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Great earthquakes (Mw>8.0) generally involve rupture propagation over
large distances in a few minutes. In such cases, the point-source
approximation is only valid for ultra-long-period seismic waves such
as the W phase, which corresponds to the superposition of the first
overtones of the Earth normal modes between 100 s and 1000 s. The W phase
is conspicuous between the P wave and the surface wave train on broadband
displacement records. The W phase source-inversion algorithm was initially
developed to provide rapid characterization of the seismic source for tsunami
warning purposes and is a reliable and straightforward method to resolve the
first-order attributes of large earthquakes.
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