LANDSLIDE HAZARD ZONATION IN GARHWAL HIMALAYA USING REMOTE SENSING TECHNIQUES

Abstract

Himalayas, one of the youngest mountain ranges in the world are tectonically and seismically very active. The crustal movements along the longitudinal thrusts and transverse faults give rise to earthquakes and in turn the landslides. In addition to various static factors causing landslides earthquakes are one of the major causes of landslides in seismically active areas as no other phenomena can produce rock movements of such large size as found in the Himalayas. Uttarkashi (October, 20, 1991) and Chamoli (March, 29, 1999) earthquakes of 6.6 and 6.8 magnitudes respectively rocked the Garhwal Himalaya region and caused a number of landslides. Earthquakes not only reactivated existing landslides but also caused new ones. In seismic shaking conditions fissures and cracks develop which affect the overall strength of the rock mass making it more vulnerable for landslides. It has been observed that more than 80% of human casualties and material damages associated with the occurrence of earthquakes in mountainous regions are due to subsequent phenomena primarily landslides and collapses. The landslide hazard prevalent in Garhwal Himalaya region under the influence of seismic conditions needs detailed studies for landslide hazard mitigation measures to be undertaken in this region. It is necessary to identify and locate the landslide susceptible areas and divide them into zones based on potential of occurrence of landslides for future planning ofland use for socio-economic development in the region. The term Landslide Hazard Zonation (LHZ) refers to classifying an area into homogeneous zones of relative degree of potential of occurrence of landslides. A number of causative factors are involved in the occurrence of landslides that should be studied carefully and ranked as per their influence on the occurrence oflandslides. LHZ have been carried out for different parts of Garhwal region using different landslide iii causative, factors by various researchers. Garhwal Himalaya is seismically very active where more than 35 events of magnitude greater than 5 have occurred in the past. A considerable amount of seismic energy is released during an earthquake that add to the shear stresses in slope instability mechanism making it imperative to incorporate seismic factor while carrying out LHZ map preparation in a seismically active areas like Garhwal Himalaya. In the present study an endeavor has been made to prepare LHZ map including static as well as seismic factors in terms ofseismic shaking conditions. A part of Garhwal Himalaya which is very sensitive for the occurrence of landslides has been taken as study area for carrying out Landslide Hazard Zonation. The study area selected for the present study encompasses the macroseismic area ofChamoli earthquake (epicenter - latitude 30.492° N and longitude 79.288°) extending around 1300 km2 bounded by 30°15'N and 30°30'N latitudes and 79°E and 79°30'E longitudes. Landslide is a complex phenomenon resulting due to invariable contribution of various causative factors. It is, therefore important to identify and evaluate the various landslide causative factors for the analysis oflandslide hazard. One ofthe important steps for LHZ mapping is the data collection in the form of maps, images, photos, field visit and other information related to the causative factors. This data is then compiled, analyzed and integrated using various methods for LHZ map preparation. Anumber ofthematic maps of various landslide causative factors at common scale, resolution and coordinate system have been prepared which are used to explore quantitative and qualitative information of various themes (slope, lithology, landslide distribution, drainage etc.) for identifying their influence on the occurrence of landslides. In the present study thematic data layers have been prepared from various data sources viz. remote sensing images (LISS-III and PAN images), existing maps and attribute data etc. The LHZ maps have beenprepared for pre andpost Chamoli earthquake andthe difference IV image of post to pre earthquake LHZ maps has emphasized the inclusion of seismic effect while preparing LHZ map. The observed and theoretical displacements have been measured, which are used as one of the thematic data layers in LHZ map preparation for scenario earthquake. Probabilistic Seismic Hazard Analysis has been carried out for estimating probable seismic hazard. The seismic hazard assessment of an area necessarily involves some of the basic steps viz., study of the geological and tectonic setup of the area, compilation of seismicity data, division of area into independent seismogenic sources based on the geological, geophysical, tectonic and seismicity of the sources and then assessing the seismic hazard parameters using appropriate methodology (deterministic and/or probabilistic) and finally estimation of strong ground motion at the requisite site or an area. The seismotectonic modeling is carried out by studying the geological setup of the area, tectonic setup of the area and association of the occurrence of the earthquakes with the seismogenic sources present in the area. In this study, an implementation of a simple approach to incorporate probabilistic ground shaking conditions at pixel level in terms of Peak Ground Acceleration (PGA) for different time periods with other static landslide causative factors have been adopted to produce LHZ maps at probable seismic hazard. The study shows that Earthquakes play a major role in occurrence of landslides and landslide occurrences follow the trend of isoseismal map. A comparative study of LHZ maps prepared from Subjective Weight and Rating Scheme and Info Val method shows that Info Val derived LHZ map give more accurate results in terms of increasing susceptibility of occurrence of landslides in higher hazardous zones. Post Chamoli earthquake LHZ map shows a significant increase in number of pixels in High and Very High Hazard Zones. The difference image of post to pre Chamoli earthquake LHZ maps shows that after Chamoli earthquake most parts of the low, moderate and high zones have v been transformed into moderate, high and very high zones respectively. It is observed that seismic motions generated from various seismogenic sources are more in SW part of the study area; therefore, in seismic shaking conditions higher landslide hazard zones have been shifted in SWpart of the study area. The LHZ map prepared for the study area in seismic motion conditions for 20 yrs and 50yrs return periods will be useful for planners to make preparatory attempt to mitigate landslide hazards.