Evaluation of Heavy Rainfall Operation in Reactivation of Paleolandslide in Hossein-Abad Kalpoosh Village

The term landslide has various meanings and applications in general and specialized fields. Landslide is a process of changes in stress-strain state of a slope ground mass leading to a mass separation and ground movement downslope and paleolandslides are mass movements that predate the historical period and are documented using geologic and geomorphologic evidence. Based on the time of occurrence, landslides are divided into four groups: Ancient (Before Holocene), Old (Early Holocene),Recent (Late Holocene) and Present-Day. Landslides belong to Ancient, Old and Recent groups are known as Paleolandslides.Landslides often occur sudden and catastrophic. According to the International Disasters Database of records in 2017, have been died 2312 people as landslides in the world, Only 830 cases have been reported for the 9-year period between 2007-2016. This sudden increase in human casualties is correlated with heavy rains during climate change and large-scale global warming.Most slope erosion occurs in many humid mountain environments due to heavy rainfall and earthquake. Also landslides often occur in places that have occurred in the past. So, the aim of this study is to identify paleolandslides and monitor their relocation at the present time, focusing on the dominant trigger factors in the region such as heavy rainfall, by Persistent Scatterer Interferometry technique and Sentinel 1A/B images.

The study area, with an area of about 60 hectares, is located in the North of Semnan province. Kalpoosh Catchment situated in the geological zone of Kopeh Dagh and at the point of its collision with Eastern Alborz. Karstic limestones of Chaman Bid and Mozdoran formations were exposed in the area and Oghan River, which is one of the main tributaries of Gorganrud River, passes through this area. Hossein Abad village ,with a population of 3514 people, is the largest village in this region.First, a map of distribution of modern landslides and paleolandslides was prepared using old Aerial Photos, Google Earth and field visits and a landslide geomorphology map was drawn Then, cumulative precipitation and rainfall intensity were determined based on daily and monthly precipitation data. finally the displacement maps has been prepared by Persistent Scatterer Interferometry technique using 68 images of Sentinel 1A/B in 2018-2019 and the vertical displacement is evaluated by field survey.

4 paleolandslides have been identified in Hossein-Abad Kalpoosh Village . Paleolandslides have been occurred near dam lake on limestone and marl limestone. The mechanism of occurrence of paleolandslides in the region in the past goes back to the trigger of heavy rainfall and erosion in the karst environment, which due to tectonic conditions and the occurrence of several earthquakes after 1968, have occurred completely.In the winter of 2019, the region received a total of 672 mm of rainfall and this amount of rainfall is 3.5 times more than the seasonal average of previous years. The total daily rainfall in 3 days from March 2019 was 240 mm and as a result, the intensity of rainfall on March 19, with 134 mm of rainfall, has been in the heavy rainfall group.According to satellite images and field surveys, the buildings located in the paleolandslide of Hossein Abad village, after heavy rainfall, have been displaced in the sliding area to toe in different directions of horizontal and oblique movement and move down the slope. The displacement maps of Sentinel 1A/B shows reactivation of paleolandslides in the area between 2018-2019 . The ascending and descending tracks had an annual displacement of -12 - 19 mm and -22 - 16 mm, respectively. we estimated 38 mm of elevation in the deposite area.

Landslides often occur where they have occurred in the past, therefore, to predict the possible dangers of landslides in the future, it is very important to identify paleolandslides and study their behavior.The results show that paleolandslides have the most landslide activities in Hosseina Abad Kalpposh village at the present time. Also, the trigger factor of rainfall has had a great effect on the reactivation of paleolandslides . Heavy rainfall, in addition to causing the slope failure surface and occurring landslide in the area, has caused increasing the water level of Hossein Abad Dam Lake and the infiltration of water into the paleolandslide mass and has reactivated it.In addition, the range of displacements in the prepared map is in full accordance with the spatial-spatial characteristics of the new landslides in nature. The greatest fall is in the sliding area and the toe of the landslide. Also, the highest uplift and accumulation of soil is at the foot of the landslide, which has caused the floor of buildings to rise and their relative destruction. In future research, we examine the role of Kalposh dam lake and other human factors in Reactivation paleolandslide.