Discrimination of a preparatory stage leading to <i>M</i>7 characteristic earthquakes off Ibaraki Prefecture, Japan

<jats:p>An <jats:italic>M</jats:italic>7.0 earthquake struck off Ibaraki Prefecture, Japan, on 8 May 2008. This event was interpreted as belonging to a characteristic earthquake series fracturing the same asperities on the upper boundary of the Pacific plate with a quasi‐regular interval of about 22 years. The previous <jats:italic>M</jats:italic>7.0 occurred in 1982, 26 years before the latest one. I examined the background microseismicity changes preceding these two events by drawing maps of earthquake frequency ratios between two successive periods, and I found a significant similarity between them. This similarity could be expressed numerically by calculating correlation coefficients between the two maps. Surveying the temporal transition of the correlation coefficients, I confirmed that the similarity appeared only for the combination of those maps just before two characteristic earthquakes; it never appeared in the other cases. This result implies the existence and repetition of a typical preparatory stage leading to the characteristic earthquake, which might be caused by a quasi‐static slip on minor asperities in a hierarchical constitution and which forms a specific pattern of seismicity change. This evidence makes it possible to propose a definitive procedure for the practical prediction of the next characteristic earthquake, which is anticipated to occur around 2030. Two kinds of correlation coefficients (r<jats:sub>A</jats:sub> and r<jats:sub>B</jats:sub>) are calculated by treating each seismicity change map just before the 1982 and 2008 events as references. When both r<jats:sub>A</jats:sub> and r<jats:sub>B</jats:sub> exceed the prefixed threshold r<jats:sub>0</jats:sub>, it is possible to announce the time of immediate occurrence of the target earthquake.</jats:p>