Abstract

Geohazards (earthquakes and landslides) cause huge loss of lives and infrastructure every year in Iran. Many settlement areas (urban & rural) as well as Tehran, the capital city of Iran are located in the hazardous area. This research deals with the geohazard assessment and mapping based on recent Remote Sensing information on a GIS plat form. The study area is part of Central Alborz in southern Caspian Sea called Marzanabad area, it is a potentially high-risk zone as several earthquakes and landslides have occurred in the past. The study’s main objective is to identify and prepare an inventory of geohazard types and mapping Geohazard risk zones in the area of study. The set of geomorphologic, geological and structural information forms an input as an integrated database in the analysis of the Landslide Hazard Zonation Map (LHZM) and Earthquake Risk Map (ERM) using remote sensing and GIS technique

The landslide hazard and earthquake risk maps were classed into five different categories from very high to very low. The amount of weight that is given to a certain factor and the way this factor is classified is highly subjective. The different hazards with their respective weighted values were combined into a Geohazard map. The individual Earthquake risk map and Landslide hazard zonation with equivalent categories were analyzed and integrated to derive a Geohazard Map. The spatial distributions of the geohazard are reclassified into five categories. The Geohazard map elaborated in this research work will be a useful tool for urban and regional planning of future activities in the area.

Introduction

Geohazards cause huge loss of lives and infrastructure every year in the world and are dramatically increasing since last few decades in Iran. Several damaging Geohazards (earthquakes and landslides) that have occurred in Iran during the last two decades have clearly demonstrated their catastrophic consequences. The disastrous impact of earthquakes and landslides in Iran has been starkly illustrated in Bam (2003), Ardabil (1998) Ghaenat (1996), Manjil (1990) where thousands of people lost their lives.

Many settlement areas (urban & rural) as well as Tehran, the capital city of Iran are located in the hazardous area. The present area of study, a part of Central Alborz located in North of Iran, is a high-risk area with several settlements and establishments. Disastrous earthquake and landslides have been recorded in this high geohazard potential area.
Remote sensing and GIS techniques, with other available different geological data are essential tools in detecting geo-hazards of different types and to check their activities and define their potential areas. Their effect on nearby sites could also be estimated and mitigated by certain precautions. The main objective of this research is to assess the geohazard and make a large scale Geohazard map for the study area, based on recent Remote Sensing information on a GIS platform

Study Area

This research incorporates investigation of Geohazard in part of central Alborz called Marzanabad. It is located in the Central Alborz at a distance of 30 km to Caspian Sea in the north and 100 km to the capital city of Tehran in the south. It covers an area of about 1048 sq km and is located between Latitudes 36º 15’00” N to 36º 35’00” N Longitudes 51º 07’30” E to 51º 27’30” E. The study area is a rugged hilly terrain largely covered by Forest and grasslands with low ‘hills’ in the middle part (Kelardasht & Marzanabad town). The total population of settlement area according to published data by ISC (Iranian Statistics center) is around 44965 persons in two satellite towns Marzanabad and Kelardasht which includes 134 villages. In May 2004 an earthquake on magnetite 6.3 mw jolted all settlement located in this area and round and destroys proximity 28 villages. (Sharifikia et.al 2006) (Figure 1)


Figure 1: The study area
Material and Methodology

The necessary data input such as digital thematic maps (toposheet ,geology and geomorphology, etc), remote sensing ( Arial photo , IRS LiIII & PAN, Lansat TM & ETM, Radarsat, ERS , NOVA) and other obtainable data about the study area are providing and analysis for this research work.

This work has been done using a pragmatic method based on available data, which is focused on describing the factors and conditions for geohazards incidence in terms of zonation and assessment. The study was divided into two parts. In the first part the aim was to understand the geomorphology, geology and geo-structure of the study area. The second part is related to data processing and analysis of several thematic maps, visible and Radar satellite data to generation the two inaudible geohazard maps (Earthquake Risk Map and Landslide hazard zonation) as basic data for integrate and providing Geohazard map of area.

Geology, Geomorphology and Structure Setting

The sets of geomorphological maps (geomorphology unite, soil, drainages, slope and aspect, land use and etc.) including the area of different lithological units with major structures (faults and lineaments) have been prepared as digital data and inputs to geohazard analysis and mapping. Those maps form to understand the sensitiveness of geology and geomorphologic processes to human interferences and the risks associated with development and settlements of hazardous sites in the area (Figure 2).


