Ryosuke Azuma

• Doctor of Science
• Tohoku University

The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M> 8 recur along the trench. The seismic gap between the source areas of the 1973 Nemuro-oki and 2003 Tokachi-oki earthquakes, which are typical underthrust earthquakes faulting with rupture velocities of ˜3 km/s, has been ruptured by the 195...

BackgroundA GPS/acoustic (GPS/A) geodetic observation technique allows us to determine far offshore plate motion in order to understand the mechanism of megathrust earthquakes. In this technique, the distance between a sea-surface platform and seafloor transponders is estimated using the two-way traveltimes (TWT) of acoustic signals. TWTs are deter...

The 2011 Tohoku-oki earthquake occurred with a rupture length of 500 km along the Japan Trench, causing a large slip (>30 m) at the shallowest portion of the plate boundary fault south of 39°N off Miyagi and a smaller slip (~10 m) north of 39°N off Sanriku, the northern part of the source area. We estimated the P wave velocity (Vp) structure around...

The 2024 Noto-Hanto earthquake with a magnitude of 7.6 occurred in the Noto Peninsula on January 1, 2024. The mainshock had a reverse fault focal solution and direction of compression axis was the northwest–southeast. In the Noto Peninsula, earthquake swarms have been observed since December 2020. In contrast to this swarm activity, the mainshock h...

P-wave velocity (Vp) structures in marine areas have been estimated using marine controlled-source seismic surveys and refraction methods. The spatial resolution of Vp structures depends on the number of receivers and shots. In marine environments, Vp resolution is often limited due to the sparse distribution of ocean bottom seismometers (OBSs). Re...

Increased observations of shallow slow earthquakes have highlighted structural irregularities that explain the generation mechanisms of slow earthquakes. However, due to considerable uncertainties in the source locations of slow earthquakes, it is unclear what controls the spatial extent of slow earthquakes and what hinders slip propagation on the...

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of th...

Seismic exploration was conducted along a profile running through the Aira caldera located in southern Kyushu, Japan. The caldera was formed by an ignimbrite eruption approximately 30 ka BP, namely, the “AT eruption,” which produced the Ito ignimbrite and widespread Aira-Tanzawa ash. This analysis aimed to clarify the detailed P -wave velocity stru...

The region off the Boso Peninsula, Japan, is a tectonically complex area where the Pacific plate is subducting beneath both the landward plate and the Philippine Sea plate (PHS) from the Japan trench and the Izu-Bonin trench as the PHS is subducting under the landward plate from the Sagami trough. It is important to better determine the structure o...

The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S‐net) is a novel cabled ocean‐bottom station network covering a broad offshore region east of northeastern Japan. To best use the S‐net data, we estimated sensor orientations of all 150 S‐net stations, because without this information the orientations of measurem...

In the region off the Boso Peninsula, Japan, the Pacific plate is subducting westward beneath both the Honshu island arc and Philippine Sea plate, while the Philippine Sea plate is subducting northwestward beneath the Honshu island arc. These complex tectonic interactions have caused numerous seismic events occurred in the past. To better understan...

The Eggvin Bank, located between the Jan Mayen Island and Greenland, is an unusually shallow area containing several submarine volcanic peaks, confined by two transforms on the Northern Kolbeinsey Ridge (NKR). We represent P and S wave velocity models for the Eggvin Bank based on an Ocean Bottom Seismometer profile collected in 2011, showing igneou...

Foreshock events, Micro Earthquakes, Hypocenter relocation, Ocean Bottom Seismograph (OBS), Tohoku-Oki Earthquake

Foreshock events, Micro Earthquake, Hypocenter relocation, Ocean Bottom Seismograph (OBS), Tohoku-Oki Earthquake

GPS/Acoustic (GPS/A) survey is the most promising way to detect crustal deformation in the ocean far from the coast, where a dense onshore GPS network is not available. Monitoring seafloor deformation is crucial to understand the tectonic state in regions of geophysical significance such as subduction zones. We, Tohoku University, together with Nag...

On December 2, 2010, MJMA4.6 shallow felt earthquake occurred beneath southeastern part of Sapporo city. Three temporal seismic stations were installed near epicentral area. Reliable hypocenter data estimated by double difference procedure with local seismic velocity structure indicated clear southeastern-dipping distribution. Fault plane from foca...

We have revealed that the Vp of the oceanic crust and upper mantle of the Pacific lithosphere is significantly reduced near the axial part of the Japan Trench, from airgun-OBS seismic experiments made at the outer rise and the inner trench regions of the trench (Azuma et al., 2009). From the spatial correlation between the Vp reduction and the deve...

The distribution of aftershocks of a large (M w = 7.0) normal faulting earthquake beneath the outer slope of the Japan Trench in 2005, measured in 2007 using ocean bottom seismographs, indicates that the earthquake was involved with a set of conjugate normal faults. Although the faults reach to the upper mantle, the estimated Vp and Vp/Vs show no r...

Recent seismic studies around the Japan trench subduction zone have revealed that both the interplate and intraplate earthquakes occur under strong influence of fluid, in particular the water. Although it is believed that the subducted slab is the most significant carrier of the water into the earth interior, it is not fully understood how the wate...

We obtained precise hypocenter distribution and 3-D seismic velocity structure around the junction of Japan and Kuril Trenches by a seismic tomographic inversion technique using data from Long-Term OBS (LTOBS) network. We conducted two observations; the first had a period of eight months using 17 LOBSs from Mar. 2005 and the second used 42 LTOBSs f...

The seismogenic zone in the southern Kuril Trench can be divided into two segments by the Kushiro Canyon, the Nemuro segment to the east and the Tokachi segment to the west. Except for the giant compound earthquake in 17th century, [e.g. Sawai et al., 2002], M8 class earthquakes have occurred repeatedly within each of these segments. The 1952 and 2...

The Kuril Trench is one of the most active seismogenic regions due to the subduction of the Pacific Ocean Plate beneath Hokkaido, where large earthquakes have occurred repeatedly such as the 1952 Off-Tokachi (Mw=8.2), the 1973 Off-Nemuro Peninsula (Mw=7.8) and 2003 Off-Tokachi (Mw=8.2) earthquakes. The result from the analysis of teleseismic body w...

In a region off Miyagi, it is suggested that a large earthquake will occur in the near future. Therefore a long-term ocean bottom seismic observation has been performed since 2002. In addition, many seismic surveys using ocean bottom seismometers (OBSs) and controlled sources were carried out. In August 2005, a large thrust-type earthquake (M=7.2)...

We conducted an airgun-OBS experiment between the Tokachi-oki and the Nemuro-oki seismogenic segments in August 2010. The seismic line is parallel to the Kuril Trench axis and run50 km landward from the trench axis with 240 km length. The experimental area includes the source area of the 1952 Tokachi-oki interplate earthquake (M8.2), where the larg...