Strong- Motion Observation and Damage Assessment in Rock and Soil Sites of Kathmandu, Nepal After April 25, 2015 Gokha Earthquake

Abstract eng:
Kathmandu basin in the Nepal Himalaya, is filled with ~ 500 m thick fluvio-lacustrine sediment and has long been considered seismically vulnerable owing to its geo-tectonic setting. Although, large earthquakes in the past have left trails of damage, strong ground-motion studies in Kathmandu were lacking. Recently, the occurrence of the M w 7.8 Gorkha Earthquake on 25th April 2015 caused infrastructure damages in east and central Nepal including Kathmandu. Followed by more than 350 aftershocks larger than Ml4 with largest one (M w 7.3) occurring on 12th May, the Gorkha Earthquake was the latest entry in the list of large damaging earthquakes. These earthquakes garnered a lot of worldwide attention resulting in a recent increase in seismicity study of the Nepal Himalaya. The Earthquake took nearly 9,000 lives all over the country with 1,739 body counts in Kathmandu alone. The official figure shows that about 13% of buildings inside the Kathmandu basin were damaged. The damage in the basin was mostly seen in masonry structures with RC buildings suffering little or no damage at most places The present study is the analysis of strong ground-motion characteristics of the Gorkha Earthquake and its aftershocks, based on the records on four accelerometers inside the Kathmandu basin. The stations were installed by Hokkaido University in collaboration with Tribhuvan University in 2011 for evaluation of seismic site effect of the basin using strong-motion records. One of the station (KTP) lies above bedrock and was considered as reference to compute spectral ratio of other stations viz. TVU, PTN, and THM on the sediment sites. The PGA of the main shock of the Gorkha Earthquake was highest (241 cm/s2) in the rock site, KTP whereas the PGV was found to be highest in sediment site of TVU. The sediment sites show dominance in long-period waves (0.2-0.4 Hz). The spectral ratios show significant amplification in 0.2-1 Hz band at these sites. The strong-motion characteristics were correlated with the damage pattern around an area inscribed by a circle of 200 m radius from the station. The damage to the buildings around the stations did show the influence of local site effect. The acceleration response spectra of the stations are much higher than the design coefficient prescribed by the Nepal Building Code. The amplification in spectral ratio, high PGV values and high acceleration response in 1-2 s period were observed in TVU. The area around TVU saw damages to 10% of buildings as opposed to KTP where only about 5% buildings were damaged. The buildings in KTP were traditional masonry structures built without proper engineering considerations, nevertheless they endured the shaking which, on the other hand, damaged RC buildings around TVU. Another sediment site PTN also showed damage to about 9% of buildings but THM, also sediment site, showed the least damage as most of the buildings were new RC structures.

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