2:15 PM - 2:30 PM
[SVC32-03] Observation of Ground Deformation in Aira Caldera Using A Seabed Benchmark System (II)
Keywords:Seabed Benchmark System, Aira caldera
We installed the Seabed Benchmark System (SBB) to establish a method for direct observation of ground deformation at sea area in the Aira Caldera. Here, we report the temporal changes in the observation data obtained by the SBB. The positions of the individual antennas are determined by PPK analysis, and used for the attitude reduction process to calculate the benchmark position. The benchmark positions are averaged on a daily basis.
The daily averaged benchmark positions are within a range of about 2 cm in the horizontal component and about 1 cm in the vertical component over a period of about one month, suggesting that SBB is capable of detecting ground deformation larger than approximately 1 cm in the future. At least three gaps are observed on the benchmark positions over two years. The gap that occurred on August 8, 2024 was about 35 cm for the southwest direction and is considered to be caused by the movement of the anchor block due to the seismic motion of the August 8, 2024 Hyuganada earthquake. These gaps were corrected using the difference in mean positions before and after the occurrence of each gap. The daily benchmark position after the gap correction shows about 5 cm sinking until July 2024 for the vertical component, and a relatively fast sinking rate until December 2023. The horizontal component moved about 5 cm to the southwest during the same period. Such movements can be interpreted as due to natural subsidence and tilt changes of the anchor block.
Comparing with the positional changes of onshore stations near the SBB, the trend of positional changes of the SBB is slightly different from that of the surrounding stations until around July 2024, but after that, the positional changes of the SBB became small and concordant with those of the neighbor stations. There also are similarities between the positional changes of SBB and neighbor stations in spring and summer, which are thought to be seasonal variations. The similarity of the positional changes of the SBB and surrounding stations since August 2024 may indicate that the bottoming condition of the anchor block had stabilized due to its movement caused by the earthquake. Further observations are needed to determine if the anchor movement has become sufficiently small and to quantitatively evaluate the seasonal variation.
This research was funded by the Nuclear Regulation Authority of Japan.
The daily averaged benchmark positions are within a range of about 2 cm in the horizontal component and about 1 cm in the vertical component over a period of about one month, suggesting that SBB is capable of detecting ground deformation larger than approximately 1 cm in the future. At least three gaps are observed on the benchmark positions over two years. The gap that occurred on August 8, 2024 was about 35 cm for the southwest direction and is considered to be caused by the movement of the anchor block due to the seismic motion of the August 8, 2024 Hyuganada earthquake. These gaps were corrected using the difference in mean positions before and after the occurrence of each gap. The daily benchmark position after the gap correction shows about 5 cm sinking until July 2024 for the vertical component, and a relatively fast sinking rate until December 2023. The horizontal component moved about 5 cm to the southwest during the same period. Such movements can be interpreted as due to natural subsidence and tilt changes of the anchor block.
Comparing with the positional changes of onshore stations near the SBB, the trend of positional changes of the SBB is slightly different from that of the surrounding stations until around July 2024, but after that, the positional changes of the SBB became small and concordant with those of the neighbor stations. There also are similarities between the positional changes of SBB and neighbor stations in spring and summer, which are thought to be seasonal variations. The similarity of the positional changes of the SBB and surrounding stations since August 2024 may indicate that the bottoming condition of the anchor block had stabilized due to its movement caused by the earthquake. Further observations are needed to determine if the anchor movement has become sufficiently small and to quantitatively evaluate the seasonal variation.
This research was funded by the Nuclear Regulation Authority of Japan.