5:15 PM - 7:15 PM
[HQR05-P01] Differences in uplift rates between Pleistocene and Holocene terraces over the past 300,000 years
Keywords:Marine terrace, Uplift rate, Sea-level change, Climatic change
Introduction.
It has been believed that if there is no change in plate motion, crustal deformation will continue at a constant rate, and if this is accompanied by seismic activity on active faults, the active faults will periodically generate seismic activity that produces unique earthquakes. This has made it possible to predict future earthquakes by determining past earthquake history and crustal deformation rates. In particular, marine terraces have been considered effective indicators as reference surfaces for clarifying the amount and mode of deformation. Ota (1968) stated that since the old shoreline indicates the past sea level and its location can be certified and its elevation measured with considerable accuracy, crustal deformation after formation can be determined relatively more accurately than using other landforms if the contrast of the old shoreline is correct. Marine terraces were formed by eustatic sea-level changes associated with climatic cooling and warming, and by relative sea-level changes caused by local land uplift and subsidence due to fault movement and other factors. In other words, the former is considered not to produce regional differences, while the latter is considered to produce regional differences due to region-specific movements, and the Pleistocene marine terrace height distribution has been considered to be an indicator of local crustal deformation with accumulated displacement. On the other hand, the Pleistocene marine terrace height distribution has been used for paleoseismic evaluation, considering that the displacement was caused by fault movement. On the other hand, hydroisostasy has been used to explain why Holocene terraces are not in harmony with Pleistocene terraces (Nakada, M. et. al.,1991). In this study, we compared the uplift rates and elevations of Pleistocene terraces and Holocene terraces across Japan, omitting local individual events, and attempted to evaluate the tectonic changes associated with climatic change.
Results and Discussion
The area covered was 79 areas.
Examination of the age of MIS7 detachment and sea level elevation: The average uplift rate was compared for each area, and the variation was large when MIS5e-9 was compared with MIS5e-7. Although the Marine Terrace Atlas assigns a water-leaving age of 214,000 years ago and a sea level elevation of 5 m to MIS7, the high sea level period of MIS7 is long, and there is room to consider other possibilities. Therefore, we examined possible combinations of MIS7 detachment age and sea level altitude. As a result, we found the least variation in the age and sea level of MIS7 when the water release was 240,000 years ago and the sea level was the same as that of the present day. In this study, this age and altitude were used for MIS7. Incidentally, 214,000 years ago, the sea level was 5 m above sea level, but this did not make a significant difference in the later considerations.
Comparing the expected Holocene terrace elevation calculated from the MIS5e-9 uplift rate with the measured one, the measured one is larger in many areas. This indicates that the uplift rate in the Holocene is faster than that in the Pleistocene. The y-intercept of the approximate line obtained from the terrace elevations in MIS5e-9 was y< 0 in many areas. This indicates that the terraces should be uplifted in the future so that y=0 before the end of the postglacial period. These two results indicate that the uplift rate during the postglacial high sea level period was higher than that during the glacial period, and that the higher uplift rate is likely to continue in the future. In other words, they indicate that changes in the activity level of active faults near the coast may be occurring in conjunction with climate change.
It has been believed that if there is no change in plate motion, crustal deformation will continue at a constant rate, and if this is accompanied by seismic activity on active faults, the active faults will periodically generate seismic activity that produces unique earthquakes. This has made it possible to predict future earthquakes by determining past earthquake history and crustal deformation rates. In particular, marine terraces have been considered effective indicators as reference surfaces for clarifying the amount and mode of deformation. Ota (1968) stated that since the old shoreline indicates the past sea level and its location can be certified and its elevation measured with considerable accuracy, crustal deformation after formation can be determined relatively more accurately than using other landforms if the contrast of the old shoreline is correct. Marine terraces were formed by eustatic sea-level changes associated with climatic cooling and warming, and by relative sea-level changes caused by local land uplift and subsidence due to fault movement and other factors. In other words, the former is considered not to produce regional differences, while the latter is considered to produce regional differences due to region-specific movements, and the Pleistocene marine terrace height distribution has been considered to be an indicator of local crustal deformation with accumulated displacement. On the other hand, the Pleistocene marine terrace height distribution has been used for paleoseismic evaluation, considering that the displacement was caused by fault movement. On the other hand, hydroisostasy has been used to explain why Holocene terraces are not in harmony with Pleistocene terraces (Nakada, M. et. al.,1991). In this study, we compared the uplift rates and elevations of Pleistocene terraces and Holocene terraces across Japan, omitting local individual events, and attempted to evaluate the tectonic changes associated with climatic change.
Results and Discussion
The area covered was 79 areas.
Examination of the age of MIS7 detachment and sea level elevation: The average uplift rate was compared for each area, and the variation was large when MIS5e-9 was compared with MIS5e-7. Although the Marine Terrace Atlas assigns a water-leaving age of 214,000 years ago and a sea level elevation of 5 m to MIS7, the high sea level period of MIS7 is long, and there is room to consider other possibilities. Therefore, we examined possible combinations of MIS7 detachment age and sea level altitude. As a result, we found the least variation in the age and sea level of MIS7 when the water release was 240,000 years ago and the sea level was the same as that of the present day. In this study, this age and altitude were used for MIS7. Incidentally, 214,000 years ago, the sea level was 5 m above sea level, but this did not make a significant difference in the later considerations.
Comparing the expected Holocene terrace elevation calculated from the MIS5e-9 uplift rate with the measured one, the measured one is larger in many areas. This indicates that the uplift rate in the Holocene is faster than that in the Pleistocene. The y-intercept of the approximate line obtained from the terrace elevations in MIS5e-9 was y< 0 in many areas. This indicates that the terraces should be uplifted in the future so that y=0 before the end of the postglacial period. These two results indicate that the uplift rate during the postglacial high sea level period was higher than that during the glacial period, and that the higher uplift rate is likely to continue in the future. In other words, they indicate that changes in the activity level of active faults near the coast may be occurring in conjunction with climate change.