Results from a new seismic survey around the JFAST drill site

Author(s): Nakamura, Y.; Kodaira, S.; Yamamoto, Y.; Fujie, G.; Obana, K.; Miura, S.; Takahashi, N.; Cook, B.; Conin, M.; Chester, F. M.; Mori, J. J.; Eguchi, N.; Toczko, S.
Author Affiliation(s): Primary:
Japan Agency of Marine-Earth Science and Technology, Institute for Research on Earth Evolution, Yokohama, Japan
Other:
University of Southampton, United Kingdom
Universitè des Antilles et de la Guyane, Guadeloupe
Texas A&M University, United States
Kyoto University, Japan
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: After the 2011 Tohoku earthquake, we have carried out several seismic surveys in the Japan Trench region. A high-resolution seismic survey collected in 2011 using a 1300-m-long streamer cable and a gun array with volume of 320 inch3 played an important role for choosing the site location and its results showed detailed structure in the Japan Trench axis area. Due to the short offset of the streamer cable, however, the seismic velocity could not be accurately determined.. Furthermore, the regional structural profiles were not obtained because of the small volume of the sounding source from the high resolution seismic survey. In January 2013, we conducted a seismic survey around the IODP Site C0019 drilled during the IODP Expedition 343 (JFAST) with air gun arrays with volume of 7800 inch3 by R/V Kairei. We used a 6000-m-long streamer cable and 4 OBSs as receivers. The shot interval was 50 m along the survey lines. The primary survey line JFD1 runs across the Japan Trench in WNW-ESE direction and the length of the line is ∼ 100 km centered at the Site C0019. The data obtained by the streamer cable were processed through the Pre-stack time migration (PrSTM) technique. On the PrSTM section of the line JFD1, a relatively strong reflection is observed at ∼ 1 s two-way travel time (TWT) below the seafloor in the landward part of the section through ∼20 km landward from the trench axis, which corresponds to the "Cretaceous unconformity". Landward-dipping reflections observed 15-30 km landward of the trench axis could be a "backstop interface". Several landward dipping reflections are imaged within the frontal prism. In the vicinity of the trench axis, imbricated structure of incoming sediments is imaged on the PrSTM profile as previously observed on the high resolution profiles. A seaward dipping reflection, which was interpreted as a part of decollement at the landward part of the trench graben, is also observed in the PrSTM section. The top of the subducting oceanic crust is clearly imaged with horst-graben structure in the seaward part of the profile, and can be traced at least 40 km landward from the trench axis. A possible Moho reflection is also recognized as landward dipping reflection at 10-12 s TWT. We employed the iterative horizon based velocity analysis during the PrSTM processing, which determines the RMS velocity for major interpreted horizons. If we converted the RMS velocity by Dix transformation assuming constant interval velocity between two interpreted horizons, the interval velocity of the frontal prism around the JFAST site was estimated as ∼ 2.2 km/s. The interval velocity within the frontal prism ranges 2.2-2.3 km/s in the Line JFD1. The OBSs were recovered in January and April 2013 and the data will be incorporated into the velocity modeling.
Year of Publication: 2013
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 20 Geophysics, Applied; Expeditions 343/343T; Geophysical methods; Geophysical surveys; IODP Site C0019; Integrated Ocean Drilling Program; Japan Trench; Japan Trench Fast Drilling Project; North Pacific; Northwest Pacific; Pacific Ocean; Seismic methods; Surveys; West Pacific
Coordinates: N375619 N375620 E1435449 E1435447
Record ID: 2015031151
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