MM - EXTENSION TO LOVE WAVES OF THE CONCEPT OF A VARIABLE-PERIOD MANTLE MAGNITUDE

We extend to Love waves the concept of the mantle magnitude Mmintroduced recently for Rayleigh waves. Spectral amplitudes X(ω) of Love waves in the 50-300 s period range are measured on broad-band records from major events. A distance correction CD, regionalized to reflect the influence of different tectonic paths, and a source correction CS, compensating for the variation of excitation with period are effected; the exact geometry and depth of the event are however ignored. The resulting expression {Mathematical expression} is expected to be an estimation of log10M0-20, where M0 is the seismic moment of the event. All quantities in this equation are fully justified from a theoretical standpoint. The analysis of a dataset of more than 300 Love records shows that Mmcorrectly describes the seismic moment, with average residuals in the range of 0.1-0.2 unit of magnitude. No significant trend with either distance or period of measurement is present. In particular, Mmdoes not saturate and continues to grow linearly with log10M0 for very large events. The combiantion of the Rayleigh and Love Mmguards in general against underestimation of M0. © 1990 Birkhäuser Verlag.