Quantifying early aftershock activity of the 2004 mid-Niigata Prefecture earthquake (Mw6.6)

We analyze the early aftershock activity of the 2004 mid‐Niigata earthquake, using both earthquake catalog data and continuous waveform recordings. The frequency‐magnitude distribution analysis of the Japan Meteorological Agency (JMA) catalog shows that the magnitude of completeness of the aftershocks changes from values around 5.0, immediately after the main shock, to about 1.8, 12 hours later. Such a large incompleteness of early events can bias significantly the estimation of aftershock rates. To better determine the temporal pattern of aftershocks in the first minutes after the Niigata earthquake, we analyze the continuous seismograms recorded at six High Sensitivity Seismograph Network (Hi‐Net) stations located close to the aftershock distribution. Clear aftershocks can be seen from about 35 s after the main shock. We estimate that the events we picked on the waveforms recorded at two seismic stations (NGOH and YNTH) situated on opposite sides of the aftershock distribution are complete above a threshold magnitude of 3.4. The c value determined by taking these events into account is about 0.003 days (4.3 min). Statistical tests demonstrate that a small, but nonzero, c value is a reliable result. We also analyze the decay with time of the moment release rates of the aftershocks in the JMA catalog, since these rates should be much less influenced by the missing small events. The moment rates follow a power law time dependence from few minutes to months after the main shock. We finally show that the rate‐and‐state dependent friction law or stress corrosion could explain well our findings.