Recent advances in GNSS-A observation technology and networks and latest observation results around Japan Islands

2020 ◽  
Author(s):  
Yusuke Yokota ◽  
Tadashi Ishikawa ◽  
Shun-ichi Watanabe ◽  
Yuto Nakamura
Keyword(s):  
Subduction Zones ◽  
Large Scale ◽  
Surface Deformation ◽  
Nankai Trough ◽  
Detection Sensitivity ◽  
Japan Trench ◽  
Observation Data ◽  
Coupling Condition ◽  
Slow Slip Events ◽  
Combination Technique

<p>Our research group has been studying advanced GNSS-A (Global Navigation Satellite System – Acoustic ranging combination) technique over two decades. In recent years, detection sensitivity of GNSS-A observations has been sophisticated by improving the accuracy and frequency of analysis technology and acoustic systems [e.g., Yokota et al., 2018, MGR; Ishikawa et al., in prep]. The current observation frequency is more than 4 times/year, the observation accuracy for each observation is less than 2 cm, and it can detect a steady deformation rate of 1 cm/year or less and an unsteady fluctuation of 5 cm or less. Also, efforts are being made to strengthen the observation network.</p><p>GNSS-A observations for the 2011 Tohoku-oki earthquake and its postseismic field revealed the details of the crustal deformation field on the Japan Trench side [Sato et al., 2011, Science; Watanabe et al., 2014, GRL]. The long-term observation data in the Nankai Trough region revealed the strain accumulation process at the interseismic period [Yokota et al., 2016, Nature; Watanabe et al., 2018, JGR; Nishimura et al., 2018, Geosphere]. Furthermore, detection and monitoring of large-scale slow slip events (SSEs) in the shallow part of the Nankai Trough was achieved by recent sensitivity improvements [Yokota & Ishikawa, 2020, Science Advances]. The detected postseismic fields, coupling condition and shallow SSEs contain universal features that should be shared in many subduction zones. Here, along with the latest observations, we discuss spatial and temporal relationships of these events, strain accumulations and releases along subduction zones around Japan by GNSS-A and its impact on slow earthquake science.</p><p>Recently, because of the need for continuous monitoring a shallow SSE, the monitoring ability of GNSS-A was also investigated. It was confirmed that relatively large-scale shallow SSE (surface deformation: > 5 cm) could be monitored. However, the ability to determine the time constant of an SSE is poor. For monitoring the detail of an SSE, it is essential to improve the observation frequency in the future. Here, we also discuss the technical issues to be considered and their solution plans (e.g., new platform and system).</p>

scholarly journals Shallow slow slip events along the Nankai Trough detected by GNSS-A

2020 ◽  
Vol 6 (3) ◽  
pp. eaay5786 ◽  
Author(s):  
Yusuke Yokota ◽  
Tadashi Ishikawa
Keyword(s):  
Subduction Zones ◽  
Nankai Trough ◽  
Plate Boundary ◽  
Low Frequency ◽  
Satellite System ◽  
Boundary Region ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Combination Technique ◽  
Slow Earthquakes

Various slow earthquakes (SEQs), including tremors, very low frequency events, and slow slip events (SSEs), occur along megathrust zones. In a shallow plate boundary region, although many SEQs have been observed along pan-Pacific subduction zones, SSEs with a duration on the order of a year or with a large slip have not yet been detected due to difficulty in offshore observation. We try to statistically detect transient seafloor crustal deformations from seafloor geodetic data obtained by the Global Navigation Satellite System-Acoustic (GNSS-A) combination technique, which enables monitoring the seafloor absolute position. Here, we report the first detection of signals probably caused by shallow large SSEs along the Nankai Trough and indicate the timings and approximate locations of probable SSEs. The results show the existence of large SSEs around the shallow side of strong coupling regions and indicate the spatiotemporal relationship with other SEQ activities expected in past studies.

scholarly journals Improving Detectability of Seafloor Deformation From Bottom Pressure Observations Using Numerical Ocean Models

2021 ◽  
Vol 8 ◽  
Author(s):  
Yoichiro Dobashi ◽  
Daisuke Inazu
Keyword(s):  
Atmospheric Pressure ◽  
General Circulation ◽  
Subduction Zones ◽  
Nankai Trough ◽  
General Circulation Models ◽  
Japan Trench ◽  
Observation Data ◽  
Bottom Pressure ◽  
Seafloor Deformation ◽  
Ocean Models

