Use of a Sparse Geo-Info Database and Ambient Ground Vibration Survey in Earthquake Disaster Risk Study − A Case of Kathmandu Valley −

Building a reliable geo-info database and using it in various civil engineering projects as well as geo-disaster risk reduction study largely depends on the accuracy of available borehole information as well as borehole distribution density. In a developing country like Nepal, the available borehole information even in the most developed urban areas is highly sparse, which does not really help achieve the purpose of generating a geo-info database out of the available borehole information. Kathmandu Valley, the capital city area of Nepal accommodates about 3 million people in densely populated urban settlements of many poorly built and over-aged houses and buildings. It consists of thick lake deposits measuring more than 500 meters at some locations. This particular study is aimed at building a geo-info database system out of sparsely distributed borehole information, using the geo-info database in reliability check of ambient ground vibration survey results, and preparing a ground shaking map for earthquake disaster risk reduction in the Kathmandu Valley. The sparsely distributed borehole locations and different purposes of boring obviously lead to a less reliable geo-info database system. So, an extensive ambient ground vibration survey, which consists of a total of 176 survey points in about 1-km grid spacing, was conducted in Kathmandu Valley to predict the ground behavior in case of seismic shaking. The results of the geo-info database system and the ambient ground vibration survey were compared for reliability check of the latter. Finally, a ground shaking map was proposed for earthquake disaster risk mitigation in the Kathmandu Valley. The main understandings are: 1) specific use of the geo-info database system built out of the available borehole information may still be in question, but a broader implication, especially in terms of predicting the nature of soil deposit in the Kathmandu Valley can be adequately made; 2) the ground structure can be broadly predicted out of the ambient ground vibration data analysis, which in case of the Kathmandu Valley has been well confirmed by comparing the previously reported ground profiles and the results of the ambient ground vibration survey; and 3) the densely populated urban areas in the Kathmandu Valley are mostly situated over the soft and thick deposits with longer period of shaking, so longer period (i.e., 1.0-2.0 s.) structures here are prone to seismic damage.