BIAS IN BODY WAVE MAGNITUDE rnb DUE TO INADEQUATE DEPTH-DISTANCE CORRECTIONS

Large deviations in mb determination at varying epicentral distances using the Gutenberg-Richter [۱] calibration terms shows that the calibration terms presently used need to be revised. New empirical global depth-distance correction terms B(A, h) for body-wave magnitude are determined using the values of scalar moment- M۰ in the Harvard CMT Catalogue to calibrate P-wave amplitude-distance curves. Comparison of the correction terms of Gutenberg-Richter, Veith-Clawson [۲۱, and Lilwall [۳], with those obtained in this study, show that rub is biased least when using the correction terms of Veith-Clawson, Lilwall, or of this study. A systematic bias in the estimated mb for distance greater than ۸۸° is observed for the Veith-Clawson and the Lilwall correction terms. Comparison of event magnitude nib calculated using the Gutenberg-Richter calibration, and that of this study, shows that the new calibration terms tend to increase small magnitudes and de-crease large magnitudes. Using Gutenberg-Richter, Veith-Clawson, and Lilwall correction terms, m۱ for deep earthquakes is systematically underestimated by about ۰,۱-۰.۱۵, ۰.۳-۰.۵, and ۰.۱-۰.۲ magnitude units respectively. Application of the new correction terms to the ISC dataset show that estimated crab is then independent, of distance and focal depth and hence provides unbiased estimates of nib in comparison with other correction terms, This observation is also a positive test. of the hypothesis that a calibration with respect to Mo gives a better estimate of m&.