Ryuta Arai

The southern Okinawa Trough provides an opportunity to investigate the early stage of back-arc rifting before the onset of oceanic spreading. We examined the Yaeyama and Yonaguni Rifts using multichannel seismic reflection data, swath bathymetry, heat flow measurements, and geomagnetic observations. The seismic profiles reveal widespread seafloor-cutting normal faults and thick trough-fill sediments in both rift segments. In the Yaeyama Rift, a localized acoustically transparent body is identified beneath the rift axis and may represent a shallow intrusive feature. However, higher-resolution seismic data or additional geophysical constraints would be required to confirm its origin. Heat flow values are generally low (40–63 mW/m ² ) except for one localized anomaly, consistent with previous observations of subdued heat flow in the southern Okinawa Trough. The geomagnetic data do not indicate extensive shallow volcanic construction, although localized magnetized bodies (e.g., small intrusive or volcanic units) may be present. These observations suggest that early rifting in the southern Okinawa Trough is dominated by distributed normal faulting under thermally subdued conditions and with only limited evidence for substantial magmatic contribution, with possible localized magmatic activity in the Yaeyama Rift. Our results provide structural and thermal constraints on the diversity of early-stage rifting processes in continental back-arc basins.

The geometry and distribution of igneous intrusions, which define melt source locations and record syn‐emplacement stress conditions, are essential for understanding tectonic and magmatic processes associated with crustal extension. However, in back‐arc settings along continental margins, structural constraints on these features remain poorly documented. In the southern Okinawa Trough—an active back‐arc basin transitioning from rifting to seafloor spreading—previous studies have identified narrow subsurface intrusions resulting from the magma ascent driven by lithospheric extension beneath the linear topographic depression known as the Yaeyama Rift. Here, we present new seismic reflection data that constrain the pseudo‐three‐dimensional geometry of intrusions beneath the Yaeyama Rift. Seven across‐rift and one along‐rift seismic profiles reveal complex rift structures, including a rapid transition from exposed magmatic bodies forming the Yaeyama Knoll to buried intrusions capped by graben structures, suggesting caldera collapse. Although the Yaeyama Rift generally exhibits low heat flow values (2 ), the deep intrusive body is linked to a high‐temperature hydrothermal venting site on the Yaeyama Knoll. Additional narrower intrusion structures are also identified beneath rift margins and off‐axis regions where a dense network of normal faults has developed and may influence local thermal structure through structural heat focusing. Nearby, we identify seismic reflectors with polarity reversal and stratigraphic layers with reduced reflection amplitudes that indicate fluid and gas accumulations at depth. These findings suggest a close interplay between heat supplied by deep plutonic intrusions and the cooling effects of thick sedimentary layers, plus seawater inflow through syn‐rift normal faults.

Recent seismological and geodetic observations have revealed shallow slow earthquakes on subduction megathrusts, offering an opportunity to explore the factors controlling megathrust slip. Identifying these controlling factors helps constrain the potential updip extent of future megathrust ruptures. Here, we investigate the factors controlling the spatial extent of shallow slow earthquakes by integrating tremor locations with uncertainty estimates, detailed bathymetry data, and multichannel seismic data for the 2020–2021 slow earthquake sequence in the Kumano-nada region of the Nankai Trough. Tremor probability maps reveal that the updip, downdip, and lateral limits of tremor activity coincide with distinct structural boundaries. These include the kink or branching of the décollement, a deep-seated strike-slip fault, and the inner–outer wedge boundary. These structures likely act as geometrical or mechanical barriers, further influenced by variations in pore-fluid pressure and by the subducted Paleo–Zenisu ridge. The results indicate that megathrust geometry, material properties of the overriding prism, fluid distribution, and ridge subduction jointly govern the spatial extent of slow earthquakes, emphasizing the need to account for these factors in assessing the potential extent of future megathrust ruptures.

