Earthquake risk analysis for Chittagong city

Abstract

Due to rapid rate of urbanization and unplanned growth of urban centers, disasters like earthquake have become a menace for the major urban centers of Bangladesh including the port city, Chittagong. The market force of this city is so high that the built structures are growing upwards very rapidly. But the scenario of built environment is exacerbating over the days due to absence of practice of building code in construction. Most of the structures are owner built, non-engineered in nature and structurally vulnerable for earthquake and are built to gain profit within a short time span through increasing commercial floor space without any essential structural measures to makc it earthquake resistant. Only few of the total structures are resistant to any moderate magnitude of earthquake. Conventional building practices especially by architects merely heed to the aesthetics of structures rather than engineering measuremcnts. Most of the urbanities are not much aware of their location vulnerability, potential losses and fatalities of carthquake on human beings, lifelines, built structures etc. From the govt. side dearth of proper initiatives, supervision, monitoring and regulations of the use of national building code in construction have given the practitioners enough room to make the situation worse. Recent repeated earthquakes in different parts of Chittagong have generated a potential threat and caused deep concern with regards to seismic hazards and resulting risk among the people of that locality. Chittagong is now considcred as a serious earthquake prone zone where a major earthquake may occur at any time. Its rapid urban growth is causing further deterioration and increasing the vulnerability of human lives, economy and infrastructures. When such natural hazard will hit this large metropolitan city, it may create catastrophe in Chittagong, and the whole country may suffer. One of our major challenges is to reduce the vulnerability to this uncontrollable and unpredicted hazard by having greater understanding about its causes and effects. At the same timc, it is very much cssential to develop an effective earthquake risk managcmcnt plan, which requires long-term plan of action and involves multidisciplinary contribution. Chittagong City Corporation has fOtty-one Wards in its jurisdiction. Initially a GIS based Ward map is developed. For microzonation purpose 120 boreholes with SPT data are collected from different relevant organizations. To verify those data twenty eight boreholes up to a depth of 30m are carried out. Three past historical earthquakes namely 1858 Sandoway earthquake, 1912 Mandalay earthquake and 1997 Bangladesh- Myanmar earthquake are used as scenario events for this study. Intensity value obtained for these events is calibrated against attenuation laws to check the applicability of the laws for the study area. Using these laws, bedrock Peak Ground Acceleration (PGA) values are obtained for study area for different earthquakes. Finally, a bedrock PGA value for the scenario events is selected. PGA in the bedrock level in the study area and boreholes SPT data are used to develop a regional combined seismic hazard map based on site amplification as well as liquefaction and landslide potential hazard. These site effects are integrated in Geographic Information System (GIS) platform for combined seismic hazard assessment. The combined effect of liquefaction, amplification and landslide is observed. It is found that if an earthquake similar to 1912 Mandalay earthquake occurs again at the same epicentral location, it is concluded that 20% area will be affected with PGA 0.6g, 20% area will be affected with PGA OAOg and 60% area will be affected for PGA 0.20g. Also, 24% area will be affected for intensity IX, 57% area will be affected for intensity VIII and 19% area will be affected for intensity VII.