QP structure of the accretionary wedge in the Kumano Basin, Nankai Trough, Japan, revealed by long-offset walk-away VSP

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doi: 10.1186/s40623-014-0175-x
Author(s): Hino, Ryota; Tsuji, Takeshi; Bangs, Nathan L.; Sanada, Yoshinori; Park, Jin-Oh; von Huene, Roland; Moore, Gregory F.; Araki, Eiichiro; Kinoshita, Masataka
Author Affiliation(s): Primary:
Tohoku University, International Institute of Disaster Science, Sendai, Japan
Kyushu University, International Institute for Carbon-Neutral Energy Research, Japan
University of Texas at Austin, United States
Japan Agency for Marine-Earth Science and Technology, Japan
University of Tokyo, Japan
University of California, Davis, United States
University of Hawai'i at Manoa, Department Geology and Geophysics, United States
Volume Title: Earth, Planets and Space
Source: Earth, Planets and Space, 67(1). Publisher: Terra Scientific Publishing Company (TERRAPUB), Tokyo, Japan. ISSN: 1343-8832
Note: In English. 38 refs.; illus., incl. sect., 1 table, sketch map
Summary: We determined the seismic attenuation structure of the Kumano Basin, a forearc basin in the central part of the Nankai subduction zone. Despite its importance for understanding the physical condition of the Earth's interior and seismic wave propagation processes, the attenuation factor Q has been poorly estimated in the crustal layers of the offshore areas of Nankai because severe attenuation occurring in the seafloor sediments prevents the reliable estimation of Q from conventional active source seismic surveys. In the present study, we derive Q values from the diminishing rate of the high-frequency contents of seismic energy during propagation through sub-seafloor layers. The records of vertical seismic profiling acquired at approximately 1,000 m below the seafloor, which have fewer effects from shallow attenuation, enabled us to elucidate depth variation of Q of P waves (QP), the attenuation factor of P waves, down to approximately 8 km below the seafloor. Assuming that the frequency dependence of Q is small and using a previously obtained P-wave velocity structure model for the basin, we inverted the fall-off rate of the spectral ratios at various shot-receiver distances to obtain QP in the three sub-bottom layers. The QP values for the upper two layers with P-wave velocity (VP) < 2.7 km/s are 34 and 57. These values are almost identical to those obtained in the North Atlantic, suggesting the broad consistency of QP within seafloor sediment. The basement layer (VP approximately 4 km/s) has a much higher QP value of 349, which is comparable to the value estimated for crustal layers exposed onshore. This QP value is higher than the value previously assumed in a simulation of strong ground motion associated with megathrust earthquakes along the Nankai margin. We interpret that the high QP, low seismic attenuation in the basement layer reflects tectonic stability of the inner wedge of the accretionary margin. Our first estimates of QP in the present study provide a strong basis for future studies of seismic structure and strong ground motion prediction. Copyright 2015 The Author(s) and Hino et al.; licensee Springer.
Year of Publication: 2015
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Accretionary wedges; Attenuation; Basins; Body waves; Earthquakes; Elastic waves; Expedition 319; Fore-arc basins; Geophysical methods; Geophysical profiles; Geophysical surveys; Ground motion; IODP Site C0009; Integrated Ocean Drilling Program; Kumano Basin; Marine sediments; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Ocean floors; P-waves; Pacific Ocean; Propagation; Q; Sediments; Seismic energy; Seismic methods; Seismic profiles; Seismic waves; Strong motion; Surveys; Tonankai earthquake 1944; Vertical seismic profiles; West Pacific
Coordinates: N332728 N332728 E1363209 E1363209
Record ID: 2017014923
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by Springer Verlag, Berlin, Federal Republic of Germany