11:15 AM - 11:30 AM
[SSS12-03] Construction of seafloor fiber optic strainmeter array for fault slip analysis of shallow slow slip event and very low frequency earthquakes in the Nankai Trough
Keywords:Nankai Trough, slow slip event, very low frequency earthquake, fiber optic, strain, array
Observations in boreholes have revealed that slow slip events (SSE) occur repeatedly in the shallow part of the plate subduction boundary region in the Nankai Trough (Araki et al., 2017). In order to clarify the occurrence of these SSEs in the Nankai Trough in a wide area, it is necessary to develop an observation network in wide area that is sensitive enough to crustal deformation, such as in a borehole. We considered that strain measurement of optical fibers deployed on the seafloor is a promising candidate for such an observation method, and we have developed a 200-m-long seafloor fiber optic strainmeter (Zumberge et al., 2018) that can be connected to a submarine cable observation network (DONET) for long-term continuous observation, and in 2019, the first observatory (2F-S2) was installed.
From January to February 2022, we observed slow extension of about 0.7 micro strain associated with shallow slow slip and changes in seafloor surface strain associated with fault movement from very low-frequency earthquakes (VLFEs) that occurred in the vicinity during this period. Observations of seafloor surface strain variations associated with these fault motions have great power to constrain the mechanism and slip-fault model of slow slip and VLFEs.
In the observation started in 2019, the fiber optic strainmeter was deployed in the subduction direction of the Philippine Sea Plate to detect slow slip on the seafloor, but it is obvious that it is difficult to constrain the slow-slip fault model with strain observations in only one direction at one location. Therefore, in November 2022, we added one 200-m strainmeter to the extension on the opposite direction of the existing strainmeter at 2F-S2 and another 200-m strainmeter in the 60-degree westward direction to form a three-component array seafloor fiber optic strainmeter, which was connected to DONET to start observations. In these new fiber-optic strainmeters, the fiber-optic cable was buried approximately 30 cm below the seafloor. This was intended to mitigate the effects of seafloor temperature changes, which have a large effect on existing strain gauges installed in the seafloor, and to improve the coupling between the optical fiber cable and the seafloor surface.
Observations with arrayed seafloor fiber-optic strainmeters showed that changes in seafloor strain due to ocean tides and infragravity waves were similar in amplitude and phase for strainmeters in the same direction, whereas they were significantly different in amplitude and phase for the 60 degrees west-direction strainmeter. This strongly suggests that the tidal strain response seen at the seafloor is caused by the sedimentary layer structure beneath the seafloor. Long-period seismic records from far-field earthquakes were obtained very clearly for each component (Figure). In the array in the same direction, the component with buried optical fibers had a slightly larger amplitude response, and the observation results for the 60 degrees west-facing component were very different in phase, indicating that this observation is useful for azimuthal analysis of seafloor strain changes This observation is effective for azimuthal analysis of seafloor strain changes.
In conjunction with the deployment of seafloor deep borehole, we plan to install and observe similar fiber-optic strainmeters in a wide area in the Nankai Trough to conduct fault-slip analysis of SSEs and other fault slip phenomena.
From January to February 2022, we observed slow extension of about 0.7 micro strain associated with shallow slow slip and changes in seafloor surface strain associated with fault movement from very low-frequency earthquakes (VLFEs) that occurred in the vicinity during this period. Observations of seafloor surface strain variations associated with these fault motions have great power to constrain the mechanism and slip-fault model of slow slip and VLFEs.
In the observation started in 2019, the fiber optic strainmeter was deployed in the subduction direction of the Philippine Sea Plate to detect slow slip on the seafloor, but it is obvious that it is difficult to constrain the slow-slip fault model with strain observations in only one direction at one location. Therefore, in November 2022, we added one 200-m strainmeter to the extension on the opposite direction of the existing strainmeter at 2F-S2 and another 200-m strainmeter in the 60-degree westward direction to form a three-component array seafloor fiber optic strainmeter, which was connected to DONET to start observations. In these new fiber-optic strainmeters, the fiber-optic cable was buried approximately 30 cm below the seafloor. This was intended to mitigate the effects of seafloor temperature changes, which have a large effect on existing strain gauges installed in the seafloor, and to improve the coupling between the optical fiber cable and the seafloor surface.
Observations with arrayed seafloor fiber-optic strainmeters showed that changes in seafloor strain due to ocean tides and infragravity waves were similar in amplitude and phase for strainmeters in the same direction, whereas they were significantly different in amplitude and phase for the 60 degrees west-direction strainmeter. This strongly suggests that the tidal strain response seen at the seafloor is caused by the sedimentary layer structure beneath the seafloor. Long-period seismic records from far-field earthquakes were obtained very clearly for each component (Figure). In the array in the same direction, the component with buried optical fibers had a slightly larger amplitude response, and the observation results for the 60 degrees west-facing component were very different in phase, indicating that this observation is useful for azimuthal analysis of seafloor strain changes This observation is effective for azimuthal analysis of seafloor strain changes.
In conjunction with the deployment of seafloor deep borehole, we plan to install and observe similar fiber-optic strainmeters in a wide area in the Nankai Trough to conduct fault-slip analysis of SSEs and other fault slip phenomena.