Abstract
Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration (PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (or chaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the PSDM analysis shows low velocity in the frontal prism and velocity reversal across the backstop interface. The PSDM velocity model around the drill site is similar to the P wave velocity model calculated from the ocean bottom seismograph (OBS) data and agrees with the P wave velocities measured from the core experiments. The average V p/V s in the hanging wall sediments around the drill site, as derived from OBS data, is significantly larger than that obtained from core sample measurements.
Highlights
The 2011 Tohoku earthquake occurred along the Japan Trench subduction zone east off the northern Honshu island, Japan
The results show that the velocity in the seaward part of the frontal prism around the Japan Trench Fast Drilling Project (JFAST) site is determined with approximately 5% uncertainty, because the velocity models with 10% perturbation cannot flatten the common reflection point (CRP) gathers in this area (Figure 5a)
The P wave velocity model derived from the pre-stack depth migration (PSDM) analysis shows that frontal prism has the low average interval velocity approximately 2.0 km/s
Summary
The 2011 Tohoku earthquake occurred along the Japan Trench subduction zone east off the northern Honshu island, Japan. The earthquake rupture likely propagated along the plate boundary up to the trench axis. Rapid response drilling by the Integrated Ocean Drilling Program (IODP) Expedition 343 (Japan Trench Fast Drilling Project (JFAST)) successfully drilled through the inferred plate boundary fault approximately 6 km landward of the trench axis. A regional scale seismic data is necessary to determine the velocity structure of the wedge and directly connect the drill site information to the background regional scale geological structure. To understand the structure and the physical properties of hanging wall sediments, a post-drilling multi-channel seismic (MCS) survey was conducted around the JFAST drill site. We modeled travel-times of converted S wave arrivals observed at the ocean bottom seismographs (OBSs) to obtain the shear-wave velocity information around the drill site, which cannot be retrieved using the MCS data. We compare the seismic-derived velocity information with drilling results and controlled pressure Vp and Vs core measurements and discuss the similarities and differences between them
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