Search

On 15 January 2022, an eruption of Hunga volcano generated an air-coupled tsunami that propagated across the Pacific Ocean. The tsunami reached Japan approximately 7 hr after the eruption and was recorded by coastal and offshore stations. We conducted a retroactive study of tsunami early warning using a data assimilation approach. We assimilated water pressure records of 44 offshore bottom pressure gauges (OBPGs) off the Japanese Islands after the volcanic eruption and forecasted tsunamis on the Shikoku Island and the Kii peninsula. The tsunami waveforms at 12 points of interest (i.e., coastal and near-coast stations) were predicted using a data assimilation approach at 15:00 (UTC) with forecast accuracy of 97%. Moreover, we discussed the effects of air-pressure variations generated by the volcanic eruption. The results showed that air-pressure variations had negligible effect on the assimilation results. This is the first time that data assimilation approach was applied to a nonseismic tsunami event for early warning. Our study indicates that data assimilation approach plays an important role in tsunami hazard mitigation. Early warnings of nonseismogenic tsunamis can be successfully achieved by assimilating offshore water pressure records of OBPGs.

Ocean bottom pressure-gauge (OBP) records play an essential role in seafloor geodesy. Oceanographic fluctuations in OBP data, however, pose as a significant noise source in seafloor transient crustal deformation observations, including slow slip events (SSEs), making it crucial to evaluate them quantitatively. To extract the significant fluctuation phenomena common to multiple observation networks, including oceanographic fluctuations and tectonic signals, we applied principal component analysis (PCA) to the 3-year Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) OBP time series for 40 stations during 2016–2019. PCA could separate several oceanographic signals based on the characteristics of their spatial distributions, although evident transient tectonic signals could not be confirmed from the observed pressure records during this observed period. The spatial distribution of the first four principal components (PCs) reflected the common component, inclined component along sea depth, longitude component, and parabola-like pattern, respectively. By subtracting each PC (in particular, PC2 and PC4) from time series, we were able to significantly reduce the sea depth dependence of OBP records, which has been pointed out in several previous studies and is also evident in this region. We interpreted PC2–4 as the reflection of the strength and meandering of ocean geostrophic currents based on a comparison with the PCs’ spatial distribution of the numerical oceanographic models. In addition, to evaluate the ability of PCA to separate transient tectonic signal from OBP time series, including oceanographic fluctuations, we conducted a synthetic ramp assuming an SSE by rectangular fault and then applied PCA. The assumed synthetic tectonic signal could be separated from the oceanographic signals and included in the principal component independently depending on its amplitude; meaning, the spatial distribution of each PC would change if the amplitude of the synthetic signal was enough to large. We propose a transient event-detection method based on the spatial distribution difference of a specific PC with or without a tectonic signal. We used the normalized inner product (NIP) between these PCs as the indicator of their similarities. This method can detect transient tectonic signals more significantly than the moment-magnitude scale of 5.9 from OBP records.

Ocean bottom pressure-gauge (OBP) records play an important role in seafloor geodesy, but oceanographic fluctuations in OBP data are a major source of noise in seafloor transient crustal deformation observations, including slow slip events (SSEs), so it is important to evaluate them properly. To extract the significant characteristics of the oceanographic fluctuations, we applied principal component analysis (PCA) to the 3-year Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) OBP time series for 40 stations during 2016–2019. PCA can separate several oceanographic signals based on the characteristics of their spatial distributions, although transient tectonic signals could not be clearly confirmed from the observed pressure records. The higher-order modes of the principal component reflected the oceanographic variation along the sea depth, and we interpreted that they were caused by the strength or weakness and meandering of ocean geostrophic currents, based on a comparison to the global ocean model ECCO2 by “Estimating the Circulation and Climate of the Ocean” (ECCO) consortium. In addition, to evaluate the ability of PCA to separate transient crustal deformation from oceanographic fluctuations, we conducted a synthetic test assuming an SSE by rectangular faults. The assumed synthetic tectonic signal can be separated from the oceanographic signals and included in the principal component independently depending on its amplitude. We proposed a transient event-detection method based on the spatial distribution variation of a specific principal component with or without a tectonic signal. This method can detect transient tectonic signals larger than moment-magnitude scale M 5.9 from OBP records.

