Search

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10-13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

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. [ABSTRACT FROM AUTHOR]

On 8 October 2023 (UTC), unique earthquakes occurred in the Izu‐Ogasawara Arc, Japan, in which the P‐ and S‐phases were barely visible and only the T‐phases were evident, followed by tsunamis that reached islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. Our estimated T‐phase source area coincides with the Sofu Seamount, which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted 1 month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera‐sized scale that occurred at the caldera can be explained as a source of the October tsunami. Plain Language Summary: On 8 October 2023 (UTC), earthquakes were followed by tsunamis in the Izu‐Ogasawara Arc of Japan that were higher than that estimated from the magnitude of the earthquake. The tsunamis were observed on islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japanese islands. Strong T‐phases that are underwater acoustic waves were observed accompanying that event. We estimated that the T‐phases originated from the area around the Sofu Seamount, which has not been recognized as an active submarine volcano until now. One month after the event, we conducted a bathymetric survey of the seamount and revealed the characteristics of a caldera and a central cone within the caldera. Compared to the bathymetric data obtained in 1987, the caldera and central cone topography have changed significantly, such as a crater forming in the central cone, indicating that this seamount is likely to have been an active submarine volcano. The topographic changes that occurred at the depth of the caldera (∼1,000 m) and its scale (diameter ∼5 km) can be associated with the source of the tsunami on 8 October 2023. Key Points: A bathymetric survey of the Sofu Seamount was conducted after the unique T‐phase dominant earthquakes and tsunamis on 8 October 2023Compared to data in 1987, the caldera and central cone topography have changed, indicating that this seamount is an active submarine volcanoThe observed seafloor displacement at the caldera scale may have contributed to the source of the tsunami on 8 October 2023 [ABSTRACT FROM AUTHOR]

We examined the possible factors affecting the spatial distribution of very low frequency earthquakes and tremors in the shallow megathrust of Nankai Trough (<30 km) using a dense network of prestack depth migrated profiles at the frontal wedge. Geometrical parameters examined were decollement roughness, taper angle, and underthrust thickness. Physical properties such as effective basal friction (μb) and pore pressure ratio (λ*) were calculated from the taper angle and p‐wave velocity. Regions of low λ* (0.39 ± 0.08) and smooth decollement showed no slow earthquake activity. In contrast, high activity of slow earthquakes was observed in areas with a rough decollement due to the presence of subducted seamounts or bathymetric highs. The low taper angle (3.8°) off Muroto where slow earthquakes also occur translates to a wide zone of low μb (0.21 ± 0.06) and high λ* (0.66 ± 0.06). However, our results also show that slow earthquakes don't always occur in areas with high λ*. Plain Language Summary: Slow earthquakes such as very low frequency earthquakes and tremors are different than typical earthquakes because they occur longer and can last for more than several days. They have been reported in numerous subduction zones around the world and the study of slow earthquakes has recently gained attention because they have been suggested as precursors to larger earthquakes albeit on rare occasions. Slow earthquakes show a clustered distribution in the Nankai Trough. We used a dense network of seismic reflection data acquired at the shallow portion of the Nankai Trough to examine why some areas have high activity and some areas have no activity. Our results showed that areas with no slow earthquake activity have a smooth decollement and low pore fluid pressure. On the other hand, areas with high slow earthquake activity have a rough decollement. The occurrence of slow earthquakes has been typically associated with high pore fluid pressure because it allows the two slabs of rock to slide easier. However, our results suggest that high pore fluid pressure may not always be a prerequisite for slow earthquakes to occur. Key Points: A smooth decollement due to absence of subducted seamounts or bathymetric highs, and low pore fluid pressure results in a slow earthquake gapSlow earthquakes don't always occur in areas of high pore pressure but in combination with other variables like decollement roughnessThe low taper angle off Muroto indicates a wide zone of low friction and high pore pressure possibly associated with subducted seamounts [ABSTRACT FROM AUTHOR]

