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We have recently developed a source inversion algorithm using W phase at teleseismic distances. W phase is a very long period phase (mainly 100s-1000s) arriving at the same time as the P wave (see figure bellow). When the displacement seismograms are filtered between 100s and 1000s, it is conspicuous between the P wave and the surface waves. Because of its long period nature and because it precedes the large amplitude surface-wave arrivals, the W phase can provide rapid and reliable estimates of the overall source parameters of large events. This contrasts with other source inversion methods which usually take several hours to determine the first order attributes of a great earthquake (i.e. Mw>8.5) even in a well instrumented region.

W phase from the 2001 Peruvian earthquake (Mw = 8.4) recorded at HRV, and the synthetic W phase computed by mode summation using the Global CMT solution.

The W phase source inversion algorithm is now running online at U.S. Geological Survey (USGS), at the Pacific Tsunami Warning Center (NOAA/NWS/PTWC) and at the Institut de Physique du Globe de Strasbourg (IPGS-EOST, CNRS/UdS). The W phase solution calculated at PTWC is issued internaly within 30 minutes after origin time (O.T.) in order to have a quick preliminary robust estimate of the event magnitude and focal mechanism of large earthquakes. The W phase solution computed at USGS is made available on line for each significant event. (e.g. Tohoku-oki 2011 earthquake).