We conducted seismic tomography for entire Japanese Islands including the Sea of Japan and the Pacific Ocean using arrival times from reflection survey as well as the routine seismic network. We successfully imaged the shallow zone along the Sea of Japan from offshore Yamagata to the Noto Peninsula by using air gun data. An extremely low-V shallow zone is imaged between Sado Island and Noto Peninsula. We also obtained detailed seismic velocity structure beneath the Pacific Ocean at depths of 20–50 km using S-net data. The 2007 Noto Peninsula, the 2007 offshore Chuetsu, and the 2019 offshore Yamagata earthquakes occurred at the boundary between high-Vp and low-Vp zones. The west side of the hypocenter of the 2019 offshore Yamagata earthquake at depths of 10–30 km has high-V corresponding to the Mogami Trough. This high-V zone passes through Awa Island and reaches Sado Basin between Sado Island and Honshu. A major rift zone in the Tohoku Arc extending from the Akita region to the Niigata region along the coast of Sea of Japan corresponds to high-V lower crust and a shallow Moho. Graphical Abstract

SUMMARY The offshore part of the North Anatolian Fault (NAF) beneath the Marmara Sea is a well-known seismic gap for future M > 7 earthquakes in the sense that more than 250 yr have passed since the last major earthquake in the Central Marmara region. Although many studies discussed the seismic potential for the future large earthquake in this region on the basis of historical record, geodetic and geological observations, it is difficult to evaluate the actual situation on the seismic activity and structure along the NAF beneath the Marmara Sea due to the lack of ocean bottom seismic observations. Using ocean bottom seismometer observations, an assessment of the location of possible asperities that could host an expected large earthquake is undertaken based on heterogeneities in the microseismicity distribution and seismic velocity structure. Specifically, seismic tomography and precise hypocentre estimations are conducted using offshore seismic data whose recording period is 11 months. About five times more microearthquakes are detected with respect to events recorded in a land-based catalogue. A comparison with previously published results from offshore observation data suggests that the seismicity pattern had not changed from 2014 September to 2017 May. The location accuracy of microearthquakes is greatly improved from only the land-based earthquake catalogue, particularly for depth direction. There are several aseismic and inactive zones of microearthquake, and the largest one is detected using land-based seismic observation, whereas other zones are newly detected via offshore observations. The obtained velocity model shows a strong lateral contrast, with two changing points. The western changing point corresponds to a segmentation boundary, where the dip angle of the NAF segments changed. High-velocity zones from tomographic images are characterized by low seismicity eastward of the segment boundary. To the east of 28.50°E, the high-velocity zone becomes thicker in the depth direction and is characterized by low seismicity. Although the low seismic activity alone could be interpreted as both strong coupling and fully creeping, the high-velocity features at the same can be concluded that these zones are consist of brittle material and strong coupling. From comparison with other geodetic and seismic studies, we interpret these zones as locked zones that had been ruptured by the past large earthquakes and could be ruptured by future ones. These zones might accumulate strain since the main shock rupture associated with the 1766 May Ms 7.3 earthquake, the latest major earthquake in this region.

We conducted seismic tomography to estimate the seismic velocity structure and to evaluate the spatiotemporal distribution of interplate earthquakes of the Kii Peninsula, central Honshu, Japan, where the Tonankai and Nankai megathrusts are located. Microearthquakes were quantitatively detected by using the data from a cable-type seafloor seismic observation network, completed in 2015. Our velocity model was consistent with the previous 2-D active-source surveys, which reported the areal extent of key structures: a high-velocity zone beneath Cape Shionomisaki, a subducted seamount off Cape Muroto, and the subducted Paleo-Zenith Ridge. The absence of any other subducted seamount with the same or larger spatial scale, than the identified key structures, was confirmed. Our velocity model also revealed that there was not a simple relationship between areas of large coseismic slip or strong interplate coupling and areas of high velocity in the overriding plate. Relocated hypocenters widely ranged from the upper plate to within the slab, while the most active region was attributed to the oceanic crust in the aftershock region of 2004 off-Kii earthquake. Compared with the results from the land-based observation network, the accuracy of the focal depth estimation was substantially improved. Furthermore, we identified the seismic activity in the vicinity of the plate boundary and determined 14 locations for interplate seismicity areas. They were primarily distributed in the range of seismogenic zone temperature (150–350 °C) along the plate boundary and were located outside of the strong interplate coupling zone. Several active areas of interplate earthquakes exhibited clustered activity during the periods of slow-slip events, observed and accompanied with shallow very-low-frequency earthquakes. Thus, regular interplate microearthquakes became active at the plate boundary in the conjunction with slow slip. In summary, as regular earthquakes provide a more accurate source location than slow earthquakes and can detect events of smaller magnitude, monitoring such interplate earthquakes may reveal spatiotemporal variations in the stick–slip conditions on the plate boundary.

The 10 July 1804 Kisakata earthquake occurred offshore Kisakata (Akita, Japan), and widespread felt shaking was reported from Matsumae (Hokkaido) to Ohmi-Hachiman (Shiga Prefecture). The earthquake caused strong ground motions that extensively damaged areas near the epicenter, such as along the coast of Kisakata, and the resultant tsunami caused extensive damage along the coast from Kisakata to Sakata. Furthermore, Kisakata lagoon was uplifted by dislocation during the earthquake, exposing the lagoon floor. Here, we performed a field survey of the uplift distribution based on microtopographic remnants of the former shoreline of Kisakata lagoon and used historical documents to re-evaluate tsunami trace heights. Using ocean-bottom reflection profiles, we estimated a fault model for the earthquake and resultant tsunami. Our model indicates that an average of 5.6 m of slip on the fault (equivalent to an Mw 7.1 earthquake) is required to explain the observed crustal deformation and tsunami height distributions, and back correction of the modeled slip reproduced well the former shoreline of Kisakata lagoon.

