This study presents new seismic imaging of the Andean subduction zone through P-wave hybrid finite-frequency and ray-theoretical tomography. We measured both differential and absolute traveltimes using broadband seismic waveforms from stations in an array of ocean-bottom seismographs near the Chile Triple Junction (CTJ) and stations within 30° from the array. These data were combined with the global traveltime dataset to obtain a global P-wave velocity structure with a focus on central to southern South America. The new tomographic image showed the Nazca slab geometry as a continuous fast anomaly, which is consistent with seismic activity and prior slab models. Furthermore, two notable structures were observed: a broad extension of the fast anomaly beneath the Nazca slab at 26–35° S and a slow anomaly east of the CTJ. The checkerboard resolution and recovery tests confirmed the reliability of these large-scale features. The fast anomaly, isolated from the Nazca slab, was interpreted as a relic Nazca slab segment based on its strong amplitude and spatial coincidence with the current Pampean and past Payenia flat slab segments. The slow anomaly near the CTJ was consistent with the previously inferred extent of the Patagonian slab window. Moreover, the active adakitic volcanoes are aligned with the southern edge of the anomaly, and the plateau basalts are located within the anomaly. Our model showed that the slow anomaly extended to a depth of up to 250 km, suggesting a depth limit that the asthenospheric window can influence.

Shear wave velocity (Vs) estimations of accretionary prisms can pose unique constraints to the physical properties of rocks, which are hard to obtain from compressional velocities (Vp) alone. Thus, it would help better understand the fluid processes of the accretion system. This study investigates the Vs structure of the Hyuga-nada accretionary prism using an array of ocean-bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green’s functions and a surface wave dispersion curve are inverted to one-dimensional Vs structures using transdimensional inversion. The results indicate the presence of a low-velocity zone 3–4 km below the seafloor. The reduced Vs is consistent with a reduced Vp feature obtained in a previous seismic refraction survey. From its high Vp/Vs ratio, we conclude that the low velocities represent high pore fluid pressure. In addition, the resolved lithological boundary exhibits a sharp offset that extends laterally across the OBS array. We attribute this offset to a blind fault below while acknowledging other possibilities, such as due to mud diapirism. The predicted fault is located at the Kyushu–Palau Ridge flank, oriented roughly parallel to the ridge axis, and thus likely caused by ridge subduction. The fracture caused by the ridge subduction may act as a fluid conduit, forming a fluid reservoir beneath the well-compacted sediment layers.

Distributed Acoustic Sensing (DAS) is a promising technique to improve the rapid detection and characterization of earthquakes. Previous DAS studies mainly focus on the phase information but less on the amplitude information. In this study, we compile earthquake data from two DAS arrays in California, USA, and one submarine array in Sanriku, Japan. We develop a data-driven method to obtain the first scaling relation between DAS amplitude and earthquake magnitude. Our results reveal that the earthquake amplitudes recorded by DAS in different regions follow a similar scaling relation. The scaling relation can provide a rapid earthquake magnitude estimation and effectively avoid uncertainties caused by the conversion to ground motions. Our results show that the scaling relation appears transferable to new regions with calibrations. The scaling relation highlights the great potential of DAS in earthquake source characterization and early warning.

A seafloor-cable seismic and tsunami observation system is ideal for marine geophysical observation because the data can be obtained in real-time. We have developed a new compact seafloor cable seismic and tsunami observation system using Information and Communication Technology (ICT) since 2005. Our new system secures reliability by using Transmission Control Protocol/Internet Protocol technology and provides observational flexibility via observation nodes with a software-based system using up-to-date electronics technology. These features contribute to cost reduction and production sustainability. The system was installed on the Pacific Ocean floor off Sanriku, northeast Japan, in September 2015, in the source area of the 2011 Tohoku-oki earthquake. Our purpose is to better monitor seismic activity and to observe tsunami activity through spatially dense observation. The obtained high quality seismic and pressure data are stored continuously by the new system.

Subducted reliefs, such as seamounts and ridges, affect fluid processes in accretionary prisms of subduction zones. The Kyushu–Palau Ridge subducts along with the Philippine Sea Plate in Hyuganada, which is one of the regions that are best suited for studying the role of subducting topography. This study investigates the shear wave velocity structure using an array of ocean-bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green’s functions and a surface wave dispersion curve are inverted to one-dimensional shear wave velocity structures using transdimensional inversion. The results indicate the presence of a low-velocity zone 3–4 km below the seafloor. The reduced shear wave velocities are consistent with a compressional velocity structure obtained in a previous seismic refraction survey. We conclude that the low velocities are representative of high pore fluid pressure. In addition, the resolved lithological boundaries exhibit a sharp offset that consistently appears across the OBS array, suggesting the presence of a blind fault beneath it. The predicted fault, which is located at the flank of the Kyushu–Palau Ridge and oriented roughly parallel to the ridge axis, is likely caused by the ridge subduction. The fracture caused by the ridge subduction may act as a fluid conduit, forming a fluid reservoir beneath the well-compacted sediment layers. The compilation of previous refraction surveys implies that the reservoir has a lateral extension of >100 km. Its spatial distribution roughly correlates with the ridge location, highlighting the significant role the ridge plays in the formation of the reservoir.

