Shuichi Kodaira

Melt re-injection after a giant caldera eruption was quantitatively investigated for the Kikai Caldera Volcano in Japan, which erupted 7300 years ago (Kikai-Akahoya eruption). Our seismic refraction survey revealed a low-velocity anomaly directly beneath the Kikai Caldera Volcano, indicating the existence of a large magma reservoir at a shallow depth of 2.5–6 km. The reservoir can be approximated by a trapezoidal shape in this 2D section, with its width being at least the same as the width of the inner caldera, and its melt fraction was estimated as 3–6%, but could be limited to 10% at most. We propose a melt re-injection model in which new melt is re-injected into this large magma reservoir at the shallow depth just beneath the caldera, which is the same magma reservoir for the Kikai-Akahoya eruption. This model may demonstrate a common feature of volcanoes that have experienced a giant caldera eruption.

Understanding how deformation is distributed within accretionary prisms is essential to constrain their structural evolution, internal organisation and seismo-tsunamigenic potential. Researchers would typically use seismic images to characterise large-scale subsurface structure, but accretionary prisms often appear internally chaotic-to-transparent in such data, lacking coherent reflectors. This is likely due to strong lateral heterogeneity and stratal disruption around the scale of the seismic resolution, resulting from intense deformation of accreted sediments and remobilised mass transport deposits.IODP Expedition 405 “JTRACK” (October-December 2024) drilled the frontal prism of the Japan Trench subduction zone at Site C0019, targeting the decollement that hosted extremely large slip (>50 m) to the trench during the 2011 Mw 9.1 Tohoku-Oki earthquake. The expedition involved continuous coring and logging-while-drilling (LWD) through the prism, resulting in an improved time-depth tie with regional 2-D seismic profiles. At this site the seismic images show a highly chaotic internal prism, which introduces large uncertainties in structural interpretation and inhibits the upscaling and lateral extrapolation of borehole-scale observations from Site C0019.Here we present an approach to characterise this heterogeneous internal structure by inverting for scale-independent geostatistical parameters (vertical and lateral correlation lengths, dip) from the combined seismic image and LWD sonic velocity data. We use a Bayesian Markov Chain Monte Carlo approach that gives spatially varying, probabilistic estimates of the geostatistical parameters. The lateral correlation length, specifically, can be used as a proxy for the degree of stratal disruption/deformation, as shorter correlation lengths reflect more intense deformation. While the aspect ratio of the correlation lengths is generally well-constrained for seismic data, the estimate of vertical correlation length from the borehole logs is necessary to resolve the other individual geostatistical parameters away from the borehole.Initial results reveal that the degree of deformation varies significantly within the frontal prism, with the lateral correlation length varying between around 10-50 m. The degree of deformation appears to be compartmentalised by steeply dipping structures that could correspond to fault zones also observed in cores from C0019. These distinct changes in lateral correlation length correspond to lithological units interpreted from core observations. They also coincide with significant changes in vertical correlation length from the sonic log, anisotropy of seismic velocity from core samples and bedding orientation from borehole images. The results demonstrate that seismically-derived geostatistical parameters can delineate internal compartmentalisation of the frontal prism, providing a framework for tectonic and mass transport deposit interpretation and for the extrapolation of core-scale observations. Future work will extend this analysis using parallel profiles along the Japan Trench axis, allowing for mapping of frontal prism internal deformation in three dimensions away from existing drill sites.

Slip behavior on the shallowest part of the subduction zone is heavily influenced by the lithostratigraphy of the incoming plate. Coseismic slip that propagates toward the seafloor is a major tsunami hazard as seen in the 2011 Tohoku-Oki earthquake. In Nankai Trough, scientific drilling combined with seismic facies mapping has allowed scientists to characterize the lithostratigraphy of the Shikoku Basin which in turn have been linked with the geotechnical properties, structural architecture, and seismicity. Seismic reflection surveys have been conducted here for more than 25 years covering over 730 x 150 km and mapping the sediment input using the traditional visual interpretation at this scale is inefficient. Taking advantage of this large seismic dataset combined with scientific drilling to map the fault properties can help improve hazard assessments. This study presents a framework for a semi-automatic classification using 2D seismic reflection profiles acquired by different surveys. Three trench-parallel lines within the accretionary wedge acquired by three different surveys were used for the case study. The amplitudes of the western and eastern profiles were first scaled based on the seafloor reflection of the central profile. Features were extracted using a rectangular window measuring 500 m above the top of the oceanic basement with varying widths measuring 1 km, 3 km, and 5 km. A total of 121 features divided into three groups were extracted. The statistical group (13) describes the strength of reflections, the spectral group (30) describes the presence or absence of internal reflections, and textural group (78) describes the continuity of reflections. The three principal components of each group were extracted and altogether subjected to K-Means clustering with 6 clusters. The 5 km window showed the most comparable classification with visual interpretation and the consistent classification in the overlap between profiles indicate a satisfactory performance of our method. Comparing the classification with previous drilling, Cluster 0 located in the overlap between the central and eastern profile is associated with turbidites occurring in basement lows. Cluster 1 is classified as noise. Cluster 2 in the western and central profile are likely siliciclastic turbidites from the Kyushu Fan. Cluster 3 and 4 in the eastern profile appears to have no drilling analog. Cluster 5 in the central profile is associated with hemipelagic mudstones. These initial results appear promising and will be tested in larger datasets and other subduction zones in the future.

