1:45 PM - 3:15 PM
[SCG52-P01] Interplate locking state close to the Nankai Trough and Nanseishoto Trench axes
Keywords:Seafloor Crustal Deformation, Nankai Trough, Nanseishoto Trench, Interplate coupling
It is important to understand the current state of interplate locking for the evaluation of megathrust earthquakes and resulting tsunamis. The observations of seafloor crustal deformation immediately above the anticipated source area are a very effective tool for this purpose. In the Nankai Trough, the Cabinet Office of Japan has proposed a model for the area from the trough axis to a plate depth of about 10 km to consider tsunami earthquakes, but observations of seafloor crustal deformation in this area are still insufficient. In the Nanseishoto Trench, the long-term seismic activity assessment was revised in 2022, but the calculation of earthquake probability was postponed due to the lack of data that would contribute to the long-term assessment. On the other hand, the observations of seafloor crustal deformation have revealed a strong locked zone along the Nanseishoto Trench to the south and southeast of Okinawa Island [Tadokoro et al., 2018]. The locked area is consistent with the source area of the 1791 tsunami event [Nakamura and Kinjou, 2013] and is located on the up-dip side of the active short-term slow-slip area [Nishimura, 2014]. However, this result of seafloor crustal deformation measurement alone does not constrain the lateral extent of the locked area.
Therefore, we are continuing to observe seafloor crustal deformation in areas where the depth of the subducting plate along the Nankai Trough and the Nanseishoto Trench is less than 10 km, to clarify the interplate locking state. We have installed a seafloor station not only on the landward side (TCA) but also on the seafloor above the subducting oceanic plate (TOA) in the Nankai Trough area. In the Nanseishoto Trench, we are continuing observations at the ocean area between Okinawa Island and Miyako Island (RKC, RKD), which is southwest of the locked zone described above, and also in the south-eastern part of Okinawa Island (RKB). We have performed 9 observations at TCA (2013-2022), 7 at TOA (2013-2021), 11 at RKB (2011-2022), 3 at RKC (2016-2021), and 3 at RKD (2016-2022).
The obtained site velocities with respect to the Amur Plate (adopting MORVEL [DeMets et al., 2010; 2011]) in the Nankai Trough area are as follows:
TCA: N75+/-25W, 36±15 mm/yr
TOA: N70+/-26W, 50±21 mm/yr
The magnitude of the site velocity at TOA station is roughly consistent with the theoretical velocity estimated from MORVEL model, 58 mm/yr toward N60W, and the magnitude of the site velocity at TCA station is about 60% of the relative motion of the Philippine Sea plate to the Amur plate by MORVEL. This site velocity indicates an interplate coupling ratio of about 70% below the TCA station based on a forward fitting adopting the dislocation model using Okada’s [1985, 1992] code.
In the Nanseishoto Trench, the obtained site velocities relative to a block between Okinawa Island and Miyako Island are as follows:
RKB: N18±7W, 21±7mm/yr
RKC: N127W, 21mm/yr (tentative value)
RKD: N88W, 10mm/yr (tentative value)
The site velocity at RKB station is consistent with the result reported by Tadokoro et al. [2018], suggesting the existence of the locked area to the southeast of Okinawa Island. Although the number of observations at RKC and RKD stations are not large enough to obtain accurate site velocities, the directions of the site velocities (tentative values) estimated from the time series of position are completely different from the direction of the oceanic plate motion, and it appears that the interplate locked zone to the southeast of Okinawa Island is not expanding in the ocean area of RKC and RKD stations (southwest direction).
We used the F3 and F5 solutions of GEONET provided by the Geospatial Information Authority of Japan. This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).
Therefore, we are continuing to observe seafloor crustal deformation in areas where the depth of the subducting plate along the Nankai Trough and the Nanseishoto Trench is less than 10 km, to clarify the interplate locking state. We have installed a seafloor station not only on the landward side (TCA) but also on the seafloor above the subducting oceanic plate (TOA) in the Nankai Trough area. In the Nanseishoto Trench, we are continuing observations at the ocean area between Okinawa Island and Miyako Island (RKC, RKD), which is southwest of the locked zone described above, and also in the south-eastern part of Okinawa Island (RKB). We have performed 9 observations at TCA (2013-2022), 7 at TOA (2013-2021), 11 at RKB (2011-2022), 3 at RKC (2016-2021), and 3 at RKD (2016-2022).
The obtained site velocities with respect to the Amur Plate (adopting MORVEL [DeMets et al., 2010; 2011]) in the Nankai Trough area are as follows:
TCA: N75+/-25W, 36±15 mm/yr
TOA: N70+/-26W, 50±21 mm/yr
The magnitude of the site velocity at TOA station is roughly consistent with the theoretical velocity estimated from MORVEL model, 58 mm/yr toward N60W, and the magnitude of the site velocity at TCA station is about 60% of the relative motion of the Philippine Sea plate to the Amur plate by MORVEL. This site velocity indicates an interplate coupling ratio of about 70% below the TCA station based on a forward fitting adopting the dislocation model using Okada’s [1985, 1992] code.
In the Nanseishoto Trench, the obtained site velocities relative to a block between Okinawa Island and Miyako Island are as follows:
RKB: N18±7W, 21±7mm/yr
RKC: N127W, 21mm/yr (tentative value)
RKD: N88W, 10mm/yr (tentative value)
The site velocity at RKB station is consistent with the result reported by Tadokoro et al. [2018], suggesting the existence of the locked area to the southeast of Okinawa Island. Although the number of observations at RKC and RKD stations are not large enough to obtain accurate site velocities, the directions of the site velocities (tentative values) estimated from the time series of position are completely different from the direction of the oceanic plate motion, and it appears that the interplate locked zone to the southeast of Okinawa Island is not expanding in the ocean area of RKC and RKD stations (southwest direction).
We used the F3 and F5 solutions of GEONET provided by the Geospatial Information Authority of Japan. This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).