17:15 〜 18:45
[SCG55-P01] マイクロアトール化石の分布から推定されたフィリピン・ルソン島西岸の完新世地殻変動履歴
キーワード：地殻変動、マイクロアトール、マニラトレンチ、完新世相対的海水準変動、古地震
This study conducted a geological survey along the Pangasinan coast, located in the central part of the western coast of Luzon Island, Philippines, aiming to reconstruct past relative sea-level (RSL) changes using coral microatolls. The Manila trench, positioned to the west of Luzon Island, marks a plate boundary where the Sunda block subducts. Despite no historical records of >M8 earthquakes along this plate boundary since 17th century, geodetic observations have revealed a rapid convergence rate of 60-80 mm/year, indicating the potential for great subduction earthquakes. Therefore, the possibility of such events triggering large tsunamis affecting coastlines bordering the South China Sea necessitates a comprehensive investigation into paleoearthquakes in this region. In a previous study, Ramos and Tsutsumi (2010) investigated Holocene marine terraces along the Pangasinan coast, identifying three terrace levels and dating the middle level to be ~2000 yBP.
We surveyed coral microatolls along the western coast of the Pangasinan Peninsula. Microatolls are coral colonies that are valuable proxies for past RSL changes because their annual growth is limited by the lowest tide level. We found groups of fossil microatolls on two beaches in the northern and southern regions of the western Pangasinan coast. The observed elevations of fossil microatolls were classified into two distinct levels, positioned at several tens of centimeters higher than the living corals. The highest points of corals in each group were within 5 cm ranges. Furthermore, the shape of each fossil microatoll tends to be higher towards the outer perimeter, implying a rise in RSL during the period of their existence, spanning multiple decades. These coral distributions are consistent with a sequence of abrupt uplifts and subsequent gradual subsidence. Currently, the exact cause of RSL changes in the corals, whether seismic or not, remains uncertain. Nevertheless, the findings of this study contribute valuable insights into the crustal deformation history along the Manila trench, elucidating earthquake recurrence patterns predating historical records.
Ramos, N. T., Tsutsumi, H. (2010). Tectonophysics, 495, 145–158. https://doi.org/10.1016/j.tecto.2010.08.007
We surveyed coral microatolls along the western coast of the Pangasinan Peninsula. Microatolls are coral colonies that are valuable proxies for past RSL changes because their annual growth is limited by the lowest tide level. We found groups of fossil microatolls on two beaches in the northern and southern regions of the western Pangasinan coast. The observed elevations of fossil microatolls were classified into two distinct levels, positioned at several tens of centimeters higher than the living corals. The highest points of corals in each group were within 5 cm ranges. Furthermore, the shape of each fossil microatoll tends to be higher towards the outer perimeter, implying a rise in RSL during the period of their existence, spanning multiple decades. These coral distributions are consistent with a sequence of abrupt uplifts and subsequent gradual subsidence. Currently, the exact cause of RSL changes in the corals, whether seismic or not, remains uncertain. Nevertheless, the findings of this study contribute valuable insights into the crustal deformation history along the Manila trench, elucidating earthquake recurrence patterns predating historical records.
Ramos, N. T., Tsutsumi, H. (2010). Tectonophysics, 495, 145–158. https://doi.org/10.1016/j.tecto.2010.08.007