We estimated the origin time of the 1854 Ansei–Tokai tsunami from the tsunami waveforms recorded at three tide gauge stations (Astoria, San Francisco, and San Diego) on the west coast of North America. The tsunami signal is apparent in the San Francisco and San Diego records, and the arrival time was 0–1 p.m. Greenwich Mean Time (GMT) on 23 December 1854, whereas the tsunami signal of Astoria is ambiguous, and the arrival time could not be determined from the waveform. The simulated waveforms on the basis of nonlinear dispersive wave theory by assuming an origin time of 0 a.m. GMT on 23 December arrived earlier than the observations. Cross-correlation functions between the observed and simulated waveforms recorded at San Francisco and San Diego showed a time gap between them of approximately 30 min. Based on these results, we concluded that the origin time of the 1854 Ansei–Tokai tsunami was approximately 00:30 a.m. GMT or 09:46 local time on 23 December. Our result is roughly consistent with reports by a Russian frigate anchored in Shimoda Bay, ranging the earthquake between 09:00 and 09:45 and the tsunami between 09:30 and 10:00. The earthquake was also reported in historical Japanese documents ranging from 8 and 10 o’clock in local time.

The spatial variation of azimuthal S-wave phase velocity anisotropies caused by differential horizontal stress along the subducting plate at the Nankai Trough was analyzed to understand the stress ...

The offshore part of the North Anatolian Fault (NAF) beneath the Marmara Sea is a well-known seismic gap for future M > 7 earthquakes in the sense that more than 250 years have passed since the last major earthquake in the Central Marmara region. Here, an assessment on the location of possible asperities to host the expected next large earthquake is done based on the heterogeneities on the seismic velocity structure. Using long-term ocean bottom seismograph (OBS) observation data, seismic tomography and precise hypocenter estimations have been conducted. As a result, about five times more microearthquakes than the events in a land-based catalog has been detected. A comparison with previously published results suggests that the seismicity pattern has not changed during the three years period between Sep. 2014 and Jun. 2017. The obtained velocity model shows strong lateral contrast whose changing points locate at 28.10°E and 28.50°E. The western corner of the area (28.10°E) corresponds to a segmentation boundary where the dip angle of the NAF segments changed. The high velocity zones in the tomographic images are characterized by low seismicity eastward from the segment boundary at 28.10°E. Eastern 28.50°E, the high velocity zone becomes thicker in the depth direction. These zones are interpreted as asperities to be ruptured by the next large earthquake which are possibly accumulating strain since the mainshock rupture associated with the May 1766 Ms7.3 earthquake.

A ship-based seismic survey was conducted close to a fiber-optic submarine cable, and 50 km-long distributed acoustic sensing (DAS) recordings with air-gun shots were obtained for the first time. We examine the acquired DAS dataset together with the co-located hydrophones to investigate the detection capability of underwater acoustic (hydroacoustic) signals. Here, we show the hydroacoustic signals identified by the DAS measurement characterizing in frequency-time space. The DAS measurement can be sensitive for hydroacoustic signals in a frequency range from $$10^{-1}\,\text {Hz}$$ 10 - 1 Hz to a few tens of Hz which is similar to the hydrophones. The observed phases of hydroacoustic signals are coherent within a few kilometers along the submarine cable, suggesting the DAS is suitable for applying correlation analysis using hydroacoustic signals. Although our study suggests that virtual sensor’s self-noise of the present DAS measurement is relatively high compared to the conventional in-situ hydroacoustic sensors above a few Hz, the DAS identifies the ocean microseismic background noise along the entire submarine cable except for some cable sections de-coupled from the seafloor.

The damage and loss of life caused by tsunamis can be reduced by timely warnings, which predict the arrival time and maximum height of tsunamis, to support evacuations and other mitigating actions. We have developed a real-time tsunami prediction system based on data from the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) that has been implemented in some local governments along the Pacific coast of Japan. The system generates estimates of tsunami arrival times and the height, inundation areas, and worst case using selected fault rupture models. The main objective of this paper is to show the possibility of applying the above system for a complicated topography area, and we report a successful application of the system in Sakaide, a city on the Shikoku coast of the Inland Sea, using a simulated great plate-boundary earthquake in the Nankai Trough. The simulated tsunami propagates to Sakaide by complicated routes between several islands. According to calculated tsunami waveforms of 1,506 cases, waveforms of tsunamis propagating to the Inland Sea have a relatively uniform frequency, regardless of the magnitude of the causative event, after running through the narrow straits in the Inland Sea. At the same time, waves are amplified as they pass between the islands of Shodoshima and Shikoku by an interaction with reflected waves. These effects are compatible with this prediction system, and we confirmed that our predicted tsunami is consistent with the final result from a model of a magnitude 9 Nankai Trough earthquake.

