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Rupture directions and directivity effects of moderate‐sized intraslab and interplate earthquakes off the coast of Fukushima Prefecture were estimated using seafloor S‐net records and onshore K‐NET and KiK‐net records. The azimuthal dependance of the source spectral ratio of each earthquake relative to a reference earthquake observed at the same station was modeled based on a ω−2 model assuming unilateral rupture. The azimuthal dependance of the source spectral ratio observed for most earthquakes reached a maximum at 2–3 Hz and weakened at higher and lower frequencies. The rupture directivity effect contributes to stations located up to the maximum hypocentral distance of 150 km used in this study. All four down‐dip compressional intraslab earthquakes were estimated to rupture toward the up‐dip, i.e., the land. In addition, the six interplate earthquakes were estimated to rupture either up‐dip or in the opposite direction of the strikes, i.e., between south and southeast directions. Only one interplate earthquake was estimated to rupture down‐dip, i.e., toward the land. In the case of intraslab earthquakes, the regions of large radiation patterns and large rupture directivity overlap in the onshore coastal area. Therefore, when only onshore records were used, the estimates of short‐period spectral levels were biased. [ABSTRACT FROM AUTHOR]

Long-term continuous observation of seafloor pressure is effective for detecting seafloor vertical deformations that are associated with transient tectonic phenomena such as slow slip events. Since the aseismic slip event prior to the 2011 Tohoku earthquake, several discoveries have been made on spontaneous slow slip events and various other types of slow earthquake along the Japan and Kuril Trenches. Seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net) is expected to provide invaluable information on slow slip activities via geodetic signals that are detected by pressure observation. This study inspects the quality of the S-net pressure data in view of seafloor geodesy by comparison with records obtained by more than 100 autonomous ocean bottom pressure recorders (OBPRs) deployed along the Japan Trench. OBPRs have long been standard tools in seafloor geodesy, and the data collected are considered a benchmark in terms of quality. Most of the S-net stations showed noise levels that are considerably higher than those of the OBPRs over periods of more than 2 d. We speculate that a strong correlation between pressure and temperature accounts for much of the long-term noise. In this study, the temperature-dependent fluctuation component was estimated by prediction filtering and removed from the original data, leading to a significant reduction in the noise level at 51 stations, which reached levels almost equivalent to those of OBPRs. Although no significant pressure changes have been identified as associated with the 2018 Boso SSE or repeated tremor bursts in the northern Japan Trench thus far, our findings indicate that these stations are sufficiently sensitive to detect slow slip events occurring nearby. [ABSTRACT FROM AUTHOR]

Tsunamis with amplitudes of up to 40 cm, related to the Mw 7.1 normal‐faulting earthquake off Fukushima, Japan, on November 21, 2016, were clearly recorded by a new offshore wide and dense ocean‐bottom pressure gauge network, S‐net, with high azimuthal coverage located closer to the focal area. We processed the S‐net data and found some stations included the tsunami‐irrelevant drift and step signals. We analyzed the S‐net data to infer the tsunami source distribution. A subsidence region with a narrow spatial extent (∼40 km) and a large peak (∼200 cm) was obtained. The other near‐coastal waveforms not used for the inversion analysis were also reproduced very well. Our fault model suggests the maximum stress drop across the fault plane of >∼10 MPa and the average of 4.2 MPa, whereas the shear stress increase along the fault caused by the 2011 Tohoku earthquake was only ∼2 MPa. Past studies have suggested that horizontal compressional stress around this region switched to horizontal extensional stress after the Tohoku earthquake due to its stress perturbation. The present result, however, suggests that the horizontal extensional stress was locally predominant at the shallowest surface around the focal area even before 2011. The present study demonstrates that the S‐net high‐azimuthal‐coverage pressure data provide a significant constraint on the fault modeling, which enables us to discuss the stress regime within the overriding plate at the offshore. Our analysis provides an implication for crustal stress states, which is important for understanding generation mechanisms of intraplate earthquakes. Plain Language Summary: On November 21, 2016 (UTC), a large earthquake occurred within the continental plate off Fukushima, Japan, and a new seafloor tsunami network, S‐net, recorded its tsunamis with much higher azimuthal coverage and with shorter epicentral distance than any of the previous networks. We analyzed the S‐net data to reveal the rupture process of this earthquake. Our result explained all of the S‐net data and the other tsunami network data very well. According to past studies, the continental plate in northeastern Japan was under horizontal compression before the 2011 Tohoku earthquake due to the pushing force by the subducting oceanic plate. However, our rupture modeling result suggested that the plate around the earthquake rupture area was horizontally stretched even before the Tohoku earthquake, so that the off‐Fukushima earthquake occurred. Our study demonstrated that the S‐net, which has high spatial coverage, makes it possible to reveal the rupture model of offshore earthquakes, which was difficult in the past before S‐net became available. The S‐net will also enable us to discuss the impact of the Tohoku earthquake on the crustal stress, which is necessary for understanding the earthquake generation mechanics. Key Points: Tsunamis due to the 2016 off‐Fukushima shallow normal‐faulting earthquake were observed by the S‐net wide and dense pressure gauge networkUse of the near field and the high‐coverage array significantly improved the constraint of the fault modeling of the 2016 earthquakeHorizontal extensional stress predominant even before the 2011 Tohoku earthquake should be the main cause of the earthquake and tsunami [ABSTRACT FROM AUTHOR]

