Ranjit Das

Unbiased Estimation of Moment Magnitude from Body‐ and Surface‐Wave Magnitudes

Abstract For regression of variables having measurement errors, general orthogonal regression (GOR) is the most appropriate statistical procedure that yields a linear relation between the true values of the dependent ( y t ) and independent ( x t ) variables. However, the GOR procedure to obtain unbiased estimate of the dependent variable for a given error‐affected value of the predictor variable is not well addressed in the literature. In the conventional GOR approach, the error‐affected value of the predictor variable is substituted as such in the GOR relation, yielding biased estimates of y t . In another approach, the orthogonal projections of the given points on the GOR line are used to first estimate x t and then y t . In this study, a procedure making use of true points on the GOR line is proposed to obtain improved estimates of y t . The proposed GOR procedure is applied to the magnitude conversion problem between m b to M w and M s to M w , using real data set. The absolute average differences of the estimates obtained and their standard deviations are compared, indicating that the proposed GOR procedure provides improved estimates of the dependent variable ( y t ) compared with the conventional GOR approach. The improved unified magnitudes obtained using the proposed GOR procedure will result in more realistic seismic hazard for a given catalog and seismotectonic environment.

Probabilistic seismic hazard assessment of NW and central Himalayas and the adjoining region

The Himalayan region has undergone significant development and to ensure safe and secure progress in such a seismically vulnerable region there is a need for hazard assessment. For seismic hazard assessment, it is important to assess the quality, consistency, and homogeneity of the seismicity data collected from different sources. In the present study, an improved magnitude conversion technique has been used to convert different magnitude scales to moment magnitude scale. The study area and its adjoining region have been divided into 22 seismogenic zones based upon the geology, tectonics, and seismicity including source mechanism relevant to the region. Region specific attenuation equations have been used for seismic hazard assessment. Standard procedure for PSHA has been adopted for this study and peak ground motion is estimated for 10% and 2% probability of exceedance in 50 years at the bed rock level. For the 10% and 2% probability of exceedance in 50 years, the PGA values vary from 0.06 to 0.36 g and 0.11 to 0.65 g, respectively considering varying b-value. Higher PGA values are observed in the southeast part region situated around Kaurik Fault System (KFS) and western parts of Nepal.