It is well known that magnitude mb shows a large variation between different stations and agencies for the same event. In this study we investigate the various causes of this variation and we have found that apart from the regional variation caused by the Earth’s 3D structure, a major contribution to the variation is the way the data are processed: automatic or manual determination of amplitude and period is different for the same signal at different agencies or processing systems and/or there are errors in the amplitude measurements when correcting the signal from the simulated World Wide Standard Seismic Network Short Period (WWSSN‐SP) response. Part of the problem to objectively investigate the causes for the variation is that few agencies use the International Association of Seismology and Physics of the Earth’s Interior standard for reporting the observations. To get less variability in mb, a standardized way of reading amplitude and period must be developed and used.

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