This study investigates the process of melt re-injection into a shallow, large magma reservoir after a giant caldera eruption, based on observational evidence. We chose the Kikai Caldera Volcano as our target because it experienced a giant caldera eruption, the Kikai-Akahoya (K-Ah) eruption, 7,300 years ago. High-resolution seismic reflection surveys and analyses of submarine deposits revealed that the uppermost seismic unit is a pyroclastic deposit produced by the K-Ah eruption, with its estimated volume of >71 km³ (Shimizu et al., 2024). The total bulk volume of the K-Ah eruption was then estimated to be 133–183 km³ in DRE (Dense Rock Equivalent), suggesting that it was probably the largest Holocene eruption. To investigate the current state of the magma reservoir beneath the Kikai Caldera Volcano, we conducted a seismic refraction survey using 39 ocean bottom seismometers and an airgun array along a 175 km survey line across the volcano. The results of the 2D P-wave velocity structure revealed a low-velocity anomaly with a reduction rate of over 15% (maximum 22%) directly beneath the volcano, indicating the existence of a large magma reservoir at a shallow depth of 2.5–6 km. This reservoir is approximately trapezoidal in shape, with a width at least equal to that of the inner caldera. The low-velocity anomaly enabled us to estimate the melt fraction to be 3–6%, but it could be limited to 10% at most. Integrating these results with petrological evidence allows us to propose a model of melt re-injection to form a magma reservoir in the same location as the shallow magma reservoir during the giant caldera eruption. The estimated magma depth during the K–Ah eruption period was 3–7 km (Saito et al., 2001, 2003), and the estimated magma depth for the post-caldera central lava dome formed after 3,900 years ago was 2–4 km (Hamada et al., 2023). These depths overlap with the 2.5–6 km depth of the present magma reservoir identified in this study. Furthermore, the central lava dome exceeds 32 km³ in volume and has a different rock composition from that of the K–Ah eruption magma (Tatsumi et al., 2018), suggesting that different melts were reinjected into the magma reservoir at the same location. The model suggests the following scenario: (1) the K–Ah eruption occurred 7,300 years ago, ejecting approximately 160 km³ of material; (2) caldera formation occurred immediately afterwards; (3) after 3,900 years ago, new melt was injected to form a shallow magma reservoir at the same location, with at least 32 km³ of lava reaching the surface to form the central dome; and (4) this is reflected in the current low-velocity anomaly, characterised by increased melting rates in the magma reservoir. This model may demonstrate a common feature of volcanoes that have experienced a giant caldera eruption, and the temporal variation in low-velocity anomalies in the shallow crust is a crucial indicator for eruption prediction.

Understanding the structure of the incoming Pacific Plate is essential for reconstructing the tectonic evolution of the Japan Trench. International Ocean Discovery Program Expedition 405, “JTRACK,” drilled through the frontal prism and plate boundary fault zone of the Japan Trench and into the underlying Pacific Plate at Site-C0019, located close to the hypocentre of the 2011 Mw 9.0 Tohoku‑Oki earthquake. Here, we present new petrographic and geochemical results from five basaltic samples recovered from below the plate boundary fault zone (930–946 mbsf), which show the first direct geological evidence for a subducted volcanic body beneath this segment of the margin.Core and image logs from Expedition 405 indicate that the drilled interval comprises a repeated sequence of sheeted dykes, massive basalts, and pillow lavas, with at least one interbedded sedimentary horizon separating the successions. This architecture is incompatible with simple ocean-plate stratigraphy and instead indicates a later phase of volcanic activity interacting with pre-existing crust. Of the five samples analysed, one was taken from below the intercalated sedimentary horizon and four from above it. The four samples from above the sediments are systematically more evolved than the sample below. In the samples above, plagioclase shows anorthitic cores overprinted by albitised rims, pyroxenes are Mg-rich, and ilmenite is abundant. Several samples contain K-rich clays, disseminated sulphides, and Zr-rich domains, including possible baddeleyite. These features suggest interaction with sedimentary cover and progressive evolution of magma chemistry during the later stages of emplacement, alongside post-emplacement hydrothermal alteration. Whole-rock major and trace element data show enrichment in incompatible elements (K, Pb, Cs, Rb, Ba, Th, U) relative to typical MORB. K-metasomatism discrimination diagrams indicate that these signatures are not produced by alteration, supporting a primary magmatic origin. Together, the mineralogical and chemical characteristics point to a volcanic body formed off-axis on the Pacific Plate rather than at a spreading ridge.Integration with seismic interpretations suggests that this volcanic body, likely a seamount chain, was faulted prior to subduction and now lies partially perched on a horst, forming a structural high directly beneath the plate boundary fault zone. These results provide the first physical confirmation of a subducted, faulted seamount chain beneath the Japan Trench, offering new constraints on the structure and evolution of the incoming Pacific Plate.

The 2011 moment magnitude 9.1 Tohoku-oki earthquake is exceptional among great earthquakes for having peak slip of ~50 to 70 meters on the shallowest portion of the plate boundary megathrust. Drill cores and geophysical logs from International Ocean Discovery Program Expedition 405 demonstrate that the megathrust preferentially develops at the top or base of the pelagic clay in the sedimentary layers present on the incoming Pacific Plate, where pronounced contrasts in physical properties occur. This preference results in a narrow, weak fault located at a major mechanical contact between frontal prism mud and subducted sediments, which enhances the tendency for shallow seismic slip, suggesting that the Japan Trench may be more susceptible to ruptures with large shallow slip than are margins without weak clays.

Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK)