[S-SS13] Active faults and paleoseismology

[SSS13-P16] Active thrusting inferred from geomorphic analysis and terrestrial in situ cosmogenic nuclide dating of marine terraces: a case study from the northeastern coast of Sea of Japan

*MA Bowen¹, Tatsuya Ishiyama¹, Sachi Wakasa² (1.Earthquake Research Institute, University of Tokyo, 2.Institute of Regional Innovation(IRI), HIROSAKI University)

Northeast (NE) Japan is one of the most active convergence zones in the world where deformed marine terraces are well developed and potentially record long-term permanent deformation data of the overriding plate above subducting Pacific plate. Based on the seismic reflection profiles and boreholes, previous studies revealed the existence of multiple west-trending thrust faults in Fuka’ura (Sato et al., 2021). However, the structural relationship between uplifted marine terraces and underlying thrusts remains unclear. In this study, we estimated the ages of marine terraces to compare patterns of uplifted rates with previous seismic interpretations to understand linkages between deformed terraces and underlying thrusts. Four pairs of uplifted marine terraces with former shorelines ranging in elevation from 25 ± 2 m to 170 ± 4 m above m.s.l. have been identified and mapped using topographic maps (at a scale of 1:2,5000), stereo pairs interpretations, and 2 to 5 m grid Digital Elevation Models (DEMs). We also carried out terrestrial in-situ cosmogenic nuclide ¹⁰Be dating to estimate exposure ages of strath surfaces of middle marine terraces; three samples (Yu-1, Yu-2, and Yu-3) were collected from outcrops comprised of Miocene quartz-rich rhyolite at the Yukiaizaki site in Fuka’ura. Estimated exposure ages of 85,476 1184 yr for Yu-1, 79,941 1139 yr for Yu-2, and 77,502 1867 yr for Yu-3 considering with 0.002 mm yr^-1 erosion rates, which were correlated with Marine Isotope stage (MIS) 5a. Long-term uplift rates ranging from 0.5 0.1 mm yr ^-1 to 0.8 0.1 mm yr^-1 have been suggested using eustatic sea level for the study area. MIS5c, MIS5e, and MIS7 marine terraces distributed along the western margin of the Tsugaru Plain correspond to the hanging wall of the fault, indicating that fault movement continued into the Late Quaternary. The patterns of estimated discrete uplift rates across thrust faults are mostly consistent with deformed Tertiary strata in the seismic profile, indicating that spatial patterns of these marine terraces are associated with active thrusting.