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Slow slip occurs repeatedly in the shallow part of the plate subduction boundary region in the Nankai Trough (Araki et al., 2017). To investigate its activity in wider area, we developed and installed a seafloor fiber-optic strainmeter connected to a submarine cable observation network (DONET) in west of the previous observation network in the borehole in 2019. In January-February, 2022, the seafloor fiber-optic strainmeter observed slow transient extension of about 0.7 μstrain over 3 weeks. Very low-frequency earthquakes (VLFEs) were observed in the vicinity during this period. The fiber-optic strainmeter was able to observe slow step-like changes for many of the VLFEs that occurred near the strainmeter. We observed strain change about 20 n strain for a VLFE of Mw 4.4. The observed strain change polarity and transitions were consistent with those expected from the focal mechanisms of the VLFEs, suggesting the possibility to investigate slip processes of each individual VLFE by seafloor fiber strain record.The strain changes by the local VLFEs were insufficient to account for the total slow 3-week strain change observed by the fiber optic strainmeter. Therefore, we propose that shallow slow slip occurred in the areas where the VLFEs were observed, and that the slow slip may have loaded individual VLFE faults into a burst-like cluster of VLFEs.We further expanded our strainmeter observations by adding two strainmeters in array and in different direction in November 2022, where we anticipate more robust and detailed fault slip analysis will be performed in the next similar slip occurrence., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Marine volcanoes, i.e., both above-surface- and submarine-volcanoes can be threat for human lives and social activities because of not only volcanic eruption itself but also they sometimes excite tsunamis. Long-term continuous monitoring is important to predict the eruptions, and therefore we attempt to measure the seafloor deformation near the volcanic island using an in-situ pressure sensor and some bench marks. The measurement period was to be five years and it is designed that the communication port is available to recover the measurement dataset whenever we access the pressure sensors during pressure calibration. The present target is Izu-Oshima Island, one of the Izu-Ogasawara (Bonin) volcanic chains, categorizing an active volcano. We calibrate the pressure sensor once a year and evaluate the deformation at the point of pressure sensor and the bench marks over time. To perform this concept, firstly we select a pressure sensor indicating small sensor drift based on the laboratory experiment pressurized by a pressure balance. Then, we have installed the pressure sensor at the seafloor near the island in 2021. The first in-situ pressure calibration was conducted in 2022 after one year of the first deployment of the pressure sensor. We recovered the dataset and confirmed the initial sensor drift. The second in-situ pressure calibration will be conducted in May of 2023 and it is not until that the sensor drift can be removed. In the presentation, we introduce the laboratory experiment and the in-situ pressure measurement during the first one-year observation., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

We use ambient noise records obtained by a vertical array of seismometers at borehole and seafloor in the Kumano basin of the Nankai subduction zone, Japan, and examine whether S reflections from deep interfaces can be persistently detected or not. The vertical array contains a geophone at the seafloor and a borehole broadband sensor at a depth of 900 m from the seafloor. Our results show the retrieval of S reflections from the top of high velocity body above the megasplay fault, the megasplay fault itself, and the top of the subducting oceanic crust. The depths of these boundaries are 5–10 km from the sea surface. The arrival time of the S reflection for a polarization direction of N70ºE was faster, indicating that the direction is the fast polarization direction. We also investigated the temporal variation of the amplitudes of the S reflections at a slow polarization direction (N160ºE), and obtained temporal variations in amplitudes of the S reflections from the top of the high velocity body and megasplay fault for 1 year after 2016 off-Mie earthquake (Mw5.9) whose epicenter is ~30 km away from the vertical array. This may be related to fluid migration around these boundaries., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

[CRUISE REPORT] This document is a preliminary report of KS-22-J05 Leg1 on R/V SHINSEI MARU from 2022-06-03 to 2022-06-11 and KS-22-J05 Leg2 on R/V SHINSEI MARU from 2022-06-12 to 2022-06-20.

2011年3月11日に，太平洋プレートと日本列島を乗せた陸側のプレートとの境界で2011年東北地方太平洋沖地震が発生した．この地震は，日本周辺では観測史上最大のマグニチュード9という巨大地震だった．本震発生後には多数の余震が発生するが，大地震発生のメカニズムを解明するためには，正確な余震分布を調べることが重要である．全国の6つの大学と海洋研究開発機構，気象庁気象研究所は，本震発生直後から共同で100台以上の海底地震計を用いて余震観測を行った．2011年6月中旬までのデータから，震源域全体で約3か月間の精度の良い震源分布が得られた．余震の震源の深さは，全体的に陸に近づくにつれて深くなっていた．震源分布からは，本震時に大きくすべったプレート境界では余震活動が低いことがわかった．上盤の陸側プレート内では余震活動が活発で，正断層型と横ずれ型が卓越していた．太平洋プレート内の余震も多くが正断層型か横ずれ型だった．このことから，日本海溝付近の太平洋プレート内の深部と上盤の陸側プレート内では，本震の発生によって応力場が圧縮場から伸張場に変化したことが示唆される．

Observation of micro-earthquakes using Ocean Bottom Seismometers (OBSs) was carried out from May 17 to July 2, 2007 to obtain seismic activities beneath the area off Cape Erimo, Hokkaido. In this neighboring region, large earthquakes with magnitude around 8 occurred at intervals of 40̃60 years. In this study area, clusters of micro-earthquakes were detected immediately before the 2003 Tokachi-oki earthquake (M8.0) by a previous OBS observation (Takanami et al., 2005). The purpose of this study is to investigate the seismic activity after the 2003 Tokachi-oki earthquake by comparing with the results by Takanami et al. (2005). Most of the hypocenters determined in this study are located at shallower depth than those by the Japan Meteorological Agency (JMA). This means that it is difficult to obtain detailed micro-seismicity in offshore area by using land stations only. Therefore, ocean bottom seismographic observations are necessary to detect micro-earthquakes in the region far from the land. According to the depth of the subducting Pacific plate, which is about 10km beneath this study area, it is interpreted that most of these earthquakes occurred at the plate boundary. Seismic activities in this study area were lower than those by Takanami et al. (2005), but some concentrated earthquakes were detected. However, there was no temporal concentration. This suggests that cumulated stress was released by the 2003 Tokachi-oki earthquake and the following post-seismic slip and that the seismicity of this area returns to the steady state.