Suchergebnisse

Abstract. The probable mitigation and management issues of seismic hazard necessitate seismic microzonation for hazard and risk assessment at the local level. Such studies are preceded with those at a regional level. A comprehensive framework, therefore, encompasses several phases from information compilations and data recording to analyses and interpretations. The state-of-the-art methodologies involve multi-disciplinary approaches namely geological, seismological, and geotechnical methods delivering multiple perspectives on the prevailing hazard in terms of geology and geomorphology, strong ground motion, site amplification, site classifications, soil liquefaction potential, landslide susceptibility, and predominant frequency. The composite hazard is assessed accounting for all the potential hazard attributing features with relative rankings in a logic tree, fuzzy set or hierarchical concept.

Abstract. The city of Kolkata is one of the most urbanized and densely populated regions in the world and a major industrial and commercial hub of the eastern and northeastern region of India. In order to classify the seismic risk zones of Kolkata we used seismic hazard exposures on the vulnerability components, namely land use/land cover, population density, building typology, age and height. We microzoned seismic hazard of the city by integrating seismological, geological and geotechnical themes in GIS, which in turn are integrated with the vulnerability components in a logic-tree framework for the estimation of both the socioeconomic and structural risk of the city. In both the risk maps, three broad zones have been demarcated as "severe", "high" and "moderate". There had also been a risk-free zone in the city that is termed as "low". The damage distribution in the city due to the 1934 Bihar–Nepal earthquake of Mw = 8.1 matches satisfactorily well with the demarcated risk regime. The design horizontal seismic coefficients for the city have been worked out for all the fundamental periods that indicate suitability for "A", "B" and "C" type of structures. The cumulative damage probabilities in terms of "none", "slight", "moderate", "extensive" and "complete" have also been assessed for the predominantly four model building types viz. RM2L, RM2M, URML and URMM for each seismic structural risk zone in the city. Both the seismic hazard and risk maps are expected to play vital roles in the earthquake-inflicted disaster mitigation and management of the city of Kolkata.

Abstract. Seismic microzonation is a process of estimating site-specific effects due to an earthquake on urban centers for its disaster mitigation and management. The state of West Bengal, located in the western foreland of the Assam–Arakan Orogenic Belt, the Himalayan foothills and Surma Valley, has been struck by several devastating earthquakes in the past, indicating the need for a seismotectonic review of the province, especially in light of probable seismic threat to its capital city of Kolkata, which is a major industrial and commercial hub in the eastern and northeastern region of India. A synoptic probabilistic seismic hazard model of Kolkata is initially generated at engineering bedrock (Vs30 ~ 760 m s−1) considering 33 polygonal seismogenic sources at two hypocentral depth ranges, 0–25 and 25–70 km; 158 tectonic sources; appropriate seismicity modeling; 14 ground motion prediction equations for three seismotectonic provinces, viz. the east-central Himalaya, the Bengal Basin and Northeast India selected through suitability testing; and appropriate weighting in a logic tree framework. Site classification of Kolkata performed following in-depth geophysical and geotechnical investigations places the city in D1, D2, D3 and E classes. Probabilistic seismic hazard assessment at a surface-consistent level – i.e., the local seismic hazard related to site amplification performed by propagating the bedrock ground motion with 10% probability of exceedance in 50 years through a 1-D sediment column using an equivalent linear analysis – predicts a peak ground acceleration (PGA) range from 0.176 to 0.253 g in the city. A deterministic liquefaction scenario in terms of spatial distribution of liquefaction potential index corresponding to surface PGA distribution places 50% of the city in the possible liquefiable zone. A multicriteria seismic hazard microzonation framework is proposed for judicious integration of multiple themes, namely PGA at the surface, liquefaction potential index, NEHRP soil site class, sediment class, geomorphology and ground water table in a fuzzy protocol in the geographical information system by adopting an analytical hierarchal process. The resulting high-resolution surface consistent hazard, liquefaction and microzonation maps are expected to play vital roles in earthquake-related disaster mitigation and management of the city of Kolkata.