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[SCG56-P01] Crustal Movement of Cape Muroto Associated with the Earthquake Cycle at the Nankai Trough: Does the residual uplift exist?
Keywords:Cape Muroto, Nankai earthquake
Cape Muroto is located at the southeastern tip of Shikoku in Japan, facing the Nankai Trough where a large earthquake occurs every 100~150 years. It is known that Muroto Cape uplifts with the earthquake and subsides between the earthquakes. Yoshikawa et al. (1964) considered that coseismic uplifts are not canceled by subsidence between the earthquakes and residual uplifts accumulate over time, forming well-developed marine terraces. On the other hand, Maemoku (2001) studied fossil calcareous assemblages of organisms along the coast of Cape Muroto. He analyzed the height and date of them and considered that the residual uplifts of Nankai earthquakes do not exist but Cape Muroto uplifts by inland earthquakes every 1000~2000 years. In this study, we analyzed the vertical motion of Cape Muroto associated with the 1946 Nankai earthquake by using long-term geodetic data including GNSS, and we discuss the existence of the residual uplifts through the earthquake cycle at the Nankai Trough.
In this study, we used GNSS, leveling, and tidal data. We used GNSS of the F5 solution of GEONET. Tidal data are provided by the Coastal Movement Data Center. We used monthly mean tide records of the Muroto tidal station from 1972 to 2020, which were corrected for the effects of atmospheric pressure and sea state using the method of Kato and Tsumura (1979). We also used annual mean tides at Kochi after 1911. The leveling data was also provided by GSI.
To begin with, we calculate the vertical movement rate using the least square method to the ellipsoidal height of daily GNSS data from July 2003 to December 2021. The subsidence rate is 3~5 mm/yr at Muroto Cape. Then we calculate the velocity of the change of height related to the sea level at Muroto tidal station in 1980-2020 using the least square method, and the velocity is 8.1mm/yr. This value is apparently inconsistent with one of the GNSS data, however, considering the velocity of sea level rise in Japan after 1980 (2.7mm/yr) according to Japan Meteorological Agency (2021), the velocity becomes 5.4mm/yr, consistent with GNSS data. On the other hand, we also calculate the velocity of the relative height change of BM5142 at Muroto with respect to BM5163 at Aki in 1965-2019 using leveling data. The estimated subsidence rate is 6.0mm/yr. We correct the vertical rate (1.9mm/yr) at Aki with GNSS data, then the velocity becomes 4.1mm/yr, consistent with the GNSS result. Therefore, we consider that the interseismic subsidence at Muroto Cape is stable in the long term at about 5mm/yr.
Generally, the crustal motion immediately after a large earthquake is different from the interseismic pattern because of the postseismic deformation. In order to estimate the absolute vertical motion after the 1946 Nankai earthquake, sea level data is necessary for the height reference. The closest tidal station to Muroto Cape at that time was the one in Kochi. Then we estimate the vertical motion at Muroto Cape in 1947-1965 to be a slight uplift of +49mm or +16mm with two different ways to combine annual sea level change at Kochi and leveling data. Assuming 5mm/yr as the average subsidence late of Muroto, we obtain the total subsidence in 100, 120, and 150 years as 50 cm, 60 cm, and 75 cm. With such subsidence, the coseismic uplifts of more than 1 m cannot be completely canceled and the residual uplifts exist. However, the calculated residual uplift rate over the last 3 Nankai earthquakes is too large compared with the present height of the actual marine terraces.
In this study, we used GNSS, leveling, and tidal data. We used GNSS of the F5 solution of GEONET. Tidal data are provided by the Coastal Movement Data Center. We used monthly mean tide records of the Muroto tidal station from 1972 to 2020, which were corrected for the effects of atmospheric pressure and sea state using the method of Kato and Tsumura (1979). We also used annual mean tides at Kochi after 1911. The leveling data was also provided by GSI.
To begin with, we calculate the vertical movement rate using the least square method to the ellipsoidal height of daily GNSS data from July 2003 to December 2021. The subsidence rate is 3~5 mm/yr at Muroto Cape. Then we calculate the velocity of the change of height related to the sea level at Muroto tidal station in 1980-2020 using the least square method, and the velocity is 8.1mm/yr. This value is apparently inconsistent with one of the GNSS data, however, considering the velocity of sea level rise in Japan after 1980 (2.7mm/yr) according to Japan Meteorological Agency (2021), the velocity becomes 5.4mm/yr, consistent with GNSS data. On the other hand, we also calculate the velocity of the relative height change of BM5142 at Muroto with respect to BM5163 at Aki in 1965-2019 using leveling data. The estimated subsidence rate is 6.0mm/yr. We correct the vertical rate (1.9mm/yr) at Aki with GNSS data, then the velocity becomes 4.1mm/yr, consistent with the GNSS result. Therefore, we consider that the interseismic subsidence at Muroto Cape is stable in the long term at about 5mm/yr.
Generally, the crustal motion immediately after a large earthquake is different from the interseismic pattern because of the postseismic deformation. In order to estimate the absolute vertical motion after the 1946 Nankai earthquake, sea level data is necessary for the height reference. The closest tidal station to Muroto Cape at that time was the one in Kochi. Then we estimate the vertical motion at Muroto Cape in 1947-1965 to be a slight uplift of +49mm or +16mm with two different ways to combine annual sea level change at Kochi and leveling data. Assuming 5mm/yr as the average subsidence late of Muroto, we obtain the total subsidence in 100, 120, and 150 years as 50 cm, 60 cm, and 75 cm. With such subsidence, the coseismic uplifts of more than 1 m cannot be completely canceled and the residual uplifts exist. However, the calculated residual uplift rate over the last 3 Nankai earthquakes is too large compared with the present height of the actual marine terraces.