Analysis of seafloor seismograms of the 2003 Tokachi‐Oki earthquake sequence for earthquake early warning

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 M w 8.1 Tokachi‐Oki earthquake and its aftershocks. A wavelet transform of the first ∼4 s 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 M w 6.0 indicating that, at least for the mainshock (M w 8.1) and largest aftershock (M w 7.1), the final size of a rupture could have been estimated from the initial portion of the seismogram.