To evaluate the crustal structure of a site proposed for International Ocean Discovery Program drilling, the Japan Agency for Marine-Earth Science and Technology carried out seismic surveys in the Izu rear arc between 2006 and 2008, using research vessels Kaiyo and Kairei. High-resolution dense grid surveys, consisting of three kinds of reflection surveys, generated clear seismic profiles, together with a seismic velocity image obtained from a seismic refraction survey. In this paper, we compare the seismic profiles with the geological column obtained from the drilling. Five volcaniclastic sedimentary units were identified in seismic reflection profiles above the 5 km/s and 6 km/s contours of P-wave velocity obtained from the velocity image from the seismic refraction survey. However, some of the unit boundaries interpreted from the seismic images were not recognised in the drilling core, highlighting the difficulties of geological target identification in volcanic regions from seismic images alone. The geological core derived from drilling consisted of seven lithological units (labelled I to VII). Units I to V were aged at 0–9 Ma, and units VI and VII, from 1320–1806.5 m below seafloor (mbsf) had ages from 9 to ~15 Ma. The strong heterogeneity of volcanic sediments beneath the drilling site U1437 was also identified from coherence, calculated using cross-spectral analysis between grid survey lines. Our results suggest that use of a dense grid configuration is important in site surveys for ocean drilling in volcanic rear-arc situations, in order to recognise heterogeneous crustal structure, such as sediments from different origins.

We constructed a real-time tsunami prediction system using the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET). This system predicts the arrival time of a tsunami, the maximum tsunami height, and the inundation area around coastal target points by extracting the proper fault models from 1,506 models based on the principle of tsunami amplification. Since DONET2, installed in the Nankai earthquake rupture zone, was constructed in 2016, it has been used in addition to DONET1 installed in the Tonankai earthquake rupture zone; we revised the system using both DONET1 and DONET2 to improve the accuracy of tsunami prediction. We introduced a few methods to improve the prediction accuracy. One is the selection of proper fault models from the entire set of models considering the estimated direction of the hypocenter using seismic and tsunami data. Another is the dynamic selection of the proper DONET observatories: only DONET observatories located between the prediction point and tsunami source are used for prediction. Last is preparation for the linked occurrence of double tsunamis with a time-lag. We describe the real-time tsunami prediction system using DONET and its implementation for the Shikoku area.

Both the geometry and the depth of the seismogenic zone of the North Anatolian Fault under the Marmara Sea (the Main Marmara Fault (MMF)) are poorly understood, in part because of the fault's undersea location. We recorded 10 months of microseismic data with a dense array of ocean bottom seismographs and then applied double-difference relocation and 3-D tomographic modeling to obtain precise hypocenters on the MMF beneath the central and western Marmara Sea. The hypocenters show distinct lateral changes along the MMF: (1) both the upper and lower crust beneath the Western High are seismically active and the maximum focal depth reaches 26 km; (2) seismic events are confined to the upper crust beneath the region extending from the eastern part of the Central Basin to the Kumburgaz Basin; and (3) the magnitude and direction of dip of the main fault change under the Central Basin, where there is also an abrupt change in the depth of the lower limit of the seismogenic zone. We attribute this change to a segment boundary of the MMF. Our data show that the upper limit of the seismogenic zone corresponds to sedimentary basement. We also identified several seismically inactive regions within the upper crust along the MMF; their spatial extent beneath the Kumburgaz Basin is greater than beneath the Western High. From the comparison with seafloor extensometer data, we consider that these regions might indicate zones of strong coupling that are accumulating stress for release during future large earthquakes.

