Earthquake Early Warning (EEW) algorithms estimate the magnitude
of an underway rupture from the first few seconds of the P-wave to allow
hazard assessment and mitigation before the S-wave arrival. Many large
subduction-zone earthquakes initiate 50-150 km offshore, potentially
allowing seafloor instruments sufficient time to identify large ruptures
before the S-waves reach land. We tested an EEW algorithm using
accelerograms recorded offshore Hokkaido in the region of the 2003 Mw 8.1
Tokachi-Oki earthquake and its aftershocks. A wavelet transform of the
first 4 seconds of the P-wave concentrates information about earthquake
magnitude from both waveform amplitude and frequency content. We find that
wavelets with support of a few seconds provide discriminants for EEW that
are both accurate enough to be useful and superior to peak acceleration or
peak velocity. Additionally, we observe a scaling of wavelet coefficient
magnitude above Mw 6.0 indicating that, at least for the mainshock and
largest aftershock (Mw 7.1), the final size of a rupture could have be
estimated in a stochastic sense from the initial portion of the seismogram.
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