(192 words) High pore pressure ratio (λ*) has been associated with the occurrence of slow earthquakes. Many studies have estimated the λ* using taper angle, P-and S-wave velocities, and drilling but the extent is not yet clearly defined. This study utilized a recently published high-resolution P-wave velocity model off Muroto derived from a two-step tomographic inversion of ocean-bottom seismograph data to determine the λ* using empirical relationships between velocity, porosity, and effective mean stress. We determined an extensive zone of high λ* (>0.4) from the frontal thrust up to ~65 km landward and to a depth of 8 km with three characteristic observations. First, the underthrust sediments in the outer wedge show patches of overpressured aquifers where λ* >0.6, consistent with previous drilling results. Second, the high λ* (>0.6) region in the inner wedge coincides with previously reported underplated sediments composed of fluid-rich trench-fill sediments dragged down by seamounts. The high λ* may be caused by tectonic compression from the newly subducted seamounts. Lastly, vertical columns of high λ* were observed in many of the thrust faults with some faults showing a negative polarity, which are interpreted to be evidence of fluid flow.

The eastern part of the Nankai Trough is a region of intense tectonic activity where the Philippine Sea Plate subducts beneath the Eurasian Plate. Large interplate earthquakes, such as the Tonankai and Tokai earthquakes, have repeatedly occurred in this area. A notable feature of the region is the paleo-Zenisu Ridge – a ridge subparallel to the trough axis – believed to be subducting beneath the accretionary prism. Previous studies have suggested that the extent of the paleo-Zenisu Ridge is closely linked to the distribution of rupture zones for megathrust and slow earthquakes. However, the precise extent and topography of the paleo-Zenisu Ridge remain poorly constrained.Within the long-term collaborative project we aim to reconstruct the detailed seismic velocity structure of the eastern Nankai Trough using several dense 2-D wide-angle OBS datasets to better constrain the condition of the paleo-Zenisu Ridge and evaluate its critical role in the region’s tectonic and seismic dynamics. In this study, we apply first-arrival traveltime tomography and time-domain full-waveform inversion to recover high-resolution velocity models along two parallel OBS profiles (spaced ~20 km apart) acquired by JAMSTEC in the eastern Nankai Trough. These profiles integrate OBS data from two legacy seismic dasets (KR07-10_B and KR12-12_Z04; ~4.8 km OBS spacing) with recently acquired coincident profiles (KM23-13), where OBS instruments were deployed between the receiver positions of the legacy surveys. This integration results in datasets consisting of 80 and 97 OBS per profile, with an improved spacing of ~1.6 km.The dense OBS spacing of the combined datasets enables stable FWI application up to 8 Hz, allowing for detailed reconstruction of the underlying structure of the accretionary wedge and subducting oceanic crust. Our results, combined with other high-resolution velocity models from the Tokai area of the Nankai Trough, reveal variations in the topographic relief of the paleo-Zenisu Ridge along the subduction front axis. These findings suggest a non-uniform impact of the subducting ridge on the overlying wedge, as well as variations in stress distribution, fluid migration, and seismic coupling along the subduction interface. By leveraging additional OBS profiles and advanced inversion techniques, this study enhances our understanding of the paleo-Zenisu Ridge and its role in shaping the tectonic framework of the eastern Nankai Trough.

Mud volcanoes, common seafloor features in subduction zone forearcs, provide crucial insights into deep hydrogeological systems associated with plate subduction. Despite their widespread occurrence, the factors governing their uneven distribution remain unclear, and their internal physical properties are poorly understood. Using multiple seismic reflection profiles and high-resolution P-wave velocity (Vp) models, we examined the distribution and physical properties of mud diapirs in the Hyuga-nada subduction zone, southwest Japan, impacted by the Kyushu-Palau ridge seamount chain entering the junction of the Nankai Trough and the Ryukyu Trench. Our seismic reflection data revealed more than 60 diapiric structures, some breaching the seafloor, while others remain buried beneath shallow sediments. These diapirs predominantly occur where the subducting slab is 10–20 km deep, mainly on the leading edge of a large subducting seamount. Notably, there is a significant absence of diapirs directly above the seamount and within approximately 50 km of the trench axis. The findings suggest that once the slab reaches depths greater than 10 km, thermogenic hydrocarbon gas production, which gains significant buoyancy, drives upwelling fluid flows in the overriding plate. The ridge subduction also contributes to diapir distribution by enhancing dehydration of underthrust sediments on the downdip side and creating a porous fracture network in the upper plate. The Vp structure of an exposed mud volcano indicates that it contains methane levels similar to those in the Kumano-nada region of the central Nankai Trough. Buried mud diapirs, rooted in fluid- and gas-rich reservoirs at 1–5 km depths below the seafloor, exhibit significantly lower seismic velocities (Vp < 3.0 km/s), indicating higher fluid and hydrocarbon gas concentration at depth. These observations suggest the forearc wedge in the Hyuga-nada area contains more extensive fluid/gas reservoirs than previously recognized.

