16:15 〜 16:30
[MGI25-08] Paleotsunami records in the remanent magnetization of reworked boulders, Kingdom of Tonga
キーワード：津波石、年代測定、残留磁気、トンガ
Extreme coastline flooding of tsunamis results in the onshore deposition of large boulders. These boulders are important geological evidence, and then, their size, position, and reworked age have been examined at various locations around the world. In addition to such studies, tracking their movement history is important for reconstructing past geologic dynamics. However, challenges remain in determining whether the boulder has undergone repeated reworkings. Although radiometric dating provides single reworked ages of boulders, assuming that the death of attached organisms is synchronous with the reworking, these approaches cannot track multiple movements or intricate transport histories of individual boulders. Paleomagnetic information has been used as a marker for detecting movements (e.g., Sato et al., 2022). When a boulder is reworked and fixed in a new orientation, a subset of magnetic particles gradually acquires a new remanent magnetization aligned in the geomagnetic field direction. Therefore, paleomagnetic investigations of boulders reworked by tsunami events provide important constraints on the frequency and magnitude of tsunami events in areas with repeating events.
There is relatively little evidence of prehistoric tsunamis in the Kingdom of Tonga partly because an adequate survey area of sandy tsunami deposits, such as flat plains, is rare on such small island. The most significant paleo-tsunami evidence is large coral boulders on the western and eastern coastlines of the main island, Tongatapu. Previous studies suggested that the tsunami generated by volcanic activity, such as flank collapse event and caldera-forming eruption, is the origin of the boulders (Frohlich et al., 2009, Lavigne et al., 2021). The timing of boulder transport, as obtained from the comparison of U/Th dating of marine organisms, modeling, and sea level changes, was found to be either within the past 7,000 years or ca. 1.22 × 105 years, rather than during intermediate times when the sea level was 15–120 m below the present level (Frohlich et al., 2009). In contrast, radiocarbon ages of the sediment below the boulders indicate that they were moved during a 15th century event (Lavigne et al., 2021). The discrepancy in the interpretation of the reworked age suffers from significant limitations in radiometric dating techniques applicable to the boulders. Moreover, the paleo-tsunami evidence prior to the latest reworking event in the island has not been reported although the area has experienced tsunamis in historic times. Here, we report the remanent magnetization records in the boulders and establish a chronology of boulder reworkings including the stepwise transport records. Two boulders had a new remanence after the rework, and two other larger boulders lacked new remanence but had a stable remanence component. Both new and stable remanences deviate from the geomagnetic field direction. These observations indicate that the (1) the boulders with a new remanence component were reworked prior to the latest reworking event, which could have reworked all boulders, and (2) magnitude of the latest event was larger than that of an earlier event. The timing of boulder transport, as obtained from the comparison of the age estimation of the new remanence and previously reported radiometric ages, shows that the earlier event occurred between 3,000 years ago and the 15th century. The difference in the minimum tsunami wave height required to move boulders on the eastern and western coasts suggests that a potential source of the earlier tsunami was likely an eruption event due to ubiquitous volcanoes along the Tonga Ridge.
There is relatively little evidence of prehistoric tsunamis in the Kingdom of Tonga partly because an adequate survey area of sandy tsunami deposits, such as flat plains, is rare on such small island. The most significant paleo-tsunami evidence is large coral boulders on the western and eastern coastlines of the main island, Tongatapu. Previous studies suggested that the tsunami generated by volcanic activity, such as flank collapse event and caldera-forming eruption, is the origin of the boulders (Frohlich et al., 2009, Lavigne et al., 2021). The timing of boulder transport, as obtained from the comparison of U/Th dating of marine organisms, modeling, and sea level changes, was found to be either within the past 7,000 years or ca. 1.22 × 105 years, rather than during intermediate times when the sea level was 15–120 m below the present level (Frohlich et al., 2009). In contrast, radiocarbon ages of the sediment below the boulders indicate that they were moved during a 15th century event (Lavigne et al., 2021). The discrepancy in the interpretation of the reworked age suffers from significant limitations in radiometric dating techniques applicable to the boulders. Moreover, the paleo-tsunami evidence prior to the latest reworking event in the island has not been reported although the area has experienced tsunamis in historic times. Here, we report the remanent magnetization records in the boulders and establish a chronology of boulder reworkings including the stepwise transport records. Two boulders had a new remanence after the rework, and two other larger boulders lacked new remanence but had a stable remanence component. Both new and stable remanences deviate from the geomagnetic field direction. These observations indicate that the (1) the boulders with a new remanence component were reworked prior to the latest reworking event, which could have reworked all boulders, and (2) magnitude of the latest event was larger than that of an earlier event. The timing of boulder transport, as obtained from the comparison of the age estimation of the new remanence and previously reported radiometric ages, shows that the earlier event occurred between 3,000 years ago and the 15th century. The difference in the minimum tsunami wave height required to move boulders on the eastern and western coasts suggests that a potential source of the earlier tsunami was likely an eruption event due to ubiquitous volcanoes along the Tonga Ridge.