5:15 PM - 7:15 PM
[SCG59-P03] New dense OBS seismic survey across Japan Trench to reveal detailed structural evolution of the incoming plate
Keywords:oceanic plate, bend fault, Japan Trench, seismic exploration, full-waveform inversion
In the last two decades, seismic velocity reduction within the oceanic crust and uppermost mantle prior to subduction has been confirmed at many subduction zones around the world, such as the Japan Trench, Kuril Trench, Central America Trench, South America Trench, Alaska Trench, Aleutian Trench, and Tonga Trench. The observed reduction in seismic velocity is generally considered to be caused by fracturing of the oceanic plate due to repeated plate bending-related faulting and fluid penetration through the fractures and possible hydration there. It means that the reduction in the seismic velocity is expected to be concentrated on the bend faults. However, because the seismic velocity changes in the previous studies were estimated using travel-time analysis techniques, which have a much coarser spatial resolution than the bend fault spacing, the actual condition along each bend fault is not well constrained. As a result, the detailed processes that occur owing to bend faulting, such as the degree of the fracturing and hydration, have been poorly understood.
To better understand the processes occurring along the bend faults, we need to reveal more detailed seismic structural evolution due to bend faulting. Application of the full-waveform inversion (FWI) to the seismic survey data is one promising approach to develop high-resolution seismic velocity structure models. We have applied the FWI to the existing seismic survey data collected using 6-km-spacing OBSs and confirmed that 6-km-spacing data are capable of estimating Vp structure where
seismic structure does not show significant lateral variations, but is not sufficient for estimating a detailed Vp model where lateral structural variations are complicated like the trench-outer trench area. In short, we need a denser OBS array to reveal the detailed seismic structural evolution due to plate bending.
In 2024, we, thus, conducted a new seismic survey using a dense OBS array across the Japan Trench to reveal the detailed seismic structural evolution owing to plate bending-related faulting. Since the water depth in the Japan Trench is very large, we need a large number of ultra-deep OBSs that can operate in the large water depth. Unfortunately, we did not have enough ultra-deep OBSs to cover the entire trench-outer trench area. Therefore, we conducted a new seismic survey along the same line as in 2015, and deployed OBSs between the previous OBSs. In 2015, we deployed OBSs at an interval of 6-km. In 2024, we deployed two OBSs between two OBSs from the 2015 survey and re-shot the airgun array along the same line, obtaining virtually 2-km-spacing OBS array data.
The data obtained were of high quality and we were able to trace first arrivals to a distance of more than 80 km. Using the combined dataset of the 2015 and 2024 seismic surveys, we first applied first arrival tomography and confirmed that we can constrain the P-wave velocity at a depth of the uppermost mantle at the trench axis, suggesting that we will be able to reveal the detailed structural evolution within the oceanic crust and uppermost mantle in the trench outer rise region. In this presentation we will show the collected data and results of the first arrival tomography together with the preliminary results of the full waveform inversion.
To better understand the processes occurring along the bend faults, we need to reveal more detailed seismic structural evolution due to bend faulting. Application of the full-waveform inversion (FWI) to the seismic survey data is one promising approach to develop high-resolution seismic velocity structure models. We have applied the FWI to the existing seismic survey data collected using 6-km-spacing OBSs and confirmed that 6-km-spacing data are capable of estimating Vp structure where
seismic structure does not show significant lateral variations, but is not sufficient for estimating a detailed Vp model where lateral structural variations are complicated like the trench-outer trench area. In short, we need a denser OBS array to reveal the detailed seismic structural evolution due to plate bending.
In 2024, we, thus, conducted a new seismic survey using a dense OBS array across the Japan Trench to reveal the detailed seismic structural evolution owing to plate bending-related faulting. Since the water depth in the Japan Trench is very large, we need a large number of ultra-deep OBSs that can operate in the large water depth. Unfortunately, we did not have enough ultra-deep OBSs to cover the entire trench-outer trench area. Therefore, we conducted a new seismic survey along the same line as in 2015, and deployed OBSs between the previous OBSs. In 2015, we deployed OBSs at an interval of 6-km. In 2024, we deployed two OBSs between two OBSs from the 2015 survey and re-shot the airgun array along the same line, obtaining virtually 2-km-spacing OBS array data.
The data obtained were of high quality and we were able to trace first arrivals to a distance of more than 80 km. Using the combined dataset of the 2015 and 2024 seismic surveys, we first applied first arrival tomography and confirmed that we can constrain the P-wave velocity at a depth of the uppermost mantle at the trench axis, suggesting that we will be able to reveal the detailed structural evolution within the oceanic crust and uppermost mantle in the trench outer rise region. In this presentation we will show the collected data and results of the first arrival tomography together with the preliminary results of the full waveform inversion.