Four decades of seismic reflection, onshore‐offshore and ocean‐bottom seismic data are integrated to constrain a high‐resolution 3‐D P‐wave velocity model of the Hikurangi subduction zone. Our model shows wavespeeds in the offshore forearc to be 0.5–1 km/s higher in south Hikurangi than in the central and northern segments (V_P ≤ 4.5 km/s). Correlation with onshore geology and seismic reflection data sets suggest wavespeed variability in the overthrusting plate reflects the spatial distribution of Late Jurassic basement terranes. The crustal backstop is 25–35 km from the deformation front in south Hikurangi, but this distance abruptly increases to ∼105 km near Cape Turnagain. This change in backstop position coincides with the southern extent of shallow slow‐slip, most of which occurs updip of the backstop along the central and northern margin. These relationships suggest the crustal backstop may impact the down‐dip extent of shallow conditional stability on the megathrust and imply a high likelihood of near/trench‐breaching rupture in south Hikurangi. North of Cape Turnagain, the more landward position of the backstop, in conjunction with a possible reduction in the depth of the brittle ductile transition, reduces the down‐dip width of frictional locking between the southern (∼100 km) and central Hikurangi margin by up‐to 50%. Abrupt transitions in overthrusting plate structure are resolved near Cook Strait, Gisborne and across the northern Raukumara Peninsula, and appear related to tectonic inheritance and the evolution of the Hikurangi margin. Extremely low forearc wavespeeds resolved north of Gisborne played a key role in producing long durations of long‐period earthquake ground motions.

In this study, we reveal the deformational structure of the crust of the northern part of the Ryukyu Arc and Okinawa Trough using ambient noise tomography. Compared with southern Ryukyu, the northern segment exhibits a wide and shallow basin, a crust without localized thinning, slow extension rates, and highly arc‐oblique, right‐lateral retreat of the Ryukyu Arc. We present both isotropic and azimuthally anisotropic shear‐wave velocity models using data recorded by an ocean‐bottom seismometer array and nearby island stations. The isotropic model demonstrates a monotonic decrease in velocity from the backarc to the forearc, in accord with the accretionary‐prism origin of the latter. The resolved azimuthal anisotropy exhibits arc‐parallel and arc‐perpendicular fast shear‐wave polarization directions in the upper to mid‐crust and the lower crust to uppermost mantle in much of the arc and backarc, respectively. We interpret the arc‐parallel anisotropy as resulting from the anisotropic fabrics aligned by the vertical shearing imposed by the right‐lateral motion of the Ryukyu Arc. The underlying arc‐perpendicular anisotropy may be attributed to horizontal shearing driven by corner flow in the mantle wedge. We found arc‐perpendicular anisotropy in the forearc upper crust, which may reflect crack alignment caused by the collision of the Amami Plateau. The oblique arc retreat and the resolved deformation fabrics in the arc and backarc together attest to the shear‐dominant, transtensional nature of the northern Ryukyu continental rift system. Some of the features in northern Ryukyu may be better explained from the perspective of transtensional rifting.

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 megathrust. We utilize the precise locations of tectonic tremors, bathymetry data, and seismic reflection images acquired from the Kumano-nada in the Nankai Trough subduction zone to investigate structural factors that determine the spatial extent of tectonic tremors. Results suggest that an abrupt change in the dip angle of the megathrust, located at the point where the frontal thrust branches from the décollement, behaves as a geometrical barrier to along-dip slip propagation. An underplated structure likely generates a pore fluid pressure contrast near this branching point, further strengthening this barrier. In addition, we find that a similar geometrical barrier created by deep-rooted strike-slip faults may affect along-strike slip propagation. Furthermore, the absence of slow earthquakes in the eastern part of the study area is attributed to a stress shadow on the rear side of the subducting Paleo–Zenith ridge.