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The isoseismal maps of different earthquakes have been obtained from different publications. Isoseismal map of five earthquakes (Fig.1) have been taken from Nandy (2001), which are 1918 Srimangal earthquake, 1930 Dhubri earthquake, 1945 Mikirhills earthquake and 1964 Medinipur earthquake. The isoseismal map of 1897 Great Indian Earthquake and mean epicentral distance of different isoseismals have been taken from Ambraseys and Bilham (2003). Isoseismal map for 1999 Moheshkhali earthquake is given by Ansary et al. (1999). Isoseismal map (Oldham Intensity Scale) of 1885 Bengal earthquake is given by Oldham (1899) and Islam (2003) converted these intensities to MMI Scale.
Figure 1: Isoseismal map for different earthquakes (after Nandy, 2001).
Some of the intensity levels have been modified to represent more realistic intensity levels. Ambraseys and Bilham (2003) gave re-estimated intensity values for different isoseismals of 1897 Great Indian earthquake in MKS scale which is equivalent to the MMI scale. In the new isoseismal map, intensity VIII has a mean radius of around 76 km. For the 1885 Bengal earthquake (M=7.0), while converting from Oldham isoseismals, the lower bound of intensities (in MMI scale) have been used considering the relatively large distances. As for example, the isoseismal for 403 km distance is taken as III intensity (MMI scale). The 1930 Dhubri earthquake (M=7.1) affected a large area of radius 233 km at intensity V inward Bangladesh. The 1918 Srimangal earthquake (M=7.6) has shown faster attenuation with an intensity of V at a distance of 119 km, which may be due to the smaller focal depth (14 km). The isoseismals of Dhubri and Srimangal earthquakes were converted into MMI Scale from RF Scale by conversion and qualitative assessment. Medinipur and Moheshkhali earthquakes of lower magnitudes affected smaller areas. Moheshkhali earthquake had shallow focal depth of 10 km and an intensity of V at a small distance of 17.5 km.
Posted in: Isoseismals
