1:45 PM - 2:15 PM
[MIS15-01] Sedimentation and erosion by the 2011 Tohoku-oki tsunami
★Invited Papers
Sedimentation and erosion by the 2011 Tohoku-oki tsunami were investigated from various aspects not only by sedimentological and micropaleontological ways but also by geochemical and agricultural approaches. Since the possible predecessors including the AD869 Jogan event were well studied before the event, it was also possible to test the validity of the pre-event paleotsunami researches that had been done along the Pacific coast of Tohoku. Also, numerous data about tsunami such as flow depth and inundation area obtained by the efforts of tsunami researchers provided us a rare opportunity to quantitatively compare the tsunami deposit characteristics with flow dynamics. In these points, paleotsunami research after the 2011 Tohoku-oki event has drastically been changed both in its quality and quantity. Then, what did we actually learn from the 2011 Tohoku-oki event and how such knowledge has been incorporated in the paleotsunami research?
Lessons learned from the 2011 Tohoku-oki event can be classified into expected and unexpected results. For instance, sedimentary features such as grading and lamination and distribution pattern as well as the sedimentary process were more or less reasonable based on the pre-2011 knowledge since they are similar to those of other modern tsunami deposits formed by, e.g., the 2004 Indian Ocean tsunami. Also, sedimentary characteristics of the 2011 tsunami deposits were remarkably similar to that formed by paleotsunamis in this region so that identification procedures used in the pre-2011 paleotsunami researches can be evaluated as valid. As expected, diverse size of sedimentary grains from mud to meter-long boulders were transported depending on the sediment source and size of local tsunami. Considering the fact that over thousand tons of huge blocks were transported by the 2011 tsunami, even few thousand tons of enigmatic boulders reported in the world might be explained by large, but not extremely large, tsunami similar to the 2011 one.
On the other hand, some survey results were somewhat beyond the common knowledge of paleotsunami research at that time. For example, sandy deposits were not necessarily distributed up to the maximum tsunami inundation limit so that estimation of earthquake magnitude from paleotsunami research might have been underestimated. While, muddy deposits or geochemical signature, which had been generally overlooked, were important to appropriately estimate the inundation area from the deposits. Also, tsunami deposits were not necessarily of marine source so that absence of marine microfossils does not indicate the event deposit is not formed by the tsunami. These results commonly indicate that paleotsunami research before the 2011 Tohoku-oki event had a chance of underestimation for the estimation of recurrence interval and tsunami/earthquake size.
Now, ten years since the 2011 event, identification of tsunami deposit and its usage to understand paleotsunami events have been under improvement. Paleotsunami research should expand its research field to overcome the remaining difficulties. For example, scarce of modern examples of other water-related event deposits such as flood or storm deposits prevent us to understand the identification criteria of tsunami deposits. Also, technical improvements and reconsideration of research scheme might be essential for efficient, high-accuracy, and high-resolution estimation of paleotsunami recurrence interval and size.
Lessons learned from the 2011 Tohoku-oki event can be classified into expected and unexpected results. For instance, sedimentary features such as grading and lamination and distribution pattern as well as the sedimentary process were more or less reasonable based on the pre-2011 knowledge since they are similar to those of other modern tsunami deposits formed by, e.g., the 2004 Indian Ocean tsunami. Also, sedimentary characteristics of the 2011 tsunami deposits were remarkably similar to that formed by paleotsunamis in this region so that identification procedures used in the pre-2011 paleotsunami researches can be evaluated as valid. As expected, diverse size of sedimentary grains from mud to meter-long boulders were transported depending on the sediment source and size of local tsunami. Considering the fact that over thousand tons of huge blocks were transported by the 2011 tsunami, even few thousand tons of enigmatic boulders reported in the world might be explained by large, but not extremely large, tsunami similar to the 2011 one.
On the other hand, some survey results were somewhat beyond the common knowledge of paleotsunami research at that time. For example, sandy deposits were not necessarily distributed up to the maximum tsunami inundation limit so that estimation of earthquake magnitude from paleotsunami research might have been underestimated. While, muddy deposits or geochemical signature, which had been generally overlooked, were important to appropriately estimate the inundation area from the deposits. Also, tsunami deposits were not necessarily of marine source so that absence of marine microfossils does not indicate the event deposit is not formed by the tsunami. These results commonly indicate that paleotsunami research before the 2011 Tohoku-oki event had a chance of underestimation for the estimation of recurrence interval and tsunami/earthquake size.
Now, ten years since the 2011 event, identification of tsunami deposit and its usage to understand paleotsunami events have been under improvement. Paleotsunami research should expand its research field to overcome the remaining difficulties. For example, scarce of modern examples of other water-related event deposits such as flood or storm deposits prevent us to understand the identification criteria of tsunami deposits. Also, technical improvements and reconsideration of research scheme might be essential for efficient, high-accuracy, and high-resolution estimation of paleotsunami recurrence interval and size.