震度データによる１９１４年秋田仙北地震の短周期地震波発生域と地震規模の推定および１８９６年陸羽地震との比較

Abstract

The 1914 Akita-Senboku earthquake was investigated using seismic intensity data estimated from damage records to clarify their magnitudes and source areas related to short-period seismic waves. We evaluated the most appropriate fault model of the Akita-Senboku earthquake on the basis of grid search analysis and other existing research works such as hypocenter distribution of micro earthquakes and S-wave velocity perturbations [Okada et al. (2010)]. The seismic intensity inversion analysis for the Akita-Senboku earthquake assuming an east-dipping fault model (depth: 6-13 km) indicated that the short-period radiation zone was located in the north and deep part of the fault plane and there was a low S-wave velocity zone in the lower crust beneath it. The magnitude of 6.5.6.6 yielded the least evaluation error. The magnitude was also estimated from the area of seismic intensity .ve-lower or more using empirical relationship [Muramatsu (1969)]. It resulted in M=6.6.∼6.7 and reinforced the accuracy of the intensity inversion result. It is concluded that the magnitude of the Akita-Senboku earthquake should be considerably less than M=7.1 after Utsu (1979) even if considering the estimation error. The 1896 Rikuu earthquake was also analyzed to be compared with the Akita-Senboku’s results and to consider their dieffrences. The fault location of the Rikuu earthquake was already identi.ed from surface faults. We assumed two conjugate faults for the northern fault zone of the eastern margin of the Mahiru mountains and the northern fault zone of the eastern margin of the Yokote basin. The most appropriate magnitude was estimated to be M=7.1, which is equivalent to that after Utsu (1979). Therefore, our results indicate that the Rikuu earthquake was considerably larger than the Akita-Senboku earthquake. The short-period radiation zone of the Rikuu earthquake, which might be less accurate due to the lack of intensity data compared to the Akita-Senboku earthquake, was located in shallow zone at the intersection of two conjugate faults. This location contrasts with that of the Akita-Senboku earthquake without surface faults.