Characteristics of Moho Reflections Identified by MCS Reflection Records in the Western Pacific Ocean and Effects of Moho Transition Zone Properties

Abstract

The thickness of the Moho transition zone (MTZ) at the boundary between the Earth's 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⊖10 km 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 130 km west of D00-D, the PmP clearly shows a high-amplitude continuous reflection near the seamount's 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 1 km for the dominant frequency of 4 Hz, then PmP can be observed with the current MCS survey equipment. If the dominant frequency of the MCS reflection survey is ～15 Hz, 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 ～ 100 m to more than a few kilometers. This is consistent with petrological observations in ＊ 日本大陸棚調査(株) ＊＊ 地球科学総合研究所 ＊＊＊ 独立行政法人 石油天然ガス・金属鉱物資源機構 ＊＊＊＊ 静岡大学理学部地球科学教室