Seismic microzonation of Cox's Bazar municipal area

Abstract

The purpose of this study is to develop geotechnical microzonation maps using Gcographical Infonnation Systems (GIS). As study area, Cox's Bazar Municipal Area, locatcd in the Southcastern region of Bangladesh, beside the Bay of Bengal, has been chosen. The model inputs include site amplification, liquefaction potential study and slope stability analyses. It is now well known, and widely accepted amongst the earthquake engineering community, that the errects of surracc geology 0" seismic motion exist and can be large. Nearly all recent destructive earthquakes have hrought additional evidence of the dramatic importance of site effects. Accounting for such "site eflCcts" ill scismic regulations, land use planning or design of critical facilities thus has become one goal or earthquake hazard reduction programs. Cox's Bazar area, having been a great tourist resort, has experienced a rapid urbanization ill the last few decades including various establishments, construction of significant number or buildings ""d other structures in an unregulated manner and without seismic design considerations. Landslide amI related casualties have also become very common in the hilly areas or the locality. In order to asscss seismic vulnerability based on ground susceptibi~ity and adopt mitigation strategies for urban areas. seismic microzonation is considered to be the first step. This study deals with the microzonation of the Cox's Bazar Municipal Area using geographic information system (GIS) where reflection of ground shaking and the site attributes of soil amplification, liquefaction and landslide are the salicnt reature,. The probable earthquake hazard and expected ground motion for this area wcre assesscd nsing probabilistic approach. The liquefaction potential was estimated from Standard Pcnctration Tcsl (SPT) following the methods suggested by Seed and Idriss combined with Japanese Code of Bridgc Design. SHAKE analysis was perfonned for estimation of ID site amplification. Slope stability analyses were perfonned for samples from the hilly regions of the area using the program XSTAlll .. The result, obtained for site amplification. liquefaction and landslide potential were exported in GIS envirolll1lent and presented as microzonation maps. The findings of the study show that the rock level Peak Ground Acccleration (PGA) orthe area is 0.1 Sg for a 7.5 magnitude earthquakc having a retum period 01'200 years. The surl:,ce PGA conld be a, high as OAlg for an average 2.3 times amplification factor if extreme or most severe cOIJ(jilioll i'i considered. For this, ground shaking amplificd by 2, 2.5 and 3 times can arrect 47%. 42% and I I'h, "I' the municipal area respective I)'. 87% of the study area is highly susceptible to liquefaction and approximately S% of the municipality consists of hilly region whose 97% is very unsafe regarding natural slope stability. On the other hand, surface PGA will be O.31g for all average <lmplilicatioll factor of 1.7 if a refined hazard condition is assumed based 011 ~.I\'eragehorizontal spectral accc1cration technique. 89% area will be affected by I.7 limes amplification of groond shaking and 58% area will be prone to high liquefaction potential. 96% of the hilly region will bccomc vulncrablc if high landslide associates with only 1.7 times amplification of ground shaking.