Tsunamis caused by offshore active faults and their deposits

Abstract This paper reviews the variable features of coastal tsunamis caused by offshore active faults along the eastern margin of the Japan Sea (EMJS) based on the historical records and modern observations of earthquakes and tsunamis. Tsunami deposits discovered along the EMJS coast were also introduced and categorized depending on their sedimentary environments. The offshore active faults, which are densely distributed along the EMJS and generate tsunamis, are related to the opening of the Japan Sea back-arc basin during the late Oligocene to Miocene period. The magnitudes of the earthquakes that caused large tsunamis in the EMJS were under 8.0. The size of the fault plane (approximately equal to the tsunami source area) was approximately 10 × 100 and several tens of kilometers in length. The slip distance of the reverse-type faults was less than 20 m. The sedimentary characteristics and depositional processes of tsunami deposits were obtained from deposits in various environments, such as coastal lowlands, marine terraces, coastal lagoons, and inland ponds. Certain unique approaches used to identify tsunami deposits were reviewed, such as gravel layers that intercalate inorganic soils, and a Holocene marine terrace, dinoflagellate cysts, and foraminiferal linings, which are potentially derived from the sea, and chemical features (e.g., the Ca2+/Mg2+ ratio) as indicators of seawater inflow into a coastal lagoon. The spatial distribution and recurrence of earthquakes and tsunamis in the EMJS were found based on previous paleo-tsunami studies. The difference between tsunami deposits caused by offshore active faults and those by megathrust earthquakes only manifests itself in the horizontal and vertical directions, but not in the sedimentary environments and structures. The data thus inferred can be used for the estimation of offshore active fault models and validation of previous tsunami disaster reconstructions in the future.