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Title: Removal of bias in global seismic magnitude determinations
Author: Rezapour, Mehdi
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1999
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Bias in global seismic magnitude determinations caused by inadequacies in distance/depth calibration functions is reduced, by developing new formulae for surface-wave magnitude Ms, and new distance/depth calibration terms for body-wave magnitude mb . Bias in Ms and mb is investigated using the complete ISC and NEIC datasets between 1978 and 1993. Analysis of the ISC dataset shows that the density function for magnitude against frequency of Ms values is smooth but significantly asymmetric, while that for mb is symmetric and is close to normally distributed. Examination of Ms : mb for this dataset reveals some anomalous earthquakes which plot as explosions according to the Ms : mb discriminant. Also, the frequency-distance plot for reported surface wave amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances. This censoring via the body waves represents a large deficit in the number of potentially usable surface-wave amplitude observations, particularly in the P-wave shadow zone for 120° < Δ < 100°. To reduce bias in surface-wave magnitude two new formulae are proposed, with constants obtained using all ISC data: Mes = log (A/T) max + 1.155 log (Δ) + 4.269 Mts = log (A/T) max + 1/3 log (Δ) + 1/2 log (sin Δ) + 0.0046Δ + 5.370. For Mes the conventional logarithmic dependence of the distance correction is retained, while for Mts the theoretically-known relationship for the dispersion and geometrical spreading contributions is exploited. Comparison of these formulae with other work confirms the inadequacy of the distance-dependence term in the Gutenberg (1945) and Prague formulae. The Mes formula, as well as that of Herak and Herak (1993), give less bias at all epicentral distances to within the scatter of the observed dataset. Mts provides an improved overall distance correction, especially beyond Δ=145°.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available