Figure 2: virtual 3D view of study area , facing North

The south Caspian, site of intense oil exploration, is possibly the deepest sedimentary basin in the world, with a thickness of 20 km of Jurassic and younger sediments overlying mafic basement (Neprochnov, 1968; Berberian, 1983; Devlin et al., 1999). Overall structural relief from the Alborz Mountains to the southernmost Caspian basement is ~25 km. It is a part of the Alpine-Himalayan organic belt in Western Asia, which is bounded by Caspian Sea on the north and Central Iran on the south (Jackson et al., 2002). The study area is a small part of the central Alborz Mountain consisting of dolomite, limestone, shale and sandstone. It is marked by a number of thrusts that are oriented in east-west and northeast-southwest direction. The major structures such as thrust and faults that have been considered as important parameters for the geohazard studies have been discussed. The area of investigation is traversed by number of thrust of regional extent, which generally shows NW-SE with northeasterly dips varying 30-59 degree. The various litho-units of area have been displaced by a number of faults. Most of these faults are transverse with reference of the thrusts. It is observed that geology, geomorphology and geo-structure condition in area causing the vulnerable zone for earthquake and landslide hazard risk. Further any change in this condition had brought about adverse environmental implications.

Data Processing for Earthquake Risk Mapping

Earthquake hazard analysis requires an assessment of earthquake hazard parameters and the future of earthquake potential in a region .The development of Earthquake risk map of the study area is based on computation by remote sensing and GIS techniques using the historical earthquakes data, geology, tectonics, fault activity to identify high-risk areas to the settlements and infrastructure in the study area. This research work involved fault identification and classify the risky area around the fault by applying different buffer over the GIS software specifying distance of the source to fault location. The flute identification has been done using RS & GIS techniques for lineament extraction and analysis. The lineaments have been defined in terms of abrupt change of slope gradient that can be expressed in SRM (shaded relief map) provided by microwave satellite image with 10m resolution and digital contour map with 10m resolution as well as image filtration.

Linear features as fault scarps, river valleys or ridgelines can be defined in terms of terrain parameters. Enhancement of these lineaments requires different approaches in each DTM (Digital Terrain Modeling). On superimposing lithology and geomorphology over the lineaments in a GIS environ, number of probable faults were identified which were previously marked as lineaments. A modified fault map has been prepared for use in ERM consisting of thrusts, previous faults, confirmed new fault and several probable faults. On basis of modified fault map, classify the risky area around the fault by applying different buffer over the GIS software, the study area were divided into five earthquake risk zones.The result of this buffering on earthquake risk map by GIS show that 12.57% of area is located in Strongly high-risk zone. This zone covers about 31.34% of settlements with population of 13815 of the total 44969 people living in the area of study. i.e. (30.72%). The second risk zone defined as High-risk zone involves 19.69% of the total area with 35.07% settlements with a 21.1% of total population. The Moderate risk zone covers 29.08% of the total area with 33.58% of total settlements and a population of 21664. Moderately low risk cover 38.67% of total area this zone. It is free of settlements as well as population (Table 1) (Fig 3)


Table 1: Earthquake risk zonation of the study area


Table 3: Earthquake hazard risk map of the study area
Data processing for Landslide Hazard Mapping

In most part of Alborz zone as well study area, landslides are one of the most devastating Geohazards, especially all along the road network. Landslides coupled with earthquake in this high-risk area act as double disaster. The Several data sets such as Digital thematic maps, high resolution optical and SAR satellite data, etc. have been used for carrying out landslide hazard assessment. The landslide map of the area has been prepared in this study on the basis of Arial photographs, satellite (Visible and Radar) and field investigations. The area has been subdivided into old landslide, active landslide; reactive landslides and rock falls. the out come of this map has been used as occurrence landslide in zonation method .

The zonation is based on information value method (Yin and Yan, 1988; Wu et al., 2000) which has to designate in to five categories from High to low hazard zones. This is the probabilistic approach based on the observed relationship between each factor and distribution and occurrence of the past and present landslides in the area of study. A LHZ map was prepared showing the five zones, namely “very low hazard”, “low hazard”, “moderate hazard”, “high hazard” and “very high hazard”. The calculation and analysis of landside hazard zonation map over the GIS environment for risk assessment shows that Very high and High landslide hazard zones involves a 7.95 % of total population living over 37.60% of the total area with 14.9% of total settlements. Where the Low and very Landslide hazard zone cover 35 % of total area with 25.12 % of habitat and population of 40.39%


Table 2: Landslide hazard in relation to no. of villages and population at risk in the area


Figure 4: Landslide hazard map of the study area

Geohazard Mapping

The individual Earthquake risk map and Landslide hazard zonation were analyzed and integrated to derive a Geohazard map. The landslide hazard and earthquake risk maps were classed into five different categories from very high to very low. The amount of weight that is given to a certain factor and the way this factor is classified is highly subjective. The different hazards with their respective weighted values were combined into a Geohazard map. The different earthquake risk and landslide hazards with equivalent categories were added with the help of spatial operation tools in GIS environ to arrive at different combinations of geohazard. The spatial distributions of the geohazard are reclassified into five categories.

The final output of the research is the geohazard map depicting that a total population of approximately 49% and about 77% settlements in an area of 50% of the total area are living in very high hazard zone (Fig. 5 & 6). The Geohazard map elaborated in this research work will be a useful tool for urban and regional planning of future activities in the area.


Figure 5: Area, settlement and population in different geohazard zone


Figure 6: Geo- Hazard map of the study area

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