Seismic attenuation structure of the accretionary prism in the Kumano Basin, Nankai Trough, Japan, revealed by walk-away VSP

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Author(s): Hino, R.; Bangs, N. L.; Sanada, Y.; Park, J.; von Huene, R.; Moore, G. F.; Tsuji, T.; Araki, E.; Kinoshita, M.
Author Affiliation(s): Primary:
Tohoku University, Researth Center Earthquake Prediction, Miyagi Sendai, Japan
University of Texas at Austin, Austin, TX, United States
Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
University of Tokyo, Kashiwa, Japan
University of California at Davis, Davis, CA, United States
University of Hawaii, Honolulu, HI, United States
Kyushu University, Fukuoka, Japan
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The Kumano Basin is a forearc basin located on the landward slope of the Nankai Trough. It lies directly above the large coseismic slip area of the 1944 Tonankai earthquake (M 8.1). A number of seismic surveys have revealed the structural heterogeneity of the accretionary prism relevant to the evolution of the margin including the dynamic behaviors of the megathrust and megasplay faults. In this paper, we report on the seismic attenuation structure of the accretionary prism, which has been poorly known, especially in the seismic frequency band (≈ 10 Hz), by analyzing a walk-away vertical seismic profile (WVSP) obtained during IODP Exp. 319 in 2009, as a part of the NanTroSEIZE project. A 16 channel vertical geophone arrays installed in the deep (from 907 to 1,135 m below seafloor) borehole provided seismic waveform records with broad frequency band and enabled us to estimate an attenuation parameter (Q) through calculations of the amplitude fall off rates of the high frequency component of the recorded signal. By taking the spectral ratio between up-coming P-waves from large offset shots and down-going waves from near offset shots, we could estimate Qp of the prism down to ≈ 5 km below seafloor. Preliminary estimated value of Qp was 80 ≈ 120 in frequency range of 10 to 50 Hz, assuming frequency independent Q. The smaller estimated Qp in Nankai indicates that the seismic attenuation in the accretionary prism is larger than in the onshore part of the NW-Japan (Q ≈ 250 at ≈ 10 Hz), but Qp is larger in Nankai than in the Costa Rica margin wedge (Q = 25 ≈ 75) indicating substantially less attenuation in Nankai than Costa Rica.
Year of Publication: 2012
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 20 Geophysics, Applied; Accretionary wedges; Expedition 319; Geophysical methods; Geophysical profiles; Geophysical surveys; Integrated Ocean Drilling Program; Kumano Basin; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; Seismic methods; Seismic profiles; Surveys; West Pacific
Coordinates: N324900 N333000 E1365300 E1363200
Record ID: 2014082007
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