11:15 AM - 11:30 AM
[S06-06] Investigating Japan Trench Outer Rise Structure off Sanriku and Miyagi Using Multichannel Seismic Reflection Survey
Introduction
A number of historically large earthquakes with magnitude higher than 8 occurred in the Japan Trench subduction zone, with the March 2011 Tohoku earthquake (M 9.1) being the latest one. Obana et al. (2012) used ocean bottom seismograph observations after Tohoku earthquake and realized that the mechanism of intraplate earthquakes beneath the outer rise of Japan Trench has changed at depths below 20 km from a reverse faulting to a normal faulting pattern. Their observation suggested that the stress regime of the Pacific plate has changed to a tensional stress down to a depth of 40 km after the Tohoku earthquake. This could have a direct impact on the occurrence of the large normal faulting earthquakes which could trigger a Tsunami. In order to investigate the detailed geological setting of the outer rise of Japan Trench off Sanriku and Miyagi of northeast Japan, we have conducted a high resolution multichannel seismic reflection survey in April, 2019.
Data Acquisition and Processing
We used research vessel Shinsei-Maru for survey KS-19-05 to acquire seismic reflection data along two seismic lines. Line number 5 off Miyagi passes closely to the epicenter of Tohoku earthquake, and line number 1 off Sanriku is in the northern part of the study area. A streamer was towed at a nominal depth of 6 m with 288 channels at intervals of 6.25 m to record the data with a maximum offset of 1800 m. Two GI airguns (2 x 355 cubic inches) at a depth of 5 m were fired simultaneously at shot intervals of 50 m and 37.5 m for line 5 and line 1, respectively. The total length of seismic line was 100 km and 150 km for line 5 and line 1, respectively. Due to a problem with acquisition tools, the depth of streamer could not be fixed in line 5 and a significantly strong swell noise contaminated the reflection data. Moreover, a large amount of spike noise occurred due to the recording system issues, which demanded advanced noise suppression methods. We used a time-variant band-limited envelope thresholding technique to suppress the strong swell noise. The spiky traces were manually edited to be excluded from data processing, and FX deconvolution filter was applied to further reduce the noise effect. Stacked sections and post-stack time-migrated sections illustrated the high resolution subsurface structure of the Japan Trench outer rise.
Results and Future Work
We could recognize a horst and graben pattern which starts from the seaside and continues toward the Japan Trench axis with clear associated normal faults. There are three major vertical displacements along line 5, the line close to the Tohoku earthquake epicenter, which are of significant importance for further investigation. We also observed a series of horst and graben structures that are characterized by normal faults with displacements of several hundreds of meters along line 1. We found slumping-like structure containing turbidites in the major grabens, which we believe were produced by the normal faulting earthquakes. Our future work is to apply advanced velocity model building and depth imaging techniques including reverse time migration (RTM) for a detailed and more accurate imaging of the subsurface structure.
Acknowledgments
We are grateful to the crew of the R/V Shinsei-Maru for their support in acquiring the seismic reflection data. This work was supported by JSPS KAKENHI Grant Number 18H03732. We thank Paradigm/Emerson (http://www.pdgm.com) for providing seismic data processing software.
References:
Obana K., G. Fujie, T. Takahashi, Y. Yamamoto, Y. Nakamura, S. Kodaira, N. Takahashi, Y. Kaneda, and M. Shinohara, 2012, Normal-faulting earthquakes beneath the outer slope of the Japan Trench after the 2011 Tohoku earthquake: Implications for the stress regime in the incoming Pacific plate, GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L00G24.
A number of historically large earthquakes with magnitude higher than 8 occurred in the Japan Trench subduction zone, with the March 2011 Tohoku earthquake (M 9.1) being the latest one. Obana et al. (2012) used ocean bottom seismograph observations after Tohoku earthquake and realized that the mechanism of intraplate earthquakes beneath the outer rise of Japan Trench has changed at depths below 20 km from a reverse faulting to a normal faulting pattern. Their observation suggested that the stress regime of the Pacific plate has changed to a tensional stress down to a depth of 40 km after the Tohoku earthquake. This could have a direct impact on the occurrence of the large normal faulting earthquakes which could trigger a Tsunami. In order to investigate the detailed geological setting of the outer rise of Japan Trench off Sanriku and Miyagi of northeast Japan, we have conducted a high resolution multichannel seismic reflection survey in April, 2019.
Data Acquisition and Processing
We used research vessel Shinsei-Maru for survey KS-19-05 to acquire seismic reflection data along two seismic lines. Line number 5 off Miyagi passes closely to the epicenter of Tohoku earthquake, and line number 1 off Sanriku is in the northern part of the study area. A streamer was towed at a nominal depth of 6 m with 288 channels at intervals of 6.25 m to record the data with a maximum offset of 1800 m. Two GI airguns (2 x 355 cubic inches) at a depth of 5 m were fired simultaneously at shot intervals of 50 m and 37.5 m for line 5 and line 1, respectively. The total length of seismic line was 100 km and 150 km for line 5 and line 1, respectively. Due to a problem with acquisition tools, the depth of streamer could not be fixed in line 5 and a significantly strong swell noise contaminated the reflection data. Moreover, a large amount of spike noise occurred due to the recording system issues, which demanded advanced noise suppression methods. We used a time-variant band-limited envelope thresholding technique to suppress the strong swell noise. The spiky traces were manually edited to be excluded from data processing, and FX deconvolution filter was applied to further reduce the noise effect. Stacked sections and post-stack time-migrated sections illustrated the high resolution subsurface structure of the Japan Trench outer rise.
Results and Future Work
We could recognize a horst and graben pattern which starts from the seaside and continues toward the Japan Trench axis with clear associated normal faults. There are three major vertical displacements along line 5, the line close to the Tohoku earthquake epicenter, which are of significant importance for further investigation. We also observed a series of horst and graben structures that are characterized by normal faults with displacements of several hundreds of meters along line 1. We found slumping-like structure containing turbidites in the major grabens, which we believe were produced by the normal faulting earthquakes. Our future work is to apply advanced velocity model building and depth imaging techniques including reverse time migration (RTM) for a detailed and more accurate imaging of the subsurface structure.
Acknowledgments
We are grateful to the crew of the R/V Shinsei-Maru for their support in acquiring the seismic reflection data. This work was supported by JSPS KAKENHI Grant Number 18H03732. We thank Paradigm/Emerson (http://www.pdgm.com) for providing seismic data processing software.
References:
Obana K., G. Fujie, T. Takahashi, Y. Yamamoto, Y. Nakamura, S. Kodaira, N. Takahashi, Y. Kaneda, and M. Shinohara, 2012, Normal-faulting earthquakes beneath the outer slope of the Japan Trench after the 2011 Tohoku earthquake: Implications for the stress regime in the incoming Pacific plate, GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L00G24.