Earthquake Related Landslides in the Indian Himalaya: Experiences from the Past and Implications for the Future

Abstract

Most parts of the Indian Himalaya fall in seismic zone V and IV, indicating a high degree of susceptibility to earthquakes. Although numerous studies on earthquake risk assessment have been done by different researchers yet very few of these studies and reports have focused on landslides related to earthquakes. It has been observed globally that many casualties during an earthquake in a hilly terrain are attributed to the incidences of landslides triggered by the earthquake and the response actions are also hurdled by the rockfalls/landslides along the highways. Field observations have indicated that such landslides are often associated with earthquakes of magnitude 4 or more. About 20–25 % losses during earthquakes in hilly terrains have been attributed to landslides. The earthquake triggered landslides have affected even the structures and buildings which were well constructed but adversely located in the ground. However, a perusal of seismic zonation studies indicate that landslides have not received due attention. Similarly most of the landslides hazard zonation studies do not consider the impacts of earthquakes in generating numerous and large landslides. Hence, the present paper emphasizes the significance of earthquake related landslides in the hilly terrains through experiences from the past incidences of landslides during earthquakes along with their impact and proposes its consideration in future earthquake risk management programmes as well as in landslides hazard zonation studies for effective risk reduction strategies. The significant earthquakes that affected the Himalayan terrain include Assam (1897), Kangra (1905), Bihar-Nepal (1934), Shillong (1950), Bihar-Nepal (1988), Uttarkashi (1991), Chamoli (1999), Kashmir (2005) etc., that caused heavy damages/losses as well as casualties which were found partly related to the ground and slope failures during these earthquakes. A study of landslides associated with earthquakes has lead to identification of morphological, lithological, tectonic, hydrological and landuse conditions that govern the occurrence of such landslides. For example, most of earthquake triggered landslides/rockfalls happened on convex slopes whereas rain-induced landslides are more common on concave slopes. The concentration of landslides and their size has also been found proportional to the magnitude of the earthquake to some extent. An attempt has also been made to differentiate freshly triggered and reactivated co-seismic landslides in earthquake affected areas as well as post-seismic landslides.