Broadband records of the first passage of Rayleigh waves at Papeete, Tahiti from large circum-Pacific earthquakes are used to recover in real time their seismic moments. The magnitude concept is used at periods varying from 50 to 300 s. Adequate corrections are made for distance and period. Typical standard deviations are 0.2 orders of magnitude on M0. This mantle scale Mm can be extended continuously with no additional corrections over 4 orders of magntiude. It can be measured in the frequency- or time-domains, and we describe briefly the automation of the method, with obvious applications in tsunami warning. ¿ American Geophysical Union 1987