We investigated ocean bottom pressure (OBP) observation data at six plate subduction zones around the Pacific Ocean. The six regions included the Hikurangi Trough, the Nankai Trough, the Japan Trench, the Aleutian Trench, the Cascadia Subduction Zone, and the Chile Trench. For the sake of improving the detectability of seafloor deformation using OBP observations, we used numerical ocean models to represent realistic oceanic variations, and subtracted them from the observed OBP data. The numerical ocean models included four ocean general circulation models (OGCMs) of HYCOM, GLORYS, ECCO2, and JCOPE2M, and a single-layer ocean model (SOM). The OGCMs are mainly driven by the wind forcing. The SOM is driven by wind and/or atmospheric pressure loading. The modeled OBP was subtracted from the observed OBP data, and root-mean-square (RMS) amplitudes of the residual OBP variations at a period of 3–90 days were evaluated by the respective regions and by the respective numerical ocean models. The OGCMs and SOM driven by wind alone (SOMw) contributed to 5–27% RMS reduction in the residual OBP. When SOM driven by atmospheric pressure alone (SOMp) was added to the modeled OBP, residual RMS amplitudes were additionally reduced by 2–15%. This indicates that the atmospheric pressure is necessary to explain substantial amounts of observed OBP variations at the period. The residual RMS amplitudes were 1.0–1.7 hPa when SOMp was added. The RMS reduction was relatively effective as 16–42% at the Hikurangi Trough, the Nankai Trough, and the Japan Trench. The residual RMS amplitudes were relatively small as 1.0–1.1 hPa at the Nankai Trough and the Chile Trench. These results were discussed with previous studies that had identified slow slips using OBP observations. We discussed on further accurate OBP modeling, and on improving detectability of seafloor deformation using OBP observation arrays.

Complexity of low-frequency earthquakes activity in western Shikoku

2020 ◽  
Author(s):  
Natalia Poiata ◽  
Jean-Pierre Vilotte ◽  
Nikolai Shapiro ◽  
Mariano Supino ◽  
Kazushige Obara
Keyword(s):  
Subduction Zones ◽  
Large Scale ◽  
Active Faults ◽  
Low Frequency ◽  
Space Time ◽  
Interaction Patterns ◽  
Slow Slip Events ◽  
Time Activity ◽  
Tectonic Tremor ◽  
Slow Earthquakes

<p>Short-duration transient seismic events known as low-frequency earthquakes (LFEs) are a component of the slow earthquakes family observed in the transition zone, at the root of seismogenic regions of the subduction zones or active faults. LFEs are the signature of impulse seismic energy radiation associated to and often mixed within complex tectonic tremor signal. Detailed analysis and characterization of LFE space-time activity in relation to other slow earthquake phenomena can provide important information about the state and the processes of fault interface.</p><p>We derive a catalog of LFEs in western Shikoku (Japan) by applying a full waveform coherency-based detection and location method to the 4-year continuous data covering the period of 2013-2016 and recorded at Hi-net seismic stations of NIED. The obtained catalog of over 150,000 detected events allows looking into the details of LFE space-time activity during the tectonic tremor sequences and inter-sequence periods.</p><p>We use this catalogue of LFEs to perform a systematic statistical analysis of the event occurrence patterns by applying correlation and clustering analysis to infer the large-scale (long temporal ~ 1-2 day duration) space-time characteristics and interaction patterns of activity and its potential relation to the structural complexity of the subducting plate. We also analyze the correlation between the migration of clustered LFE activity during energetic tremor sequences and short-term slow slip events occurring in the area during the analyzed period.</p>

Towards assessing the link between slow slip and seismicity with a Deep Learning approach

2020 ◽  
Author(s):  
Giuseppe Costantino ◽  
Mauro Dalla Mura ◽  
David Marsan ◽  
Sophie Giffard-Roisin ◽  
Mathilde Radiguet ◽  
...  
Keyword(s):  
Deep Learning ◽  
Subduction Zones ◽  
Surface Deformation ◽  
Short Term Memory ◽  
Ground Deformation ◽  
Japan Meteorological Agency ◽  
Joint Analysis ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Seismicity Rate

