Fuzzy Clustering Analysis of HVSR Data for Seismic Microzonation at Lahore City

Abstract

A microzonation study deals with the classification of hazards in a town or city in terms of surface ground motions that result from amplification and resonance frequency in soils against seismic tremors. This paper presents the result of a microzonation study in terms of resonance frequency and peak amplitude for Lahore city, Pakistan. In order to recognize the local soil effects of the covered geology at 159 sites in Lahore city, the horizontal-to-vertical spectral ratio (HVSR) Nakamura method was implemented. A fuzzy C-mean (FCM) clustering algorithm was adopted to obtain the best cluster solution of the analyzed HVSR parameters. The results of the Silhouette Index suggest that the FCM clustering solution of observation data points is more consistent. The results of clustering reveal three solutions. Clusters 1 and 2 reveal that a major part of the research area possesses low to moderate frequencies (0.66–1.03 Hz) with a peak amplitude of 2.25–4.38 mm, indicating the presence of soft to hard rock and thick alluvial sedimentary cover. Cluster 3 reveals the presence of soft to compact rocks (with frequencies and amplitudes of 0.73–1.03 Hz and 3.02–4.11 mm, respectively) overlaying the bedrock. Lahore city has 60% of soil cover with an amplitude of 2–3 mm (for the central part) and about 40% of 3–4 mm in the northern, southern, and southwest portions. According to the NEHRP soil classification code of 1997, a major part of the city has stiff nature of the soil, while a few places reveal the presence of very dense soil. The maps produced in this study will provide expected ground motion-related useful information to reduce the seismic risk for infrastructure in Lahore city.