Calibrating Global Measurements of Surface-Wave Magnitude, Ms.

The accurate determination of surface-wave magnitude, M_s, is important in two fields of seismology. In seismic hazard assessment, M_s can be used to estimate the energy release of shallow earthquakes where more sophisticated techniques cannot be applied. To the forensic seismologist, accurate M_s measurements are a vital part of event-screening, that is, identifying definite earthquakes in a population of seismic disturbances of unknown origin. It has been known since the earliest attempts to measure M_s that observations at particular stations may show systematic biases, and that propagation effects in differing tectonic regions can be large. Here we present preliminary work based on the M_s reports in the USGS NEIC monthly bulletin, in an attempt to provide calibration information which can be used to improve the accuracy of M_s measurements. These results should enable us to investigate issues such as station calibration problems, the estimation of errors in M_s values determined from a small number of observations and possible regional variations in log ₁₀M₀ - M_s and m_{b/M_s}. Since M_s measurements are made from Rayleigh waves at periods near 20s, the results should also cast light on lateral variations in shallow Earth structure. We demonstrate that a large part of the variance in M_s observations can be explained by a system of station and path corrections.