Around Japan, some oceanfloor network systems were installed, which are dense oceanfloor network system for earthquakes and tsunamis (DONET), seafloor observation network for earthquakes and tsunamis along the Japan trench (S-net), and so on. These systems have seismometers and pressure sensor, respectively, and are indispensable for early detection of earthquakes and tsunamis around huge events with a magnitude of over eight around the subduction zone. We have developed a real-time tsunami prediction system using above oceanfloor network systems and it was already installed on some local governments and private company of the infrastructure. The prediction items are tsunami arrival time, maximum tsunami height and inundation area. The principal of the tsunami prediction system is based on the tsunami amplification. We set total 1,506 models changing a magnitude, depth, and dip along the Nankai trough area from the off the Boso Peninsula to Hyuganada of the off Kyushu, and calculated tsunami waveforms of the DONET observatories and target points of the tsunami prediction near the coast line. Our system has a tsunami database including these calculated waveforms and inundation areas for each target point, and the system extracts the worse cases from the database corresponding to input pressure data observed by DONET. The calculated tsunami waveforms of extracted models are also shown with above predicted information. The accuracy of the tsunami prediction depends on the correlation between the maximum height of the target point and the pressure value observed by DONET. And the correlation is influenced by seafloor topography and shape of the coastal line. When diffracted tsunami waves only propagate to the target point, the predicted tsunami height is relatively small. In cases with the same directions of the bay mouse and the tsunami prediction, the tsunami amplification becomes large. Therefore, it is important to select fault models from the database. We use triggering orders of the seismometers and the pressure data for the selection of models based on the source direction and keep accuracy of the prediction. Considering the simple principle for the prediction, tsunami propagation is one of important elements affecting the accuracy of the prediction. In cases with some complicated propagation routes, the tsunamis traveling different routes are stacked and possible large tsunami is locally generated. Through social implementation of the prediction system, we confirm the effectiveness around the rias coast around the Kii Peninsula, small islands area around the Shima Peninsula, and possible area expected some complicated propagation routes around the inland sea. However, oceanfloor network systems do not cover all seismogenic zones around Japan. For such areas, tentative real-time system using a buoy is effective like the DART system installed in worldwide. Around Japan with very strong current like the Kuroshio, the buoy observation system for tsunami and crustal displacement adopting slack mooring has been developed. The real-time tsunami prediction can be applied for the buoy system. With mobility of the buoy system, the deployment point for the buoy system can be selected as needed for the disaster prevention. We investigated that the tsunami height calculated using above models at a point far from a target point. The tsunami height at the estimated buoy location has strongly correlation for that of the target point.

We detected long-term hydrostatic pressure changes at ocean-bottom stations of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) along the Nankai trough, off southwestern Japan. We detected these changes after removing the contributions of ocean mass variations and sensor drift from the records. In addition, we detected a decrease in the background seismicity rate of a nearby earthquake cluster that was synchronous with the hydrostatic pressure changes. We interpreted these observed hydrostatic pressure changes to reflect vertical deformation of the ocean floor of 3–8 cm, and we consider the cause of the seafloor crustal deformation to be a slow slip event (SSE) beneath the stations. Because the pressure changes were observed at stations with distances less than 20 km to each other, we inferred that the SSE occurred in the shallow part of the sedimentary wedge, such as on a splay fault system. The synchronous observation of an SSE and a seismicity rate change suggests that both were triggered by a change in the regional stress that may be associated with stress accumulation and release processes occurring along the Nankai trough. These data show that continuous and careful monitoring of crustal activities by DONET stations provides an effective way to detect seismic and geodetic signals related to the occurrence of megathrust or other types of large earthquakes.

Mega-thrust earthquakes are anticipated to occur in the Nankai Trough in Southwest Japan. In order to monitor seismicity, crustal deformations, and tsunamis in earthquake source areas, we deployed the seafloor seismic network DONET (Dense Ocean-floor Network System for Earthquakes and Tsunamis) in 2010 (Kaneda et al., 2015; Kawaguchi et al., 2015). The DONET system consists of a total of 20 stations that are composed of multiple types of sensors, including strong-motion seismometers and quartz pressure gauges. These stations are densely distributed at an average distance of 15‐20 km and cover from near the trench axis to coastal areas. Observed data are transferred to a land station through a fiber-optic cable and then to the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) data management center through a private network in real time.After the 2011 earthquake off the Pacific coast of Tohoku, each local government close to the Nankai Trough sought to devise a disaster prevention scheme. These local governments requested that JAMSTEC disseminate the DONET data along with other research capabilities so that they could exploit this important earthquake information. In order to provide local government access to the DONET data, which are recorded ostensibly for research purposes, we have developed a web application system, REIS (real-time earthquake information system), that provides seismic waveform data to some local governments close to the Nankai Trough. In the present paper, we introduce the specifications of REIS and its system architecture.

The vertical structure of a tropical rain forest is complex and multilayered, with strong variation of micro-environment with height up to the canopy. We investigated the relation between morphological traits of leaf surfaces and tree ecological characteristics in a Malaysian tropical rain forest. The shapes and densities of stomata and trichomes on the abaxial leaf surfaces and their relation with leaf characteristics such as leaf area and leaf mass per area (LMA) were studied in 136 tree species in 35 families with different growth forms in the tropical moist forest. Leaf physiological properties were also measured in 50 canopy and emergent species. Most tree species had flat type (40.4 %) or mound type (39.7 %) stomata. In addition, 84 species (61.76 %) in 22 families had trichomes, including those with glandular (17.65 %) and non-glandular trichomes (44.11 %). Most leaf characteristics significantly varied among the growth form types: species in canopy and emergent layers and canopy gap conditions had higher stomatal density, stomatal pore index (SPI), trichome density and LMA than species in understory and subcanopy layers, though the relation of phylogenetically independent contrasts to each characteristic was not statistically significant, except for leaf stomatal density, SPI and LMA. Intrinsic water use efficiency in canopy and emergent tree species with higher trichome densities was greater than in species with lower trichome densities. These results suggest that tree species in tropical rain forests adapt to a spatial difference in their growth forms, which are considerably affected by phylogenetic context, by having different stomatal and trichome shapes and/or densities.

