Initial 30 seconds of the 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0)—amplitude and τ c for magnitude estimation for Earthquake Early Warning—

We analyzed the waveforms of the mainshock ( M w 9.0) and three foreshocks of the 2011 off the Pacific coast of Tohoku Earthquake during the initial 30 s after P -wave onset to determine the maximum amplitudes of acceleration, velocity, and displacement, and τ c (the period parameter of the waveform). The amplitudes for the M w 9.0 event were quite small for the first several seconds, as small as those of the M w 6 foreshocks, and the τ c value was also as small as those of the foreshocks. For the first 30 s, the amplitude of the M w 9.0 event was larger than that of the M w 7.3 foreshock whereas τ c was smaller. These results suggest that it is difficult to determine the eventual magnitude for very large earthquakes from the initial several seconds, that an updating procedure is important for Earthquake Early Warning using ongoing waveforms, and that τ c might not be reliable for magnitude estimation at least for the main shock.