[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-P04] Well and seismic reflection data integration/correlation at JTRACK drill sites

*Yasuyuki Nakamura¹, Jonathan Ford², Uisdean Nicholson³, Ayumu Miyakawa⁴, Yohei Hamada¹, Marianne Conin⁵, Patrick Fulton⁶, Jamie Kirkpatrick⁷, Christine Regalla⁸, Shuichi Kodaira¹, Kohtaro Ujiie⁹, Nobuhisa Eguchi¹, Lena Maeda¹, Natsumi Okutsu¹, Sean Toczko¹, IODP Expedition 405 Science Party - (1.Japan Agency for Marine Earth Science and Technology, 2.University of Hamburg, 3.Heriot-Watt University, 4.National Institute of Advanced Industrial Science and Technology, 5.University of Lorraine, 6.Cornell University, 7.University of Nevada, Reno, 8.Northern Arizona University, 9.University of Tsukuba)

Keywords:Japan Trench, JTRACK, Seismic-well tie, IODP

During IODP Exp. 405 (JTRACK), we established seismic-well ties using the sonic logging data from the Logging-while-drilling (LWD), and seismic reflection data acquired by JAMSTEC. LWD data were obtained from the seafloor down to the basalt layer at both the prism site (Site C0019, Hole C0019H) and the input site (Site C0026, Hole C0026A). Even though the LWD dataset from C0026 was limited to the real-time data provided via mud pulse telemetry because the tool could not be recovered at this site, we could correlate the drilling results to the seismic data. Time-domain synthetic seismograms were computed from the P-wave velocity data obtained with LWD and tied with the major reflections in the seismic profiles. We used two different seismic datasets at C0019: higher resolution seismic data (HD33L) and deeper penetration seismic data (JFD1). The seismic-well tie was established for each seismic data respectively. Only the higher resolution seismic dataset (HDMY001) was used at Site C0026. We slightly modified the time-depth relationship to align the major reflections. The adjustment amount was ~30 ms at Site C0019, and ~2 ms at C0026. The drilling results in the depth-domain were converted to time-domain using the adjusted time-depth relationship to correlate with the seismic profiles.

Within most of the prism at Site C0019, the correlation between the seismic reflection profiles, both JFD1 and HD33L, and synthetic seismograms were poor, especially in the wavelet phases. However, intervals with higher amplitude synthetic seismograms correspond to those with relatively high amplitudes in seismic profile JFD1. This might be attributed to the offset between the location of the borehole and seismic profile, and the complicated structure observed in the JTRACK core samples.

At Site C0026, most of the reflections in the synthetic seismograms were adequatelycorrelated with the seismic reflection data within the chert interval. In the shallow part, the correspondence between the synthetic seismogram and seismic profile is poor; however, seismic characteristics such as reflection amplitude variations can be correlated with the prescribed lithological units in some intervals. Noticeably, the lithological unit boundary between Unit 1 (mud) and Unit 2 (clay) corresponds to the boundary between the interval with weak and strong seismic reflection within the Seismic Unit 2.

Seismic-well ties established from the LWD and seismic reflection data, combined with the JTRACK drilling results and various post-cruise research will contribute to understanding the shallow plate boundary slip and the tectonics in the Japan Trench subduction zone.