[S-CG56] 変動帯ダイナミクス

[SCG56-12] Multi-scale subduction zone deformation history from observations of coastal geomorphology in western Luzon Island, the Philippines

*小森 純希¹、Aron Meltzner^1,2、Noelynna Ramos³ (1.Earth Observatory of Singapore, Nanyang Technological University、2.Asian School of the Environment, Nanyang Technological University 、3.National Institute of Geological Sciences, University of the Philippines)

キーワード：沈み込み帯、マニラトレンチ、海岸段丘、相対的海水準変動、標高数値モデル、マイクロアトール

Prehistoric crustal deformation along subduction zones provides essential information for hazard assessment and for understanding subduction dynamics. Processes and time scales of interest range from abrupt crustal uplift due to paleo-earthquakes to long-term deformation exceeding 100,000 years. However, deformation over different periods is often analyzed using different approaches and is rarely explained using comprehensive and consistent models. We investigated the three types of coastal features along the western coast of Pangasinan, central Luzon Island, northern Philippines: Pleistocene marine terraces, Holocene marine terraces, and coral microatolls. First, the Pleistocene marine terraces in Pangasinan are 50–150 m in elevation and were formed due to the eustatic sea-level changes in the late Pleistocene (80–120 ka). We obtained the elevation distributions of paleo-shoreline angles (the past coastlines) using a 2.5 m grid digital elevation model (DEM) and evaluated the average uplift rate using a marine terrace formation simulation. Second, the Holocene marine terraces, uplifted in the past ~6,000 yr, are also distributed along the western Pangasinan coast and indicate the past abrupt relative sea-level changes due to earthquakes related to the subduction. At last, coral microatolls have the potential preserving annual relative sea-level changes. The vertical growth limit of coral microatolls is precisely limited by the sea surface, and therefore their fossils can be a proxy for the past relative sea-level curve, which can be used to estimate past interseismic vertical deformation rates, information that is not otherwise preserved by marine terraces. Analysis of the Pleistocene terraces suggests an average uplift rate along the western coast of Pangasinan of 0.9–1.3 m/ka, with a peak at Macabuboni, the central coast of western Pangasinan. The uplift pattern observed in the Pleistocene terraces is similar to that for the Holocene terraces, documented in earlier work. This result strongly suggests an irregular geometry along the Manila subduction zone. Future modeling of the subduction zone should consider an irregularity on the plate interface, such as a subducted seamount.