Recent research has shown that shallow tremors tend to occur in spatial association with heterogeneous conditions, which are formed by seamounts or ridges on the subducted oceanic crust. In the southern Kuril Trench, shallow tremors have been observed on the western side, although no apparent heterogeneity in the subducted oceanic crust has been identified. To investigate structures spatially associated with these shallow tremors, we processed reflection seismic data acquired along seven profiles. We examined structures ranging from the seafloor to the top of the oceanic crust, within a depth of up to 20 km. Our results reveal that undulations observed at the top of the subducted oceanic crust are related to the distribution and number of shallow tremors. The undulations would play a crucial role in the distribution of shallow tremors. Plain Language Summary: Shallow tremors, often occurring near subducted seamounts and ridges underground, are small earthquakes that take place at shallower depths of a plate boundary compared to megathrust earthquakes. While the mechanisms behind these two types of earthquakes are not yet fully comprehended, there is an observed correlation, particularly in terms of their locations. At the southern Kuril Trench, shallow tremors have only been detected on the western side, where there aren't any subducted seamounts or ridges. Seismic surveys carried out in the southern Kuril Trench have provided detailed images of the subsurface structures. These images indicate that shallow tremors are more likely to happen in areas where the subducted oceanic crust has pronounced undulations. It's worth noting that only these undulations on the oceanic crust—and not other significant structures—have been linked to the occurrence of shallow tremors. Key Points: Seismic reflection surveys in the Kuril Trench image undulations in the subducted oceanic crust, as well as strong overlying reflectorsAmplitudes and wavelengths of undulations correlate with shallow tremor occurrencesUndulations and the distribution of strong reflections on the plate boundaries shown in previous studies also generally correspond [ABSTRACT FROM AUTHOR]

Off Cape Muroto area, along the Nankai Trough in southwest Japan, is a typical area with adjacent occurrences of slow and megathrust earthquakes. High‐resolution monitoring of slow earthquakes is necessary to understand tectonic conditions. In the off Muroto area, distributed acoustic sensing (DAS) measurement, which provides high‐density strain data, has been conducted using offshore fiber‐optic cable. We observed shallow tremors, a type of slow earthquakes, using DAS measurement for the first time. The characteristics of the tremor signals recorded by DAS were longer durations than those recorded in seismographs and composed of several phases with apparent velocities of several hundreds of m/s to several km/s that are coherent only in tens of meters. By combining DAS and seismograph data, we located these tremors around a subducted seamount peak. Therefore, spatial relationship between slow earthquakes and structural characteristics is suggested. This is a pioneering study of slow earthquake observation using DAS. Plain Language Summary: Distributed acoustic sensing (DAS) measurement is a recent technology that uses a fiber‐optic cable as a strain sensor array. As DAS provides high‐density observation data, it has often been used for the observation of regular earthquakes in recent years. However, few studies have used DAS measurement to observe slow earthquakes, which have longer characteristic durations compared to regular earthquakes. We observed shallow tremors, a type of slow earthquake, using DAS for the first time off Cape Muroto, where slow and huge regular earthquakes occur in the neighboring areas. The characteristics of tremor signals in DAS data are long durations and composition of several phases with variable apparent velocities that are coherent only in tens of meters. We located tremors by picking the arrival of signals by visually checking the waveforms of DAS and an offshore seismograph network and locating them at the point where the difference between synthetic and observed arrival times was the smallest. Most tremors occurred around 134.7°E, 32.8°N, which corresponds to a subducted seamount peak; therefore, a spatial relationship between slow earthquakes and a subducted seamount is suggested in this region. Key Points: We first observed shallow tremors by distributed acoustic sensing (DAS) using offshore fiber‐optic cable in the Nankai TroughTremor signals of DAS are composed of several phases of variable apparent velocities that are coherent only in tens of metersSpatial relationship between shallow tremors and subducted seamount is suggested along the dip direction [ABSTRACT FROM AUTHOR]

Three-dimensional seismic reflection data reveal the presence of a low seismic velocity zone (LVZ) with weak reflectivity character along the Nankai accretionary prism. This LVZ is intercalated between an upper, offscraped layer and a lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the LVZ, which is estimated to be a maximum of ~2 km thick, ~15 km wide, and ~120 km long. The LVZ could be an underthrust package underplated in response to the lateral growth of the Nankai accretionary prism. Underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward. The LVZ could have elevated fluid pressure, leading to rigidity reduction of the entire outer accretionary wedge. The rigidity-lowered outer wedge, containing the LVZ, may be more easily uplifted and thus eventually foster tsunami generation during a Nankai megathrust earthquake. If the fluid-rich LVZ supplies a significant amount of the fluid to the megasplay fault zone at depth, it may affect stick-slip behavior of the fault. doi: 10.1130/G30205.1