<p>The deployment of increasingly dense geophysical networks in many geologically active regions on the Earth has given the possibility to reveal deformation signals that were not detectable beforehand. An example of these newly discovered signals are those associated with low-frequency earthquakes, which can be linked with the slow slip (aseismic slip) of faults. Aseismic fault slip is a crucial phenomenon as it might play a key role in the precursory phase before large earthquakes (in particular in subduction zones), during which the seismicity rate grows as well as does the ground deformation. Geodetic measurements, e.g. the Global Positioning System (GPS), are capable to track surface deformation transients likely induced by an episode of slow slip. However, very little is known about the mechanisms underlying this precursory phase, in particular regarding to how slow slip and seismicity relate.</p><p>The analysis done in this work focuses on recordings acquired by the Japan Meteorological Agency in the Boso area, Japan. In the Boso peninsula, interactions between seismicity and slow slip events can be observed over different time spans: regular slow slip events occur every 4 to 5 years, lasting about 10 days, and are associated with a burst of seismicity (Hirose et al. 2012, 2014, Gardonio et al. 2018), whereas an accelerated seismicity rate has been observed over decades that is likely associated with an increasing shear stress rate (i.e., tectonic loading) on the subduction interface (Ozawa et al. 2014, Reverso et al. 2016, Marsan et al. 2017).</p><p>This work aims to explore the potential of  Deep Learning  for better characterizing the interplay between seismicity and ground surface deformation. The analysis is based on a data-driven approach for building a model for assessing if a link seismicity – surface deformation exists and to characterize the nature of this link. This has potentially strong implications, as (small) earthquakes are the prime observable, so that better understanding the seismicity rate response to potentially small slow slip (so far undetected by GPS) could help monitoring those small slow slip events. The statistical problem is expressed as a regression between some features extracted from the seismic data and the GPS displacements registered at one or more stations.</p><p>The proposed method, based on a Long-Short Term Memory (LSTM) neural network, has been designed in a way that it is possible to estimate which features are more relevant in the estimation process. From a geophysical point of view, this can provide interesting insights for validating the results, assessing the robustness of the algorithms and giving insights on the underlying process. This kind of approach represents a novelty in this field, since it opens original perspectives for the joint analysis of seismic / aseismic phenomena with respect to traditional methods based on more classical geophysical data exploration.</p>

Scientific Drilling Program of Drilling Vessel Chikyu and Drilling Data Acquisition for Future Technical Development

2011 ◽  
Author(s):  
Tomoya Inoue ◽  
Kazuyasu Wada ◽  
Eigo Miyazaki ◽  
Tsuyoshi Miyazaki
Keyword(s):  
Data Acquisition ◽  
Subduction Zones ◽  
Large Scale ◽  
Nankai Trough ◽  
Technical Development ◽  
Seismogenic Zone ◽  
Validity Data ◽  
Scientific Drilling ◽  
Drilling Data ◽  
Drilling Operations

The scientific drilling vessel Chikyu has started drilling at Nankai trough under the international organization, IODP. The Nankai trough located beneath the ocean off the southwest coast of Japan is one of the most active earthquake zones on the planet and one of the best-studied subduction zones as well. The Nankai Trough Seismogenic Zone Experiment attempts for the first time to drill, sample, and instrument the earthquake-causing or the seismogenic portion of Earth’s crust, where violent, large-scale earthquakes have occurred repeatedly throughout history. Before starting the international drilling operations, an integration drilling test off Shimokita Peninsula was conducted and we acquired actual drilling data such as vessel heave, hook load, and compensator position. Confirming its validity, data acquisition systems have worked continuously in international drilling operations. It is very important to consider the actual drilling data for the drilling operation and for further technical development. This paper describes the scientific drilling programs of the drilling vessel Chikyu and the drilling data acquisition for future technical development in relation with the sample data acquired in the internal drilling operations.

scholarly journals Mantle-derived helium released through the Japan trench bend-faults

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Jin-Oh Park ◽  
Naoto Takahata ◽  
Ehsan Jamali Hondori ◽  
Asuka Yamaguchi ◽  
Takanori Kagoshima ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Japan Trench ◽  
Helium Isotope ◽  
Normal Faults ◽  
Circulation System ◽  
Deep Penetration ◽  
Oceanic Plate ◽  
Seismic Reflection Data ◽  
Reflection Data

AbstractPlate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.

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