Earthquake intensity-attenuation relationship for Bangladesh and its surrounding region

Abstract

This study presents the results of an investigation of the magnitude-intensity and intensityattenuation relationships for earthquakes in Bangladesh and its. surrounding region using available macroseismic data. When available in sufficient quantity, the macroseismic data allow the determination of very useful parameters such as intensity distribution and its attenuation, macroseismic epicentral location and depth. There is a difference between macroseismic and instrumental seismology. Instrumental seismology deals with what happened during the shaking, whereas the macro seismic seismology is interested in the effects of what happened. The complexity and the multitude of effects produced by destructive events cannot be studied on the theoretical basis only. Thus, the necessity for efficient macroseismic observations to both theoretical and practical needs. This work is based on a selected sample of isoseismal maps from 18 events. Among these isoseismal maps of 5 events namely 1869 Cachar earthquake, 1885 Bengal earthquake, 1897 Great Indian earthquake, 1918 Srimangal earthquake and 1930 Dhubri earthquake are completely revised here based on European Macroseismic Scale (EMS). Also isoseismal maps of three recent events namely, 1997 Bangladesh-India, 1997 Bangladesh-Myanmar and 1999 Moheskhali earthquakes are developed for the first time in this study. EMS intensities of the rest of the events are found using existing correlation of different intensity scales. Surface-wave magnitudes (Ms) of these 18 events vary between 4.2 to 8.7. Expression of general form for the magnitude-intensity and intensity-attenuation correlation adopted are as follows: Msc= Al + A2(1i)+ A3(Ri) + A410gRi+ crP I = BI + B2(Ms) + B3(R) + B410gR+ crP where M,c is the predicted macroseismic magnitude, Ri is the hypocentral distance that corresponds to the average epicentral radii Di = (Ri2- ho2) 1/2, in km of isoseismal of intensity Ii and ho which represents the mean focal depth for the whole data set used, cr is the standard deviation of M" and the constant P takes a value zero for 50 percent probability that the predicted parameter will exceed the real value and one for 84 percent probability. The data.- set consists of 18 events with 74 pairs of (Ii, D,). Three separate analyses are performed using (a) the whole data set, (b) northeast India and Ganges-Basin data set and (c) only northeast India data set. Thc results of this study show that the intensity-attenuation models are adequate to predict quite well the die-out of intensity with distance for Bangladesh and its surrounding region; it is also found that magnitude can be predicted accurately by calibrating isoseismal radii against instrumental surface-wave magnitude. Such magnitude-intensity relationships may be used to evaluate the magnitude of historical earthquakes in the region under survey, with no instrumental data, for which isoseismal radii and intensities are available. The intensityattenuation law developed here can help us to estimate hazard analysis at specified sites.