Beneath the Marmara Sea, Turkey, the Main Marmara Fault (MMF), the offshore part of the North Anatolian Fault (NAF), is a well-known seismic gap for future M > 7 earthquakes. However, its detailed fault geometry and microearthquake activity have been debated for several decades. Using data acquired from long-term ocean bottom seismograph (OBS) observations, we made precise hypocenter estimations based on 3-D Vp and Vs velocity structures and assessed the fault geometry beneath the western and central parts of the MMF. The results indicate a segmentation boundary between the near-vertical western part and the south-dipping eastern part located around 28.10°E. Enriched OBS locations indicate microseismicity along both the inner and outer boundary faults of the Central Basin, especially on the western side. A comparison with previously published results suggests that the seismicity pattern has not changed for at least two years, between 2014 and 2016. Using a combined dataset of this and previous studies, lateral variations in the dip angle along the MMF fault segment from 27.4°E to 28.8°E were investigated. Based on this, we depicted on-fault seismicity along the MMF and defined three inactive areas of microseismicity. Two are located in the western segment, corresponding to the rupture area of the 1912 Ms. 7.4 earthquake, and the other, the largest, is located on the eastern segment. From a comparison of previous seismic and geodetic studies, it is considered that this area is a fully locked zone and has the potential for large earthquakes. Having compared the difference between hypocenter locations determined from OBSs and land-based stations, it is proposed that the epicentral locations of the mainshock and aftershocks of the September 26, 2019, M 5.7 earthquake are located much closer to the MMF than locations reported from only land-based results.

Megathrust earthquakes have occurred repeatedly at intervals of 100 to 150 years along the Nankai Trough, situated in the southwest of Japan. Given that it has been 70 years since the last event, the occurrence of the next devastating earthquake is anticipated in the near future. On April 1, 2016, a moderate earthquake (Mw 5.9, MJMA 6.5) occurred off the southeastern coast of Mie Prefecture in the source region of the 1944 Tonankai earthquake (Mw 8.2). In this study, we investigated the influence of the 2016 earthquake on future megathrust earthquakes. We first determined the hypocenter distributions using a precise velocity structure obtained from seismic surveys in the source region. Using data obtained from the DONET ocean-bottom observation network, we found that this earthquake occurred along the plate boundary fault, which is also believed to have slipped during past megathrust earthquakes. We then performed a preliminary numerical simulation to reproduce the occurrence of a moderate earthquake in the middle of a megathrust earthquake cycle. We used a hierarchical asperity model, in which smaller asperities causing moderate earthquakes are embedded in a hyper-asperity that serves as the source region of megathrust earthquakes. The simulation shows that moderate earthquakes, caused by ruptures of a smaller asperity, occur as a result of shrinkage of strongly coupled areas in the hyper-asperity. This result is consistent with the observation that the hypocenter of the 2016 earthquake was located at the edge of a strongly coupled region along the plate boundary. The simulation also reproduced postseismic slip (afterslip/slow slip) along the plate boundary updip of the hyper-asperity, which is consistent with the observations of slow slip events and very-low-frequency earthquakes after the 2016 earthquake. Thus, the occurrence of the moderate earthquake offshore southeastern Mie Prefecture in the middle of the megathrust earthquake cycle implies that the shrinkage of the strongly coupled area along the plate boundary is occurring as a preparatory process for the next megathrust earthquake in the region.

Two caisson installation technique were conducted in the construction of DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis); one is using piston core technique [1], and the other is vibration hammer technique [2] that is developed to install the caisson using ROV. This paper describes the background noise level of seismometers installed by two caisson installation technique are discussed.

We have developed a real-time tsunami prediction system using the dense oceanfloor network system for earthquakes and tsunamis (DONET) to visualize predicted tsunami information around the Nankai coastal area. The principal of the prediction system is based on tsunami amplification according to Green's law. Tsunami waveforms of all DONET observatories and predicted target points are calculated in advance using 1506 fault models along the Nankai Trough area. The possible fastest tsunami arrival time, tsunami height, inundation area, and calculated tsunami waveforms of each target point are shown in real-time. We installed it in some local governments and an infrastructure company. Here we report the logic and strategy for social implementation.

