Ryota Hino

How and when the subduction megathrust recovers its locking state after a great earthquake is important to understanding earthquake-cycle physics and future hazards. Here we report the delineation of the relocking process after the 2011 magnitude 9 Tohoku earthquake with 14 years of postseismic seafloor geodetic observations. These data, combined with onshore geodetic observations, analyses of repeating earthquakes, and numerical modeling, reveal how the recovery of locking is interrupted by afterslip on the megathrust and accompanied by stress relaxation in the viscoelastic upper mantle. The results indicate that the megathrust has already arrived at a locking state today similar to that before the earthquake despite the ongoing stress relaxation. The acquired knowledge aids understanding of megathrust earthquake cycles at other subduction zones.

Extracting deep subsurface information beneath the ocean floor is challenged by temporally unstable oceanic noise, especially below 0.1 Hz, limiting the effectiveness of passive seismic methods such as horizontal‐to‐vertical spectral ratio (HVSR). Previously, we found that HVSR of ambient seismic noise (HVSRN) is unstable below 0.1 Hz at three broadband ocean bottom seismometer (OBS) stations offshore Fukushima in the southern Japan Trench. To overcome this limitation, we present a novel approach that estimates HVSR using the earthquake coda (HVSRC). Multiple earthquakes with long‐duration coda (≥200 s) enabled stable HVSRC estimation in the 0.05–10 Hz frequency range, thereby extending the lower frequency limit of offshore HVSR. While HVSRC and HVSRN differ below 0.1 Hz, they exhibit similar curves from 0.1 to 10 Hz, confirming the reliability of HVSRC at ocean‐bottom sites. Moreover, theoretical HVSRs below 0.2 Hz, computed using the local structure, show good agreement with observations. We inverted the averaged HVSRCs using the diffuse field assumption and the recently developed 'hvgeneralized’ algorithm, which accounts for the seawater layer, and obtained prior‐constrained high‐resolution 1D shear‐wave velocity models down to ∼10 km depth. These preferred models are broadly consistent with sedimentary and crustal layers inferred from previous geophysical constraints. At one OBS station, they include a low‐velocity zone (LVZ) of the Japan Trench plate‐interface at ∼7 km depth. These results suggest that HVSRC can mitigate low‐frequency instability in ocean‐bottom HVSR and may help extend the applicability of prior‐constrained HVSR‐based modeling from shallow to deep subsurface structure investigations.

Over the last two decades, the importance of Slow Slip Events (SSEs) in the deformation and seismic cycle of subduction zones has become more widely recognized. Knowledge of the evolution and slip distribution during SSEs can provide key insights into processes that influence SSE occurrence and their relationship to seismic slip. However, the offshore nature of many SSEs makes them difficult to observe with onshore geodetic methods alone, necessitating the deployment of seafloor-geodetic instruments to detect seabed deformation.The Hikurangi subduction zone offshore New Zealand is characterized by frequent, large SSEs, and previous experiments have shown that such events regularly produce up to a few centimetres of seabed uplift that is detectable using seafloor pressure data. We are presenting a first look on ocean bottom pressure data recovered from the most recent GONDOR deployment across the northern Hikurangi subduction zone. The 2022-2025 GONDOR project is the largest seafloor geodetic experiment to date at Hikurangi, with over 50 seafloor instruments deployed in a dense array with several kilometres spacing, of which 39 were fitted with Absolute Pressure Gauges (APG) to detect vertical displacement of the seabed. 13 of these instruments have self-calibrating A-0-A sensors, enabling mitigation of instrument drift from the pressure record. The deployment is collocated with an IODP CORK observatory, allowing for future ground truthing of the pressure data and includes two arrays of Direct-Path-Acoustic sensors, giving horizontal deformation information of SSEs. During the 2024/2025 period, at least three large SSEs have occurred beneath the array, one beneath the centre and one each beneath the northern and southern subarrays.I will present preliminary results from our analysis of seafloor pressure data, using depth-matched reference sites to mitigate oceanographic noise. I will also outline a processing workflow for pressure data analysis, including improved drift-removal techniques that do not depend on prolonged periods of oceanographic calm and are robust to sensor vertical displacement during the initial deployment phase when drift is most rapid. Further, I will explore the advantages of interspersing A-0-A instruments with conventional APGs for more robust drift mitigation to enable resolution of SSE vertical displacement during the early deployment phase.