The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) is presently the world's largest network of ocean bottom pressure sensors for real-time tsunami monitoring. This paper analyzes the efficacy of such a vast system in tsunami forecasting through exhaustive synthetic experiments. We consider 1500 hypothetical tsunami scenarios from megathrust earthquakes with magnitudes ranging from Mw 7.7–9.1. We employ a stochastic slip model to emulate heterogeneous slip patterns on specified 240 subfaults over the plate interface of the Japan Trench subduction zone and its vicinity. Subsequently, the associated tsunamis in terms of maximum coastal tsunami heights are evaluated along the 50-m isobath by means of a Green's function summation. To produce tsunami forecasts, we utilize a tsunami inversion from virtually observed waveforms at the S-net stations. Remarkably, forecasts accuracy of approximately 99% can be achieved using tsunami data within an interval of 3 to 5 min after the earthquake (2-min length), owing to the exceedingly dense observation points. Additionally, we apply an optimization technique to determine the optimal combination of stations with respect to earthquake magnitudes. The results show that the minimum requisite number of stations to maintain the accuracy attained by the existing network configuration decreases from 130 to 90 when the earthquake size increases from Mw 7.7 to 9.1. [ABSTRACT FROM AUTHOR]

A new dense and widely distributed tsunami observation network installed off northeast Japan detected millimeter‐scale tsunamis from an Mw 6.0 shallow interplate earthquake on 20 August 2016. Based on the fault model deduced from this data set, we obtained a stress drop of 1.5 MPa for this event, similar to those associated with typical interplate earthquakes. The rupture area was unlikely to overlap with regions where slow earthquakes occur, such as low‐frequency‐tremors and very‐low‐frequency‐earthquakes. The results demonstrated that this new network has dramatically increased the detectability of millimeter‐scale tsunamis. Some near‐source stations were contaminated by large pressure offset signals irrelevant to tsunami, and we must therefore be careful when analyzing these data. Nonetheless, the new array enables estimations of the stress drops of moderate offshore earthquakes and can be used to elucidate the spatial variation of mechanical properties along the plate interface with much higher resolution than previously possible. Plain Language Summary: Tsunamis are generated when an earthquake occurs beneath the seafloor. Far fewer tsunami observations have been recorded from moderate earthquakes than large to giant earthquakes because tsunamis created by moderate earthquakes have been too small to be observed. On 20 August 2016, a moderate earthquake occurred off Sanriku, in northeastern Japan, and a tsunami with a height of less than 1 cm was recorded by a new seafloor tsunami observation network. This network has many tsunami sensors distributed much closer to each other and over a much wider area than any other previous network in the world. Using these data, this study estimated the source location and size, and the slip amount of the 2016 earthquake with higher accuracy, which was impossible to achieve from past observations because they were too far away from the earthquake and the signals were too small. Using this source information, we could estimate the stress drop associated with the earthquake, which is important because the stress drop information deepens our understanding of how and why earthquakes happen. Key Points: Millimeter‐scale tsunamis from an Mw 6.0 earthquake were captured by the S‐net, a new nationwide pressure gauge array off Sanriku, JapanTsunami signals were identified from the pressure data adjacent to the source, which were contaminated by signals irrelevant to tsunamisWe inferred the stress drop of the earthquake from the tsunami data more reliably than could be done from seismogram analysis [ABSTRACT FROM AUTHOR]

National Research Institute for Earth Science and Disaster Resilience (NIED) integrated the land observation networks established since the 1995 Kobe earthquake with the seafloor observation networks established since the 2011 Tohoku earthquake and tsunami as MOWLAS (Monitoring of Waves on Land and Seafloor) in November 2017. The purpose of MOWLAS is to provide comprehensive, accurate, and rapid observation and monitoring of earthquake, tsunami, and volcano events throughout Japan and its offshore areas. MOWLAS data are widely utilized for long-term earthquake forecasting, the monitoring of current seismic activity, seismic and tsunami hazard assessments, earthquake early warning, tsunami warning, and earthquake engineering, as well as earthquake science. Ocean bottom observations provide an extension of observations to areas where no people are living and have the advantage of increasing lead time of earthquake early warning and tsunami warning. The application of recent technology advancements to real-time observations as well as the processing of MOWLAS data has contributed to the direct disaster mitigation of ongoing earthquakes. These observations are fundamental for both science and disaster resilience, and thus it is necessary to continue ceaseless operation and maintenance. [ABSTRACT FROM AUTHOR]