Progress and remaining problems in subduction paleoseismology along the Pacific coast of northeast Japan

<p>In the last two decades, tsunami geology in northeast Japan (Hokkaido and Tohoku) has focused on extending the record of tsunamis beyond the range of historical documents in the region. In Hokkaido facing to southern Kuril trench, recurrent sandy deposits interbedded with peat are regarded as evidence of historical and prehistoric tsunamis. Distribution of one of the sand layers just below a historic tephra (Ko-c2; 1694 CE), so-called 17th-century tsunami deposit, exceeds historical and recent tsunami inundations in eastern Hokkaido. Numerical simulations to reproduce the distributions first suggested a multi-segment fault model with unimodal slip (Mw > 8.4; Nanayama et al., 2003 in Nature), and later with variable slip (Mw > 8.8; Ioki and Tanioka, 2016 in EPSL). Tohoku region, facing to Japan trench, has longer historical record than Hokkaido and the oldest historical earthquake is the Jogan event in 869 CE. Numerical simulations constrained by spatial distributions of the tsunami deposits, coastal submergence, and observation of the 2011 Tohoku tsunami deposit suggest that the 869 event was a plate-boundary rupture at least 200 km long along the Japan Trench (Mw > 8.3–8.6). After the 2011 Tohoku event, a large tsunami in 1454 CE (Kyotoku event) became reexamined and considered to have been generated by a rupture area including the Miyagi-oki region (part of the Jogan rupture). If the 869. 1454, and 2011 events were similar, recurrence of earthquakes in Japan trench is more periodic than southern Kuril trench.   This presentation is based on descriptions and discussion in Sawai (2020) in Earth Science Reviews.  </p>

A study on the interaction between extreme waves and coastal development processes for identification of tsunami and storm deposits in the tsunami far-field

<p>In order to estimate the size and the origin of giant tsunamis, it is useful to investigate “tsunami far-field” as the coastal area far from the source. However, it is challenging to distinguish a tsunami deposit from an extreme storm deposit in these areas. In this study, we report sand layers induced by extreme waves on the coast of Hokkaido, Japan, facing the southern Kuril Trench. In the study area (central part of the Hidaka coast), it is said that the tsunamis caused by observed earthquakes have never exceeded the dune or beach. However, geological evidence indicates that giant earthquakes and tsunamis occurred at intervals of several hundred years in the Kuril Trench, and the traces of these tsunamis are still unclear in the Hidaka region.</p><p>The study area can be classified into the inland zone consisting of peatland and the seaward zone consisting of floodplain muds by the paleo beach ridge. We identified three volcanic ash layers and three to four sand layers with clear boundaries to the ordinary mud layer in each zone. However, there are large gaps in the ages of the sand layers discovered in both inland and seaward zones, and their distributions are limited (ranging from a few tens to 100 m from the ridge at that time) and do not overlap. To understand the peculiarities of the depositional age and distribution of the sand layers, we clarified the sedimentary environmental changes and sea-level index in the late Holocene by analyzing the diatom assemblage and CNS of the mud layer. The inland zone showed the paleoenvironments from the sandy tidal flat formed by the transgression in the mid-Holocene to the beach ridge formed by the regression, and the sheet sand layers were formed only during the period of the beach ridge development. On the other hand, the seaward zone showed various changes due to the formation of meandering rivers and beach ridges associated with the regression, and the formation of recognizable event layers is accompanied by changes in the depositional environment, such as the opening of lagoons and rapid changes to upland. Thus, especially in the tsunami far-field, the preservation potential of the event layers is strongly influenced by the coastal development and relative sea level, and such geological information will provide clues to identify the origin of the sand layer. In the presentation, the numerical simulation of the paleo-tsunami considering the reconstructed relative sea-level change and topographic development will be reported.</p>

Evaluation for hypocenter estimation error in the southwestern Kuril trench using Japan and Russia joint seismic data

The southwestern Kuril trench is seismically active due to the subduction of the Pacific plate. Great earthquakes in this zone have frequently induced fatal disasters. Seismic monitoring and hypocenter catalogues provide fundamental information on earthquake occurrence and disaster mitigation. Real-time hypocenter and magnitude estimates are extremely crucial data for tsunami warning systems. However, this region is located in the international border zone between Japan and Russia. The Japan Meteorological Agency and Russian Academy of Sciences have routinely determined hypocenters and issued earthquake information independently. Waveform data have not yet been exchanged internationally in real time. Here, we evaluated how a hypothetical Japan-Russia joint seismic network could potentially improve the hypocenter estimation accuracy. Experiments using numerical and observed data indicated that the joint network extended the distance over which hypocenters can be accurately determined over 100 km eastward compared to the Japan network only. This fact suggests that joint seismic data have the potential to improve the hypocenter accuracy in this region, which would provide improved performance in gathering disaster information at the moment of a tsunami warning.

Rediscovery and redescription of the abyssal squat lobster Munidopsis petalorhyncha Baba, 2005 (Crustacea: Decapoda: Munidopsidae) from the Northwest Pacific

A poorly known abyssal squat lobster, Munidopsis petalorhyncha Baba, 2005 [= Munidopsis subsquamosa latimana Birstein & Zarenkov, 1970], is redescribed on the basis of the holotype from the Kuril Trench at depths of 5060–5130 m and two recently collected specimens from off Japan at depths of 5353–5382 m. Examination of those specimens enabled us to reassess diagnostic characters of the species. Munidopsis petalorhyncha appears closest to M. thieli Türkay, 1975, known from abyssal plain of the eastern Atlantic, but the spinulose lateral margins of the rostrum, the presence of a distinct lateral eye spine, the presence of a distomesial spine on article 3 of the antennal peduncle and spinose ventral (flexor) margins of meri of pereopods 2–4 distinguish M. petalorhyncha from M. thieli.