Ocean-bottom pressure (OBP) gauges are a key tool in seafloor geodesy, providing time-series observations of vertical seafloor motion by continuously recording pressure variations associated with changes in water depth. However, OBP time series contain multiple superimposed signals in addition to crustal deformation, including tides, instrumental drift, and non-tidal oceanic fluctuations driven by atmospheric and oceanic processes. In particular, non-tidal oceanographic fluctuations have time scales comparable to long-term crustal deformation, making it difficult to separate them and accurately estimate long-term seafloor deformation. Ocean-model-based corrections have been proposed to isolate and remove non-tidal oceanographic components, offering a physically based approach. However, model–observation mismatches can leave large residuals after correction, making it harder to identify deformation-related signals. As a fundamental step toward understanding these model–observation discrepancies, this study quantifies the physical processes controlling OBP variability using an ocean general circulation model named OFES2 (Sasaki et al., 2020), which does not assimilate oceanic observations. As a first step, we assess the extent to which OFES2 can account for the observed OBP variability by directly comparing modeled and observed time series. The modeled OBP is compared with observed OBP records from pressure gauges deployed off northeastern Japan. Our results show that OFES2 reproduces the seasonal cycle of the observed OBP time series to some extent, whereas agreement at periods of several days to about a month remains limited in both amplitude and phase. To investigate the physical processes controlling the modeled OBP variability, we decompose the modeled OBP into four components: atmospheric pressure loading, sea surface height variability, horizontal density advection, and vertical advection, and quantify the relative contribution of each component. The decomposition indicates that atmospheric pressure loading and sea surface height variability dominate the modeled OBP fluctuations, while horizontal density advection contributes to part of the seasonal variability. Moreover, correlation analyses using the time-derivative of the observed OBP and the decomposed pressure components reveal episodic enhancements in correlation with the horizontal density advection term, suggesting that its contribution can temporarily increase during specific periods. We will extend the analysis by comparing OFES2 with longer-term OBP observations and discussing in more detail the contributions of each component to seafloor pressure variability.

Continuous observations of seafloor water pressure, which enable detection of vertical seafloor motion, are an essential technique in seafloor geodetic observations. For such geodetic applications, observation instruments equipped with Digiquartz sensors manufactured by Paroscientific Ltd. are commonly used because of their extremely high accuracy, resolution, and stability. Downsizing of observation instruments are important for making dense network allowing detection of subtle crustal deformation; however, the relatively high power consumption of Digiquartz makes it difficult to downsize self-pop-up mobile observation systems. Our group has used RBRduo3, temperature–pressure loggers as auxiliary instruments to monitor environmental conditions in other seafloor observations, such as seafloor acoustic ranging. Given that these products are compact, lightweight, and capable of easily acquiring continuous records over periods exceeding one year, we investigated their potential applicability to seafloor geodetic observations. To this end, we conducted parallel observations with ocean-bottom pressure recorders (OBPRs) equipped with high-precision Digiquartz sensors and RBR loggers. Pressure records from RBR loggers exhibit large transient variations immediately after deployment on the seafloor, with amplitudes up to about an order of magnitude larger than the typical transients observed in Digiquartz sensors. However, except for approximately the first three days after installation, this behavior can be well approximated by a time-dependent function combining an exponential term and a linear term, and no other irregular fluctuations are observed. Results from parallel observations at sites with water depths exceeding 5,000 m show that, aside from the initial post-deployment transients, pressure time series obtained by RBR loggers agree well with those from Digiquartz sensors. In contrast, at shallower sites with water depths less than about 2,000 m, the pressure time series from the two instruments differ substantially. The time series of the pressure differences closely resembles the temperature time series, suggesting that these discrepancies arise from insufficient temperature correction of the RBR pressure data. Assuming that apparent pressure variations caused by temperature changes dominate the short-period components of the pressure fluctuations recorded by the RBR logger, we estimated a coefficient for temperature correction by minimizing the power of the fluctuations. Using this coefficient, we removed the temperature-correlated component over the entire frequency band. As a result, we obtained pressure time series that agree with those from the Digiquartz sensors within approximately 0.2 hPa in the parallel observations. This demonstrates that, with appropriate temperature correction, it is possible to obtain pressure variation data from RBR loggers that are comparable in quality to those from Digiquartz sensors. By taking advantage of their compact size and low power consumption, RBR loggers could be applied as add-on instruments to ocean bottom seismometers and similar seafloor observation instruments, and are expected to contribute to an increased number of observation points in mobile seafloor observation networks.