In this research, a hydraulic experiment was carried out in order to optimally shape the surface buoy. In the hydraulic experiment, a rectangular flume was used, and the floating structure of the surface buoy and the shape of the lower structure were changed. The fluid force acting on the surface buoy was measured with a force transducer and its drag coefficient was evaluated. As the result by this experiment, it was found that the float structure has a two-dimensional dear droplet shape, and the lower structure has a surface buoy shape in which the three-dimensional dear droplet shape is optimum.

S-net is the online and real-time seafloor observation network of 150 observatories for earthquakes and tsunamis along the Japan Trench. It covers the focal region of the 2011 off the Pacific coast of Tohoku earthquake and its vicinity regions. It was established to enhance reliability of early warnings of tsunami and earthquake after the occurrence of the earthquake. Full-scale operation of the S-net has started since April 2017. The data from the 150 seafloor observatories are being transferred to the data center at NIED on a real-time basis, and then verification of data integrity are being carried out. Obtained data reveal that the S-net can observe and monitor seismic and tsunami phenomena that had been never covered only by land-based observations.

A large number of shallow low frequency events were recorded after the 2011 Mw 9.0 Tohoku-oki earthquake by the cabled network of broadband ocean bottom seismometers (DONET) deployed in the eastern part of the Nankai trough. This low frequency event activity was intense for the first few days after the great earthquake and gradually decreased. Signals of the events are most clearly visible at the frequency range around 2–8 Hz. Some of the events are accompanied by a very long frequency (VLF) signal, which is clearly observed at around 0.02–0.05 Hz. The magnitude and source duration estimated by waveform analysis for one of the largest very low frequency earthquakes (VLFEs) was 3.0–3.5 and 17 s. This source duration is extremely long compared to ordinary earthquakes of comparable magnitude. These newly detected VLFEs are likely to be normal fault earthquakes located at shallow depths within the accretionary prism, in contrast to the previously reported VLFEs that were explained by a low angle thrusting along the decollement zone. On the other hand, the low frequency events with no clear VLF signal were previously regarded as being low frequency tremors (LFTs). We show that events with and without the VLF signal likely represent the same phenomenon, and the VLF signal is only observed when a large magnitude event occurs near the station. The waveforms of VLFEs are characterized by the coexistence of long source duration and high-frequency radiation of signals, and such features were previously explained by the co-occurrence of shear failure and hydrofractures under the influence of fluid brought into the decollement zone. Our result indicates that the stress state and the mechanical environment, which promote the occurrence of VLFEs, exist not only along the decollement zone but also in the shallower part of the accretionary prism.

We improved the tsunami simulation code JAGURS, which is a paralleled version of URSGA code for a large-scale, high-speed tsunami prediction in the Nankai trough, Japan. We optimized the loop kernel for velocity update and intergrid communication on a three-dimensional torus network. Linear scaling was achieved up to the full system capability of the K computer (82,944 nodes) in a strong scaling test that used 100 billion finite-difference grid points. The measured performance on the K computer was 1.2 petaflops (11.5% of peak speed). Intergrid communication was optimized for a three-nested-grid model consisting of 0.68 billion grid points. Grid spacing in the area with the finest grid (180 km × 120 km) was about 5 m. We successfully implemented a large-scale tsunami simulation using this model that ran in about 30% of real time. We believe that this is the fastest tsunami prediction achieved to date with such a large-scale model. Our code can provide high-resolution tsunami prediction for broad regions within a reasonable time to assist emergency rescue and relief operations during future devastating tsunamis comparable to the 2004 Sumatra, 2010 Chile, and 2011 Tohoku tsunamis.

Because of improvements in offshore tsunami observation technology, dispersion phenomena during tsunami propagation have often been observed in recent tsunamis, for example the 2004 Indian Ocean and 2011 Tohoku tsunamis. The dispersive propagation of tsunamis can be simulated by use of the Boussinesq model, but the model demands many computational resources. However, rapid progress has been made in parallel computing technology. In this study, we investigated a parallelized approach for dispersive tsunami wave modeling. Our new parallel software solves the nonlinear Boussinesq dispersive equations in spherical coordinates. A variable nested algorithm was used to increase spatial resolution in the target region. The software can also be used to predict tsunami inundation on land. We used the dispersive tsunami model to simulate the 2011 Tohoku earthquake on the Supercomputer K. Good agreement was apparent between the dispersive wave model results and the tsunami waveforms observed offshore. The finest bathymetric grid interval was 2/9 arcsec (approx. 5 m) along longitude and latitude lines. Use of this grid simulated tsunami soliton fission near the Sendai coast. Incorporating the three-dimensional shape of buildings and structures led to improved modeling of tsunami inundation.

