WMm : Application of a magnitude algorithm to a large dataset of W phases, 1991-2009

The so-called W phase identified by H. Kanamori in 1993 is a combination of high-order Rayleigh overtones sampling the upper mantle at periods generally longer than 200 s and with group velocities greater than 5.5 km/s. As such, it bears considerable potential for the rapid estimation of long-period seismic moment in the context of real-time tsunami warning. We describe an algorithm ("WMm") for a W-phase mantle magnitude, directly related to seismic moment and fully justifiable from a theroreticalstandpoint. it is based on Talandier and Okal's [1989] approach for the mantle magnitude Mm and uses the spectral amplitude of ground motion,corrected at each distance and frequency for the theoretical influence of excitation and propagation. The method is applied to a dataset of more than 1800 records from 68 earthquakes with moments greater than 10**26 dyn*cm. When compared to published CMT moments, WMm features an average residual of 0.13 logarithmic units, with a standard deviation of 0.19 units, and thus appears as a promising complementary contributor to tsunami warning algorithms.