Low field anisotropy of magnetic susceptibility (AMS) and rock magnetic measurements were applied to sediments deposited by the 2011 Tohoku-oki tsunami to reveal the dynamic of the tsunami run-up and the character of the sedimentation along the Misawa coast, Aomori Prefecture, northern Japan.
In the tsunami deposits studied, elongated, coarse-grained, multi-domain titanomagnetite and magnetite minerals were the carriers of the AMS signal.
Two main types of sedimentary environment are described: a higher energy, tangential stress-dominated environment with imbrication and traction/rolling transportation and a calmer, post-peak wave environment ruled by gravitational stress.
Rapid characteristic changes in the tsunami dynamics are also described. The tsunami began with erosion of the pre-tsunami surface caused by rapidly increasing energy. Bedload features such as ripple stratification were developed by the repeated accelerations and decelerations of the tsunami wave during run-up. The arrival of the peak wave was indicated by high-density flow, "slurry-like" sediments. Following the peak wave, the decreasing energy was marked by a change in magnetic fabric (MF).
The study showed that quite similar tendencies in the character of tsunami dynamics can be identified in tsunami deposits, despite their materials being deposited in currents with quite different energy levels.