A novel enzyme, which catalyzed decarboxylation of l -lysine into cadaverine with release of carbon dioxide and oxidative deamination of l -lysine into l -2-aminoadipic 5-semialdehyde with release of ammonia and hydrogen peroxide, was found from a newly isolated Burkholderia sp. AIU 395. The enzyme was specific to l -lysine and did not exhibit enzyme activities for other l -amino acids, l -lysine derivatives, d -amino acids, and amines. The apparent K m values for l -lysine in the oxidation and decarboxylation reactions were estimated to be 0.44 mM and 0.84 mM, respectively. The molecular mass was estimated to be 150 kDa, which was composed of two identical subunits with molecular mass of 76.5 kDa. The enzyme contained one mol of pyridoxal 5′-phosphate per subunit as a prosthetic group. The enzyme exhibiting decarboxylase and oxidase activities for l -lysine was first reported here, while the deduced amino acid sequence was homologous to that of putative lysine decarboxylases from the genus Burkholderia .

Bacterial strain NYT501, which we previously isolated from soil, was identified as Stenotrophomonas maltophilia, and it was confirmed that this strain produces an intracellular β-N-acetylhexosaminidase exhibiting transglycosylation activity. Several properties of this enzyme were characterized using a partially purified enzyme preparation. Using N,N′-diacetylchitobiose (GlcNAc)2 and N,N′,N″-triacetylchitotriose (GlcNAc)3 as substrates and dried cells of this bacterium as a whole-cell catalyst, chitin oligosaccharides of higher degrees of polymerization were synthesized. (GlcNAc)3 was generated from (GlcNAc)2 as the major transglycosylation product, and a certain amount of purified sample of the trisaccharide was obtained. By contrast, in the case of the reaction using (GlcNAc)3 as a substrate, the yield of higher-degree polymerization oligosaccharides was comparatively low.

We have developed a new system for real-time observation of tsunamis and crustal deformation using a seafloor pressure sensor, an array of seafloor transponders and a Precise Point Positioning (PPP ) system on a buoy. The seafloor pressure sensor and the PPP system detect tsunamis, and the pressure sensor and the transponder array measure crustal deformation. The system is designed to be capable of detecting tsunami and vertical crustal deformation of ±8 m with a resolution of less than 5 mm. A noteworthy innovation in our system is its resistance to disturbance by strong ocean currents. Seismogenic zones near Japan lie in areas of strong currents like the Kuroshio, which reaches speeds of approximately 5.5 kt (2.8 m/s) around the Nankai Trough. Our techniques include slack mooring and new acoustic transmission methods using double pulses for sending tsunami data. The slack ratio can be specified for the environment of the deployment location. We can adjust slack ratios, rope lengths, anchor weights and buoy sizes to control the ability of the buoy system to maintain freeboard. The measured pressure data is converted to time difference of a double pulse and this simple method is effective to save battery to transmit data. The time difference of the double pulse has error due to move of the buoy and fluctuation of the seawater environment. We set a wire-end station 1,000 m beneath the buoy to minimize the error. The crustal deformation data is measured by acoustic ranging between the buoy and six transponders on the seafloor. All pressure and crustal deformation data are sent to land station in real-time using iridium communication.

From 2011 to 2012, we conducted marine seismic surveys using a multichannel seismic reflection (MCS) system and ocean bottom seismometers (OBSs) in and around the hypocentral region of the 1983 Nihonkai–Chubu earthquake. The survey covered the area from the Japan coast of the Japan Sea to the Yamato Basin and the Japan Basin. We investigated the relationship between the Nihonkai–Chubu earthquake and the crustal structure obtained from seismic reflection imaging and the P-wave velocity structure near the hypocentral region. The focus in the present paper is on seismic data that were acquired using both the MCS system and OBSs. Based on the results, the crustal structure of the Japan Sea around the hypocentral region of the 1983 Nihonkai–Chubu earthquake is divided into three types: island arc crust, thick oceanic crust, and oceanic crust. The Nihonkai–Chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust. There is a strong possibility that the east-dipping reflector detected in the crust was the source fault of the Nihonkai–Chubu earthquake. In addition, the location of anticlines formed by the east-dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the Nihonkai–Chubu earthquake was confirmed in the range of latitude from 39 ° 20 ′ to 41 ° 15 ′ N , although they were not found south of this segment. There is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the Nihonkai–Chubu earthquake. This area is located near the boundary between the thick oceanic crust and the oceanic crust.

At oceanic spreading centres, it is unclear whether plate motions drag the underlying mantle, or mantle flow pulls the overlying plates. Seismic imaging of a former speading centre in the Pacific Plate reveals strong seismic anisotropy that was generated by mantle flowing at speeds greater than plate motions, implying that mantle flow pulled this part of the plate.

To clarify the formation process of the back-arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa-shima Island. The crust beneath the northern Yamato Basin off Awa-shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P wave velocity of 5.4–6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high-velocity lowermost crust in the northern Yamato Basin with 7.2–7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25–26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.