西太平洋のマルチチャンネル地震波反射記録断面に見られるモホ反射面の特徴とモホ遷移層の性質

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The thickness of the Moho transition zone (MTZ) at the boundary between the Earths crust and the subjacent mantle has a significant effect on seismic responses from the Moho. We examined the seismic characteristics of Moho reflections (hereafter PmP) using Multi-Channel Seismic (MCS) records obtained from high-quality seismic experiments in the western Pacific by Japan Oil, Gas and Metals National Corporation (JOGMEC). The MCS records show clear reflections at ∼6-10km in depth from the ocean bottom in the north and south of Ogasawara Plateau; however, considering horizontal variations in PmP intensity, the nature of the MTZ varies by location. In seismic profile D00-D, across Ogasawara Plateau in the N-S direction, the PmP abruptly disappears far from the nearby seamount where the overlain sedimentary section shows less change. In another case, shown in D00-C located 130km west of D00-D, the PmP clearly shows a high-amplitude continuous reflection near the seamounts flank. Data acquisition is relatively constant for the Ogasawara MCS reflection lines; therefore, the difference in PmP intensity between D00-D and D00-C might relate to the nature of the Moho. We calculated synthetic seismograms to evaluate the effects of MTZ thickness on seismic reflection records. The results suggest that if the thickness of the Moho transition zone is less than 1km for the dominant frequency of 4Hz, then PmP can be observed with the current MCS survey equipment. If the dominant frequency of the MCS reflection survey is ∼15Hz, penetrating down to the Moho depth, then the thickness of the Moho required to identify the PmP should be less than a few hundred meters. Moreover, anisotropy assuming a strong olivine preferred orientation in peridotite might affect the change of PmP intensity. The MCS reflection records in the western Pacific and the western Philippine Sea Basin suggest that the thickness of MTZ varies from ∼100m to more than a few kilometers. This is consistent with petrological observations in Oman ophiolite, sections of oceanic crust, and possible mantle rock, showing that the thickness of the mafic crust to ultra-mafic mantle transition varies from an order of meters to a few kilometers. The next target of the IODP seems to be to obtain the mantle constituent materials below the Moho and to explore the nature of the Moho. Considering the large heterogeneity of MTZ even in the oceanic region, the IODP drilling site to drill to the Moho depth should be carefully selected based on an understanding of the geophysical background of the proposed sites.
社団法人東京地学協会の論文
2008-02-25

西太平洋のマルチチャンネル地震波反射記録断面に見られるモホ反射面の特徴とモホ遷移層の性質

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