9:15 AM - 9:30 AM
[SGD02-02] Uplift history of Kikai-jima considering effects of marine terrace processes and viscoelastic response
Keywords:Kikai-jima, Marine Terrace, Crustal Movement, Viscoelastic Response, Raised Coral Reef
1. Introduction
In the region of the Nansei Islands, huge earthquakes will occur along the subduction zone of the Philippine Sea Plate. However, it is difficult to evaluate the seismic potential in this region because there is no clear evidence of earthquakes. Then, previous studies pointed out the Holocene marine terrace in Kikai-jima as evidence of earthquake. Many researchers have discussed sea-level change, the formation age of each terrace, and so on based on radiometric ages of coral fossils on this marine terrace in this region. In contrast, the vertical movement of tectonic history at Kikai-jima is complicated because it is near the trench. The formation process of marine terrace by corals is expected to be closely related to the tectonic movement of the island. In this study, we try to reproduce the vertical crustal movement history of by simulating four marine terraces in Kikai-jima using a numerical analysis based on seismology and geology.
2. Method
In this study, we make a temporal change of topographic elevation model, considering nearshore processes such as erosion (Storms et al., 2002), sedimentation (Noda et al., 2018), and coral growth (Shikakura, 2014) effects. Moreover, we consider the viscoelastic effect (Ito et al., 2015) considering subsurface structure based on Finite Element Method. We assume that sea level was increased by 11 m between 6 and 8 ka to reach the current level, after which there was no change. Then, the slope of the initial topography is uniformly three degrees. Under these settings, we calculated the spatiotemporal evolution of the topography. Furthermore, we validated the model by comparing topographic change over 8000 years with the current topography.
3. Results & Discussion
This study obtained a crustal deformation model considering a viscoelastic response. Our model includes three parameters in the nearshore process model: the maximum coral growth rate, maximum erosion rate, and landward reach limit of waves. As a result, we successfully reproduce the topography during the transgressive period from 6 to 8 ka, considering coral growth and deposition. In addition, we also reconstruct the seaward deposition during the regression period from 6 ka to the present. However, our model could not reproduce four marine terrace surfaces in the current topography with the parameter settings within the grid search range. It only succeeded in making three marine terrace surfaces.
By analyzing spatiotemporal evolution, we found a once-formed marine terrace surface approaches to near sea level. This process is related subsidence by viscoelastic response after the earthquake. After that, the marine terrace becomes a part of the abrasion platform again by nearshore processes and that is why a marine terrace surface disappear. This suggests that the current topography of marine terrace may not have recorded the past large earthquakes. On the other hand, there is a possibility that we will be able to estimate more realistic parameters by finding conditions under which we obtain four marine terrace surfaces by simulation. Furthermore, these considerations may enable us to estimate viscosity, erosion rates, and so on. In the future, we plan to discuss these conditions and more.
In the region of the Nansei Islands, huge earthquakes will occur along the subduction zone of the Philippine Sea Plate. However, it is difficult to evaluate the seismic potential in this region because there is no clear evidence of earthquakes. Then, previous studies pointed out the Holocene marine terrace in Kikai-jima as evidence of earthquake. Many researchers have discussed sea-level change, the formation age of each terrace, and so on based on radiometric ages of coral fossils on this marine terrace in this region. In contrast, the vertical movement of tectonic history at Kikai-jima is complicated because it is near the trench. The formation process of marine terrace by corals is expected to be closely related to the tectonic movement of the island. In this study, we try to reproduce the vertical crustal movement history of by simulating four marine terraces in Kikai-jima using a numerical analysis based on seismology and geology.
2. Method
In this study, we make a temporal change of topographic elevation model, considering nearshore processes such as erosion (Storms et al., 2002), sedimentation (Noda et al., 2018), and coral growth (Shikakura, 2014) effects. Moreover, we consider the viscoelastic effect (Ito et al., 2015) considering subsurface structure based on Finite Element Method. We assume that sea level was increased by 11 m between 6 and 8 ka to reach the current level, after which there was no change. Then, the slope of the initial topography is uniformly three degrees. Under these settings, we calculated the spatiotemporal evolution of the topography. Furthermore, we validated the model by comparing topographic change over 8000 years with the current topography.
3. Results & Discussion
This study obtained a crustal deformation model considering a viscoelastic response. Our model includes three parameters in the nearshore process model: the maximum coral growth rate, maximum erosion rate, and landward reach limit of waves. As a result, we successfully reproduce the topography during the transgressive period from 6 to 8 ka, considering coral growth and deposition. In addition, we also reconstruct the seaward deposition during the regression period from 6 ka to the present. However, our model could not reproduce four marine terrace surfaces in the current topography with the parameter settings within the grid search range. It only succeeded in making three marine terrace surfaces.
By analyzing spatiotemporal evolution, we found a once-formed marine terrace surface approaches to near sea level. This process is related subsidence by viscoelastic response after the earthquake. After that, the marine terrace becomes a part of the abrasion platform again by nearshore processes and that is why a marine terrace surface disappear. This suggests that the current topography of marine terrace may not have recorded the past large earthquakes. On the other hand, there is a possibility that we will be able to estimate more realistic parameters by finding conditions under which we obtain four marine terrace surfaces by simulation. Furthermore, these considerations may enable us to estimate viscosity, erosion rates, and so on. In the future, we plan to discuss these conditions and more.