Characteristics of Seismic Intensity Distribution from the Earthquake (<I>M</I>=7.2) off Miyagi Prefecture, Japan, on August 16, 2005 and Its Short-Period Seismic Wave Radiation Zone

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  • 宮城県沖で2005年8月16日に起こった地震(<I>M</I>=7.2)の震度分布の特徴と短周期地震波発生域
  • 宮城県沖で2005年8月16日に起こった地震(M=7.2)の震度分布の特徴と短周期地震波発生域
  • ミヤギケンオキ デ 2005ネン 8ガツ 16ニチ ニ オコッタ ジシン M 7 2 ノ シンド ブンプ ノ トクチョウ ト タンシュウキ ジシンハ ハッセイイキ
  • Characteristics of Seismic Intensity Distribution from the Earthquake (M=7.2) off Miyagi Prefecture, Japan, on August 16, 2005 and Its Short-Period Seismic Wave Radiation Zone

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Abstract

The major earthquake of M=7.5 class, so-called Miyagi-ken-oki earthquake, has been occurred on the boundary between the Pacific Plate and North American Plate off Miyagi Prefecture every 30 to 40 years. An earthquake (M=7.2) occurred in the same source area on August 16, 2005. We compared its seismic intensity distribution with those of the Miyagi-ken-oki earthquakes in 1861, 1897, 1936, 1937, and 1978 and evaluated their short-period seismic wave radiation zones (SPRZ) using the seismic intensity inversion method. JMA seismic intensity scale (I) 6 was recorded in some extended area from the 1861, 1897, and 1978 events, while it was not recorded from the 1936 and 2005 events. The isoseismal area of I≥5 of the 1978 event is quite similar to that of the 1897 event, but that of the 2005 event is relatively small and is included in them. On the other hand, the area of the 1936 event is located in the south of other events. The SPRZ of the 2005 event is located eastward from 142°E and almost corresponds with its large slip area derived from the waveform inversion. The SPRZ of the 1936 event is adjacent to the south end of that of the 2005 event and its centroid almost agrees with the south end of the large slip area from the waveform inversion. The result indicates that the focal depth of the 2005 event is similar to that of the 1936 event in consideration of the depth contour map of the upper boundary of the Pacific Plate and has consistency in the similarity of their phase and amplitude of long-period waveforms observed in the far field. Although the SPRZ of the 2005 event is located in the east of that of the 1978 event, it is highly possible that the 2005 event ruptured a south-eastern part of the rupture area of the 1978 event. It is because that the SPRZ of the 2005 event is located in the aftershock area of the 1978 event and that there is a possibility that we could not detect the same SPRZ of the 2005 event during the 1978 event since the effect of western SPRZ near land was dominant for seismic intensity distribution. Consequently, we concluded that the rupture area of the 2005 event, which is adjacent to the north end of that of the 1936 event, occupies a south-eastern part of the rupture area of the 1978 event.

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