding nearby free-fields to explore the possibility of resonance effect. The comparison indicates that the predominant periods of about 25% tall buildings are close to the predominant periods of the corresponding grounds, and that about 35% tall buildings have almost the same predominant periods as those of the corresponding grounds. This indicates that the resonance effect is one of the major causes of the partial damage to the tall buildings in the Kathmandu Valley during the 2015 Gorkha earthquake, and hence special considerations need to be taken during design and construction of tall buildings in the Kathmandu Valley as well as in similar urban settlements.