INTEGRATED GIS STUDIES FOR DELINEATION OF EARTHQUAKE-INDUCED HAZARD ZONES IN PARTS OF GARHWAL HIMALAYA

Abstract

The Himalayan mountains are seismically highly active. Neotectonic activities due to earthquakes along numerous faults in the Himalaya have resulted in contemporary morphological adjustments. These earthquakes induce variety ofmass wasting processes, such as landslides, rock falls, slump etc., causing heavy loss to human lives, settlements and landscape. Spatial characterstics of the ground and human factors also determine the vulnerability to earthquake-induced hazards. Earthquake-induced hazards and risk analysis is therefore an important in identifying the potential consequences of an earthquake both in relation to existing facilities as well as in the planning and location of new structures. In the past various attempts have been made towards earthquake-induced hazard mapping and risk assessment. They lacked due to enormous amount of time and painstaking field work required to accomplish general requirement for hazard analysis and mapping. This study harnesses the potential of GIS, DTM and remote sensing to map seismic-induced hazard zones in a part of Garhwal Himalaya. The area around Uttarkashi, in the Garhwal Himalaya has been selected in view of several considerations, viz, (a) the area around Uttarkashi was marked by an earthquake of magnitude 6.6 on 20th October, 1991; the damage survey acts as a valuable ground truth, (b) Alarge amount of seismic data is available, (c) basic topographic and geological data are available, (d) the area forms apart of seismic-gap; therefore this study has possible applications in developmental activities. There are three steps in this study -hazard identification, spatial factors causing earthquake hazards and vulnerability analysis. Hazard identification involves / identifying major events that occur and cause destruction during an event of an earthquake. The destructive events initiated by earthquake are the function ofcertain spatial factors, such as the morphology, geology, tectonic history and related factors. Vulnerability analysis assesses susceptibility of geographic location and human settlement to an earthquake-induced hazard if an earthquake manifests in the area- Two kinds of datasets have been collected for this study viz. spatial and nonspatial. Except for remote sensing data all other data have been obtained in analogue form. Analogue data have been transferred into GIS in digital form through digitization. The data used in this study include - topographic maps, satellite images, geological maps, seismic data and damage survey maps. The two major factors identified in this context are morphological and geological factors. Each factor is comprised of various spatial parameters describing different properties of the landscape in the area. The morphological factor consists of slope, aspect, cumulative slope length, vegetation cover and curvature of slopes. Geological factor includes structural features or lineaments, textural pattern, micro-seismicity, lithology and terraces. These two major factors have been integrated using Composite Mapping Analysis by assigning variable weights to each factor. This facilitates assessing the relative importance ofeach factor in causing earthquake-induced hazards. The data collected from paper maps or in digital form have no spatial relationship amongst them. As such they have been first digitized and coregistered with the basemap. The Survey of India topographic maps have been used as base map for registering other datasets to it. All the datasets viz. geological, seismic and remote sensing images, have been rectified to the geometry of topographic map. These datasets have been subsequently processed and manipulated for the analysis. The coregistered datasets have been processed to derive various spatial parameters in each factor. Contours digitized from topographic map has been processed to derive digital elevation model (DEM). This in turn generated slope, aspect and curvature. Customized software programs have been written to generate shaded relief models (SRM). Further, new methodology, termed as 'Shaded Relief Image Substraction Technique' has been developed to normalize satellite images in a highly undulating terrain for topographic effects. Normalized satellite images provided surface cover information and helped in mapping of terraces. Visual interpretation of SRMs and images facilitated mapping of structural and morphological parameters such as lineaments and textural pattern. Lithological and seismic data have been directly incorportated for integrated analysis. jf v\<^ VMultiple factor maps have been integrated using Composite Mapping Analysis technique. This provided a logical data integration tool to merge large number of factor maps with proper weightage assigned to each of them. This is important since evaluation or the actual process of applying the decision rule can affect the vulnerability of a site to earthquake induced damages. Although there are various procedures for evaluating these variables, the most common one is the weighted linear combination of factors. It provides a simple and straightforward yet highly objective procedure of analysis in GIS. In this technique weights are derived from the principal eigen vectors of a symmetrical matrix ofa pairwise comparison between different factors. The comparison matrix describes relative importance between two variables involved. The relative values between two variables are derived from a nine-point scale, which describes importance of one variable with respect to the other one in the pairwise comparison matrix. Earthquake-Induced Hazard Zone (EIHZ) mapping has been accomplished in two steps - Factorwise analysis and Integrated analysis. Factorwise analysis integrates geological factors and morphological factors separately. In this step, analysis has been performed by varying importance of one geological factor against the other geological factor and one morphological factor against another. Thus, two maps are generated viz. in the first instance, a morphological hazard map and a geological hazard maps. During integrated analysis the above two maps have been merged. The methodology for weight assignment and integration isthe same as in the previous step. It represents relative importance among geological and morphological factors. This provides intergrated earthquake-induced hazard risk assessment. The final EIHZ map exhibits ground locations succeptibleto earthquake-induced hazards as related to a collective effect of geological and morphological factors. The Uttarkashi earthquake damage survey map has served as a ground truth for the above study. Multiple factor maps have been integrated using Composite Mapping Analysis technique. This provided a logical data integration tool to merge large numberof factor maps with proper weightage assigned to each of them. This is important since evaluation or the actual process of applying the decision rule can affect the vulnerability of a site to earthquake induced damages. Although there are various procedures for evaluating these variables, the most common one is the weighted linear combination of factors. It provides a simple and straightforward yet highly objective procedure of analysis in GIS. In this technique weights are derived from the principal eigen vectors of a symmetrical matrix of a pairwise comparison between different factors. The comparison matrix describes relatiyejmportance between two variables involved. The relative values between two variables are derived from a nine-point scale, which describes importance of one variable with respect to the other one in the pairwise comparison matrix. Earthquake-Induced Hazard Zone (EIHZ) mapping has been accomplished in two steps - Factorwise analysis and Integrated analysis. Factorwise analysis integrates geological factors and morphological factors separately. In this step, analysis has been performed by varying importance of one geological factor against the other geological factor and one morphological factor against another. Thus, two maps are generated in the first instance, a morphological hazard map and a geological hazard maps. During integrated analysis the above two maps have been merged. The methodology for weight assignment and integration is the same as in the previous step. It represents relative importance among geological and morphological factors. This provides intergrated earthquake-induced hazard risk assessment. Thefinal EIHZ map exhibits ground locations succeptible to earthquake-induced hazards as related to a collective effect of geological and morphological factors. The Uttarkashi earthquake damage survey map has served as a ground truth for the above study.