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).