10:45 〜 12:15
[SEM15-P12] Viscous remanent magnetization unveils hidden tsunami record of boulders on Itogahama beach in Beppu Bay
キーワード：津波石、残留磁気
Huge boulders are prominent geomorphic features in various landscapes. Tracking their reworking history is important for reconstructing past geologic dynamics (e.g., tsunamis); however, the reworking movements of the boulders remain unresolved. Paleomagnetic information has been used as a marker of such rock displacement. The viscous remanent magnetization (VRM) can be used to reconstruct the multiple geological events that results in significant movements of a rock. If a magnetic-mineral bearing rock is reoriented in the geomagnetic field, the magnetism of the smaller magnetic grains aligns to the field direction with time. This phenomenon is well known as Neel’s relaxation theory. There are hundreds of andesite boulders on Itogahama beach along the north coast of Beppu Bay. The mineral composition of the boulders is consistent with the Akisada pyroclastic flow deposit (0.43 Ma) that crops out just behind of the tidal flat, indicating that the boulders were probably fell from the outcrops by erosion. The maximum boulder weight decreases seaward, suggesting that some of the boulders may have been reworked seaward by backwash flows of past tsunami(s) after being fallen from the outcrops. Historical records show that the tsunami height associated with the 1596 CE Keicho-Bungo earthquake (M=7.0±1/4) was 4-5 m around the study site. Moreover, Yamada et al. (2021) identified four tsunami deposits during the past 7300 year sedimentary sequence at a coastal marsh in south coast of Beppu Bay. Therefore, we conducted paleomagnetic analysis to identify the boulders as being of tsunami origin. Although the vector plots of one-inch core samples have shown the two-magnetic components, the demagnetization temperatures of younger magnetic components were higher than 200°C, which indicates the chemical remanent magnetization due to the alteration of pre-existing magnetic minerals. To avoid such difficulties, we conduct the paleomagnetic analysis of single silicate crystal. The silicate host isolates the magnetic inclusions against chemical alteration. The thermal demagnetization result has shown the three magnetic components and the two inflection points are lower than 200°C, indicating the boulder has been moved two times. The youngest VRM component suggests the reorientation of the boulder by paleotsunami. Moreover, we have applied VRM dating protocol to date the reorientation ages of the boulders. Here we report the preliminary results of the paleomagnetic analysis of the boulders.