We proposed a reliable reference site for the site effect study in and around the Kathmandu valley, which is the location of the capital of Nepal. The data used was accelerograms recorded at two sites, DMG and KKA, and velocity seismograms co-recorded at station PKIN during nine shallow local and regional earthquakes of magnitude equal to, or greater than 5.0. The DMG site is located on the thick fluvio-lacustrine sediments of the Kathmandu valley, whereas the others are rock sites. KKA is located on the granite and gneisses of the Shivapuri Lekh about 10 km to the north-west of the capital and PKIN is in the tunnel of an old iron mine on the southern slope of Phulchwoki Hill about 15 km away toward the southeast. The spectral ratio of the ground motions of DMG compared to those of PKIN, for all earthquakes considered, confirmed that DMG has amplification ranging from 1 to 10 in the frequency range of 0.5 to 4 Hz, and the spectral ratios of KKA compared to PKIN showed that KKA has significant amplification in the frequency range of 4 to 10 Hz and its peak value is at most 10. Therefore the site amplification in and around the Kathmandu valley would be significantly underestimated in the frequency range from 4 to 10 Hz if the spectral ratios of DMG to KKA were used as a proxy for input ground motions.
Based on the above analysis, it is proposed that PKIN should be considered as a reliable reference site for the estimation of seismic hazards in and around the Kathmandu valley.