Discrimination of a preparatory stage leading to M7 characteristic earthquakes off Ibaraki Prefecture, Japan

Abstract

An M7.0 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 M7.0 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 (rA and rB) are calculated by treating each seismicity change map just before the 1982 and 2008 events as references. When both rA and rB exceed the prefixed threshold r0, it is possible to announce the time of immediate occurrence of the target earthquake.