17:15 〜 18:45
[SCG52-P03] Revealing the details of oceanic plate evolution prior to subduction
- a new data acquisition and future direction -
キーワード：海洋プレート、アウターライズ断層、日本海溝、地震探査、波形インバージョン
In the last two decades, the evolution of the oceanic plate just prior to subduction by plate bending has been confirmed at several subduction zones, such as the Japan Trench, Kuril Trench, Central America Trench, South America Trench, Alaska Trench, Aleutian Trench, and Tonga Trench, as the reduction of the seismic velocities near the trench axis. The reduction of the seismic velocities is generally thought to be caused by fracturing of the oceanic plate due to repeated bend faulting (i.e., the outer rise normal fault earthquakes) and fluid penetration through the bend faults and possible hydration there.
The degree of the hydration of the subducting oceanic plate determines the amount of water transported from the Earth's surface to its interior and affects various activities such as seismic activities, arc magmatism and controls the evolution of the island arc. However, in the preceding studies, the seismic velocity around the bend faults was estimated by traveltime tomography and the spatial resolution of the obtained velocity model is much larger than the spacing of the bend faults. As a result, the details of the structural impacts caused by bend faulting have been poorly constrained and it has been difficult to discuss quantitatively the process of the water infiltration and the degree of hydration of the oceanic plate.
To address these issues, it is essential to dramatically improve the spatial resolution of the seismic velocity structure in the area of plate bending. One approach that is expected to be effective is the full-waveform inversion (FWI) of the controlled-source seismic survey data. Because FWI can use all of the information in the observed data, including traveltime, amplitude, and phase, it has the potential to extract much more information from the subsurface structure than the traveltime tomography, which uses traveltime alone. In fact, we have recently succeeded in applying the FWI to the wide-angle seismic data obtained on the oceanic plate in the northwestern part of the Pacific plate where the bend fault has not been developed yet and confirmed that the FWI has a potential to reveal the detailed seismic velocities, such as the velocity gradient changes within the oceanic crust, without any a priori information.
To reveal the detailed seismic velocity changes in the area of the bend fault using FWI, we need densely collected wide-angle (long-offset) controlled-source seismic data. This year, we will conduct a wide-angle seismic survey in the vicinity of the Japan Trench deploying OBSs at a spacing of 2-km aiming for revealing the details of the seismic velocity changes caused by bend faulting. In this presentation, we will show the result of the FWI on the oceanic Pacific plate, introduce our seismic survey plan for this year and discuss the future work on the new seismic data.
The degree of the hydration of the subducting oceanic plate determines the amount of water transported from the Earth's surface to its interior and affects various activities such as seismic activities, arc magmatism and controls the evolution of the island arc. However, in the preceding studies, the seismic velocity around the bend faults was estimated by traveltime tomography and the spatial resolution of the obtained velocity model is much larger than the spacing of the bend faults. As a result, the details of the structural impacts caused by bend faulting have been poorly constrained and it has been difficult to discuss quantitatively the process of the water infiltration and the degree of hydration of the oceanic plate.
To address these issues, it is essential to dramatically improve the spatial resolution of the seismic velocity structure in the area of plate bending. One approach that is expected to be effective is the full-waveform inversion (FWI) of the controlled-source seismic survey data. Because FWI can use all of the information in the observed data, including traveltime, amplitude, and phase, it has the potential to extract much more information from the subsurface structure than the traveltime tomography, which uses traveltime alone. In fact, we have recently succeeded in applying the FWI to the wide-angle seismic data obtained on the oceanic plate in the northwestern part of the Pacific plate where the bend fault has not been developed yet and confirmed that the FWI has a potential to reveal the detailed seismic velocities, such as the velocity gradient changes within the oceanic crust, without any a priori information.
To reveal the detailed seismic velocity changes in the area of the bend fault using FWI, we need densely collected wide-angle (long-offset) controlled-source seismic data. This year, we will conduct a wide-angle seismic survey in the vicinity of the Japan Trench deploying OBSs at a spacing of 2-km aiming for revealing the details of the seismic velocity changes caused by bend faulting. In this presentation, we will show the result of the FWI on the oceanic Pacific plate, introduce our seismic survey plan for this year and discuss the future work on the new seismic data.