Overview | Earthquake | Drought | Fire | Flood & Cyclones | Landslide & Soil Erosion | Volcano
Seismic loss estimation for Lalitpur, Nepal
2. Case study city The city of Lalitpur, which was selected as one of the case study cities in the SLARIM project, is located in the Kathmandu valley, neigbouring Kathmandu. Lalitpur has a population of 163,000, in 35,000 households, and is divided into 22 wards (see figure 1).  Figure 1: Left: Location of the three case study cities for the SLARIM project. Right: Structure of Lalitpur Lalitpur has suffered from damaging earthquakes in the past, such as in 1255, 1408, 1810, 1833, 1934, 1980 and 1988. In the earthquake of 1934, which had a magnitude of 8.4, it was estimated that about 19,000 buildings were heavily damaged within Kathmandu valley, causing the death of more than 3800 people (JICA, 2002). Lalitpur is located on a former lake in the middle Himalayan mountain range, of which the surface materials are mainly consisting of alluvial terrace deposits on top of a thick sequence of lake sediments, with a thickness up to 400 meters (Fujii et al., 2001). Various institutions have carried out studies on earthquake hazard and risk in Kathmandu Valley (UNDP, 1994; Dixit et al., 2002). This study used high-resolution satellite imagery, together with aerial photographs and field survey in the generation of a building database for seismic loss estimation in Lalitpur. An overview of the method is used is given in Figure 2.  Figure 2: Flowchart of the procedure for seismic loss estimation in Lalitpur, Nepal. See text for explanation. 3. Generation of the dataset The Lalitpur Sub-Metropolitan City Office did not yet have a GIS section, nor did they have GIS data on the building stock and other characteristics within their municipality. A series of 1:2000 scale topomaps in AutoCad format were converted into a usable GIS database, consisting of separate layers for buildings, roads, contours and drainage. The building footprint map was prepared based on aerial photos of 1981 and 1992 and was updated in 1998. All the buildings constructed after this year as observed in the available IKONOS image from 2001 were digitized on screen to create the building data set for the year 2001. As also a CORONA image was available from 1967, this image was used to delete those buildings that were not yet present in 1967. The original digital building footprint maps did not contain any attribute information regarding the buildings within the city. Since a complete building survey would require too much time for field data collection, it was decided to use so-called homogeneous units as the basic mapping units within the city. In the field mobile GIS was used to characterize the buildings within each unit according to age (based on procedure outline earlier), occupancy class, landuse type and building type, which was a combination of construction material and number of floors. Population data were available from the latest population census in Nepal, which was held in 2001, published by the Central Bureau of Statistics (CBS) of Nepal. The average population density within different types of buildings (residential, commercial, institutional etc.) was estimated based on 196 detailed samples of buildings carried out by a local NGO, the National Society for Earthquake Technology (NSET) and the Lalitpur Sub-Metropolitan City Office. Based on these samples estimations were made of the population amounts present in different types of buildings during different periods of the day. 4. Soil response modeling In order to be able to analyze the seismic hazard in Lalitpur and its surroundings a sub-surface database was generated for the Kathmandu valley. A geological database was made for storing the information for 185 deep boreholes, with depths ranging from 35 to 575 meters, of which 36 boreholes actually reached to the bedrock, and 328 shallow boreholes (< 30 meters). All boreholes were divided into main stratigraphical units, for which the depth was determined and used in GIS for subsequent layer modeling. In this case, all sediments of the basin were divided into four layers (See Figure 3). The depth of each of the layer boundaries, including the surface elevation was used in GIS and Digital Elevation Models of each of these surfaces were obtained through point interpolation.  Figure 3: Left: Thickness maps of the three main material types in the Kathmandu valley. Right: Cross section based on the layer thickness maps. The GIS layer models were used for one-dimensional calculations of the ground response, with the help of SHAKE2000, which is derived from the original SHAKE software, used widely for soil response analysis since 1971 (Ordonez, 2002). Three earthquake scenarios were selected in line with the ones used in the study by JICA (2002). The results were calculated as Peak Ground Acceleration (PGA) as well as spectral acceleration for frequencies of 5, 3 2 and 1 Hz. These values were later linked back to the sampling points and maps were obtained through point interpolation. 5. Loss estimation For analyzing seismic vulnerability, the buildings in Kathmandu valley have been divided into a number of classes. The vulnerability curves used in the GIS analysis were derived by NSET-Nepal and JICA (JICA, 2002). For each MMI class and building type, minimum and maximum values are given of the percentage of buildings that would be heavily damaged (collapsed or un-repairable) or partly damaged (repairable, and available for temporary evacuation). The earthquake intensity maps were used in combination with the building map and the vulnerability relations to calculate the range of completely and partially damaged buildings for each earthquake intensity. In figure 4 some of the results are given. The number of casualties was estimated at homogeneous unit level for the three different earthquake scenarios mentioned earlier. The data used for this calculation were the population distribution for different periods of the day and within different occupancy classes, the building loss estimation discussed in the previous section and vulnerability and casualty ratios with respect to building damage. These casualty ratios were derived from the HAZUS methodology, which uses the widely accepted ATC-13 vulnerability curves (FEMA, 2004). With these relations, the number of casualties was estimated for the three different earthquake scenarios, and for both a daytime and nighttime scenario, with a different distribution of population over the various occupancy classes.  Figure 4: Building loss estimations for different earthquake intensities in Lalitpur. |