The Mw 9.0, 2011 Tohoku-Oki earthquake has reopened
the discussion among the scientific community
about the effectiveness of earthquake early warning for large
events. A well-known problem with real-time procedures is
the parameter saturation, which may lead to magnitude
underestimation for large earthquakes. Here we measure
the initial peak ground displacement and the predominant
period by progressively expanding the time window and
distance range, to provide consistent magnitude estimates
(M = 8.4) and a rapid prediction of the potential damage
area. This information would have been available 35 s
after the first P-wave detection and could have been refined
in the successive 20 s using data from more distant stations.
We show the suitability of the existing regression relationships
between early warning parameters and magnitude, provided
that an appropriate P-wave time window is used for
parameter estimation. We interpret the magnitude under-estimation
as a combined effect of high-pass filtering and frequency
dependence of the main radiating source during the
rupture process