The off-Ibaraki region is a convergent margin at which a seamount subducts. An intensive event location was performed around the subducting seamount to reveal the regional seismotectonics of this region. By applying a migration-based event location to an Ocean Bottom Seismic network record of both P- and S-waves, over 20,000 events were determined in the off-Ibaraki region below ~M4. The seismicity showed clear spatiotemporal patterns enough to identify the seismicity changes and geometry of the interface. At the updip side, the shallow tectonic tremors and earthquakes are shown to be spatially complementary bounded by an updip limit of the seismogenic zone. At the downdip side, a semicircular low-seismicity zone was identified, which is possibly a rupture area of the Mw7.9 event. The event depth profile exhibited a gently sloped planar downdip interface subparallel to the subducting slab. This plane appears to be stably active from 2008 to 2011. Comparison with the active source seismic survey profiles exhibits that this planar downdip interface is several kilometers deeper than the top of the oceanic crust. After the Mw7.9 event, a high-angle downdip seismic interface was activated above the planar interface. Further, below the planar downdip interface, broadly scattered events occurred with a swarm manner. We successfully illuminated the complicated subsurface structures around the subducting seamount. It is suggested that most of the event occur along or below the plate interface as the top of the oceanic crust.

Seismic waves can be significantly amplified by soft sediment layers. Large dynamic strains can trigger a nonlinear response of shallow soils having low strength, which is characterized by a shift of the resonance frequencies, ground motion deamplification, and in some cases, soil liquefaction. We investigate the response of marine sediments during earthquake ground motions recorded along a fiber-optic cable offshore the Tohoku region, Japan, with Distributed Acoustic Sensing (DAS). We compute AutoCorrelation Functions (ACFs) of the ground motions from 103 earthquakes in different frequency bands. We detect time delays in the ACF waveforms that are converted to relative velocity changes (dv/v). dv/v drops, which are characteristic of soil nonlinearity, are observed during the strongest ground motions. Moreover, the dv/v values show a strong variability along the cable. This study demonstrates that DAS can be used to infer the dynamic properties of the shallow Earth with an unprecedented spatial resolution.

The ocean floor broadband seismology has been established based on several practical observations by using broadband ocean bottom seismometer (BBOBS) and its new generation system (BBOBS-NX) in Japan since 1999. The data obtained by our BBOBS and BBOBS-NX is adequate for broadband seismic analyses, especially the BBOBS-NX enables the quality of the horizontal data comparable to land sites in longer periods (10 s –). And, the BBOBS-NX with tilt measurement function, BBOBST-NX, is in practical evaluation for the mobile tilt observation that may realize a dense seafloor geodetic monitoring with low cost. The weak point of the BBOBS-NX system lies in the intrinsic limitation of the submersible in its operation. If this system can be operated alone like as the BBOBS, it should be a true breakthrough of ocean bottom seismology. We call this new autonomous BBOBS-NX as the NX-2G in short. Several problems to realize the NX-2G have been almost cleared through test observations since 2012. The function of the NX-2G system is based on 3 stage operations as shown in the image. The glass float is added to obtain enough buoyancy to extract the sensor unit from the seafloor and also to suppress the oscillating tilt of the system in descending, not to exceed the tilt allowance of the broadband seismic sensor. In Oct. 2016, the first in-situ test of the NX-2G system was performed. The landing of the NX-2G looked well and the maximum tilt in descending was about ±2.5°, that ensured the effective suppression for the oscillating tilt by the glass float. As the final step test of the NX-2G, the one-year-long observation has been started in April 2017 with the BBOBS deployed nearby, to obtain simultaneous data for the noise level evaluation. The free-fall deployment and the transition from the landing stage to the observation stage were completed, those were monitored through the acoustic communication from the ship. This NX-2G was recovered in Oct. 2018 with the ROV, KAIKO Mk-IV, to watch the transition from the observation stage to the recovery stage at the seafloor. All function of the NX-2G at the seafloor was perfect with immediate extraction of the sensor unit. Noise level comparison with the BBOBS shows about 10 dB improvement that is not enough as expected, which may lie in a small tension of the cable between the sensor unit and the recording unit. And, scenes of the landing and the first transition were selfied by the Deep-Sea CAM.

The broadband ocean bottom seismometer (BBOBS) and its new generation system (BBOBS-NX) have been developed in Japan, and we performed several test and practical observations to create and establish a new category of the ocean floor broadband seismology, since 1999. Now, the data obtained by our BBOBS and BBOBS-NX is proved to be adequate for broadband seismic analyses. Especially, the BBOBS- NX can obtain the horizontal data comparable to land sites in longer periods (10 s –). Moreover, the BBOBST-NX is in practical evaluation for the mobile tilt observation that enables dense geodetic monitoring. The BBOBS-NX system is a powerful tool, although, it has intrinsic limitation of the ROV operation. If this system can be used without the ROV, like as the BBOBS, it should lead us a true breakthrough of ocean bottom seismology. Hereafter, the new autonomous BBOBS-NX is noted as NX-2G in short. The main problem to realize the NX-2G is a tilt of the sensor unit on landing, which exceed the acceptable limit (±8°) in about 50%. As we had no evidence at which moment and how this tilt occurred, we tried to observe it during the BBOBST-NX landing in 2015 by attaching a video camera and an acceleration logger. The result shows that the tilt on landing was determined by the final posture of the system at the penetration into the sediment, and the large oscillating tilt more than ±10° was observed in descending. The function of the NX-2G system is based on 3 stage operations as shown in the image. The glass float is aimed not only to obtain enough buoyancy to extract the sensor unit, but also to suppress the oscillating tilt of the system in descending. In Oct. 2016, we made the first in-situ test of the NX-2G system with a ROV. It was dropped from the sea surface with the video camera and the acceleration logger. The ROV was used to watch the operation of the system at the seafloor. The landing looked well and it was examined from the acceleration data. As the maximum tilt in descending was about ±2.5°, the glass float effectively suppressed the oscillating tilt. The extraction of the sensor unit was also succeeded with the total buoyancy of about 75 kgf within about 2.5 minutes. As the final step experiment, the one-year-long observation of this NX-2G system has been started in this April with the BBOBS, to obtain simultaneous data for the noise level evaluation.