Ocean Bottom Pressure gauges (OBPs) are devices installed on the seafloor to continuously measure ocean bottom pressure. They are expected to detect vertical seafloor crustal deformation caused by transient tectonic events. However, OBP data contain pressure changes originating from various sources other than tectonic events, such as ocean tides, non-tidal fluctuations due to meteorological and oceanic variations, and instrumental drift. We need to remove these non-tectonic components to identify the pressure changes related to vertical crustal deformation. However, the timescale of non-tidal fluctuations (ocean noise) is similar to that of transient tectonic signals, such as slow slip events (SSEs), which makes it difficult to separate these components.Otsuka et al. (2023) applied principal component analysis (PCA) to seafloor pressure data obtained by an OBP array to detect transient events, assuming that pressure changes due to transient events cause temporal fluctuations in the PCA results. To verify the performance of this method, they applied PCA to synthetic data and confirmed that the method can detect temporal changes in the composition of principal components (PCs). In the present study, we apply this method to OBP data obtained before the 2011 Tohoku earthquake, which includes transient events resulting from aseismic slip, as reported by Ito et al. (2013), to verify whether we can identify the event through temporal changes in the PCs decomposed from the OBP data. We performed PCA on de-tided OBP data covering about four months before the Tohoku earthquake using a short sliding time window, to examine temporal variations in the PCs.Changes in the PCs were evaluated using the normalized inner product (NIP) of the eigenvector of each PC (Otsuka et al., 2023), which measures the difference in the direction of the vector. We expect the NIPs to be stable if the OBP data do not contain any transient events, whereas evident changes in the NIPs of more than one PC would occur when transient pressure variations are included in the data. In the present study, the NIPs of PC1 (the most significant component) and PC2 (the second most significant component) remained stable over time, whereas the NIPs of PC3 and PC4 began to decrease as the moving window (60-day length) approached late January 2011 and remained low for about 40 days. Based on a comprehensive analysis of seismic and geodetic data, including the same OBP data, Ito et al. (2013) reported that an SSE lasted for about 40 days from late January 2011. The NIP changes detected in the present study may correspond to pressure changes due to this transient event.

Extracting deep subsurface information beneath the ocean is challenged due to temporally unstable oceanic noise levels, especially below 0.1 Hz, which limits the effectiveness of passive seismic methods such as horizontal-to-vertical spectral ratio (HVSR). In our previous work, we found that HVSR of ambient seismic noise (HVSRN) is unstable below 0.1 Hz at three broadband Ocean Bottom Seismometer (OBS) stations offshore Fukushima in the southern Japan Trench. To overcome this limitation, we present a novel approach that estimates HVSR using the coda of earthquakes (HVSRC). Multiple earthquakes with long-duration coda (≥200 s) enabled stable HVSRC estimation in the 0.05–10 Hz range, effectively extending the lower frequency limit for offshore HVSR. While HVSRC and HVSRN differ below 0.1 Hz, they exhibit similar curves between 0.1–10 Hz, confirming the reliability of HVSRC at ocean-bottom sites. Moreover, theoretical HVSRs below 0.2 Hz, computed using the local structure, show good agreement with observations. We inverted the average HVSRCs using the diffuse field assumption and the recently developed ’hvgeneralized’ algorithm, which accounts for the seawater layer, and obtained high-resolution 1D shear-wave velocity profiles down to 10,000 m depth. These profiles delineate sedimentary and crustal layers, and at one OBS station, we identified the Japan Trench plate interface at ~7,000 m depth. This study demonstrates that HVSRC can mitigate low-frequency instability in ocean-bottom HVSR using a straightforward approach and significantly extend its applicability from shallow subsurface investigations to deep marine crustal imaging.

The southern Kuril Trench subduction zone experienced a M9‐class megathrust earthquake in the 17th century, and another is highly probable within the next 30 years. This earthquake likely exhibited large coseismic slip at the shallower plate boundary fault, causing a devastating tsunami, while the deeper plate boundary experienced smaller slip, similar to the 2011 Tohoku‐oki earthquake. To investigate the structural factors behind the contrasting slip behaviors, we conducted a controlled‐source seismic survey across the Kuril Trench axis. We found that the boundary between the largely‐slipped‐shallow and small‐slipped‐deeper faults aligns with a Vp transition zone in the overriding plate, characterized by sharp landward Vp increase from the low‐Vp frontal wedge (0–30 km from the trench) to the high‐Vp island arc crust (>60 km), and associated with discontinuous near‐horizontal reflectors zone (RZ) above the plate boundary, exhibiting negative polarity. These findings suggest that the slip behavior boundary correlates with sudden trenchward rigidity reduction along the plate boundary, as found in other areas where seismic rupture extended to the trench. The negative‐polarity reflections in the RZ, together with the plate boundary, imply high‐pore‐pressure created by abundant fluid supply from accreted or underplated sediments consolidation and/or dehydration, and oceanic crustal crack cementation. This suggests a weak fault strength at the plate boundary beneath the transition zone, indicating weak mechanical coupling similar to the shallow fault in Tohoku‐oki. Similar to the Tohoku earthquake, a slip‐to‐the‐trench rupture may be triggered by releasing large strain energy accumulated along the deeper fault beneath the high‐rigidity crust.