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Global Navigation Satellite System–acoustic (GNSS-A) positioning is an important geodetic observation technique for detecting seafloor crustal deformation. After the 2011 Tohoku-oki earthquake, GNSS-A observational networks were extended along various subduction zones, and observational systems have been improved, especially for sea-surface platforms, such as the introduction of an unmanned vehicle, the Wave Glider. The aforementioned development of GNSS-A observations has provided a large amount of observational data. Furthermore, GNSS-A positioning methods were recently developed considering the lateral heterogeneity of the sound speed structure. Thus, it is important to develop a software that makes it easy for widespread use of the latest GNSS-A positioning methods. However, there is currently only one open-source GNSS-A positioning software, which may hinder the entry of various researchers into GNSS-A positioning analyses. Here, we developed a new GNSS-A positioning software, henceforth called “SeaGap” (Software of enhanced analyses for GNSS-acoustic positioning), that executes various positioning methods from the conventional kinematic positioning technique to the latest Markov Chain Monte Carlo (MCMC)-based static positioning technique. We introduce their methodology and demonstrate its application to actual observational data. The software newly added optional prior distributions to the unknown parameters expressing the heterogeneity of a sound speed structure into the MCMC-based static positioning method, and we also applied the new method to actual observational data. In addition to the positioning functions, the software contains various auxiliary functions, including drawing. The developed software is written using the “Julia” language and is distributed as an open-source software.

Following the 2011 Tohoku Earthquake, we constructed seafloor geodetic benchmarks for GNSS-Acoustic measurement at twenty sites along the Japan trench in September 2012 and have started repeating surveys since then. Reliable horizontal displacement rates were obtained to date for a sufficiently long period of surveys, which revealed the coexistence of viscoelastic relaxation and after slips from place to place. Further analysis to estimate vertical motion, we preliminary exposed regions of uplift and subsidence, although the expected errors were still significant. However, the pattern of vertical motion gives independent information from the horizontal ones for verifying viscoelastic models and evaluating the extent of after slips.We introduced an unmanned autonomous vehicle, called Waveglider (WG), as a surface platform instead of a ship, which overcomes the deficiency in ship-time in the sense of budget and human resources. Actually, data obtained by WG bear comparison with that by shipboard survey, and even the onboard-processed data can be transmitted to the onshore station via satellite system nearly in realtime. Moreover, significantly intensive use of WG helps increase the survey frequency, which can have a chance to identify slow slip events; for instance, marine seismometers have revealed the existence of slow events off the Sanriku region near our survey sites. The WG deficit is sometimes trapped against sea current even along the Japan trench, typically when over 2 knots, and fails into low power conditions depending on weather, season, and solar culmination altitude. Well-organized planning and operation may reduce such deficit. More practically, the slower speed of WG prevents efficient moving survey over a transponder array, especially for deep (>5000m) sites having a large footprint of the array, which is typical for our sites. Insufficient moving survey degrades the accuracy in vertical crustal movement, the importance of which increases to monitor the afterslip distribution as noted above. Then we solve this problem using a different approach that utilizes different incident angles even in point survey by employing two concentric triangles of different sizes for a six transponder site or a triangle with a centered one for a four transponder site.If WG operation would become more common and can be appropriative, a fully continuous survey will be realized at a specific site without a moored buoy. This will be valuable not only for detecting temporal phenomena like slow slip events but also for disaster mitigation to monitor offshore fault failure in realtime. In addition, such prompt measurement just after a large earthquake reveals rapid postseismic deformation in an early stage. Long-term continuous operation requires particular battery specifications and operational fashion for seafloor transponders. We are also designing such transponders at this moment.

Obvious crustal deformation is observed during a postseismic period as well as a coseismic period associated with a large earthquake. Major mechanisms of transient postseismic deformation are known as afterslip and viscoelastic relaxation. Since the viscoelastic relaxation occurs as a response to a coseismic slip, postseismic deformation provides information on coseismic deformation through the viscoelastic response. However, most previous studies have not thoroughly utilized postseismic geodetic observational data for revealing coseismic slip behaviors. In this study, we developed a slip inversion method that simultaneously estimates coseismic slip and postseismic slip distributions from coseismic and postseismic geodetic observational data using viscoelastic Green’s function (viscoelastic inversion method). We investigated the performance of the viscoelastic inversion method via two synthetic tests: one assumed a strike–slip event along an inland fault, while the other assumed a dip–slip event along a plate interface in a subduction zone. Both synthetic tests demonstrated that when extensive postseismic observational data were given, the viscoelastic inversion method provided a superior spatial resolution of coseismic slip distributions compared to conventional elastic inversion distributions. We also applied the viscoelastic inversion method to co- and post-seismic deformations associated with the 2011 Tohoku-oki earthquake. The seafloor geodetic observational network of the off-Tohoku region has been widely extended after the occurrence of the mainshock. Using this extended seafloor geodetic observational data, we successfully improved the spatial resolution of the coseismic slip distribution through the viscoelastic inversion method. Furthermore, using the seafloor observational data during the postseismic period, our inversion method enables us to obtain high spatial resolution of the coseismic slip in the offshore area and a reasonable coseismic slip distribution even if seafloor observational data during the coseismic period are unavailable. These results clarify the importance of deploying a geodetic observational network even after large coseismic events to assess past coseismic slip behaviors by considering the viscoelasticity of the Earth.

GNSS-A (combination of Global Navigation Satellite System and Acoustic ranging) observations have provided important geophysical results, typically based on static GNSS-Acoustic positioning methods. Recently, continuous GNSS-Acoustic observations using a moored buoy have been attempted. Precise kinematic GNSS-Acoustic positioning is essential for these approaches. In this study, we developed a new kinematic GNSS-A positioning method using the extended Kalman filter (EKF). As for the observation model, parameters expressing underwater sound speed structure [nadir total delay (NTD) and underwater delay gradients] are defined in a similar manner to the satellite geodetic positioning. We then investigated the performance of the new method using both the synthetic and observational data. We also investigated the utility of a GNSS-Acoustic array geometry composed of multi-angled transponders for detection of vertical displacements. The synthetic tests successfully demonstrated that (1) the EKF-based GNSS-Acoustic positioning method can resolve the GNSS-Acoustic array displacements, as well as NTDs and underwater delay gradients, more precisely than those estimated by the conventional kinematic positioning methods and (2) precise detection of vertical displacements can be achieved using multi-angled transponders and EKF-based GNSS-Acoustic positioning. Analyses of the observational data also demonstrated superior performance of the EKF-based GNSS-Acoustic positioning method, when assuming a laterally stratified sound speed structure. Further, we found three superior aspects to the EKF-based array positioning method when using observational data: (1) robustness of the solutions when some transponders fail to respond, (2) precise detection for an abrupt vertical displacement, and (3) applicability to real-time positioning when sampling interval of the acoustic ranging is shorter than 30 min. The precision of the detection of abrupt steps, such as those caused by coseismic slips, is ~ 5 cm (1σ) using this method, an improvement on the precision of ~ 10 cm of conventional methods. Using the observational data, the underwater delay gradients and the horizontal array displacements could not be accurately solved even using the new method. This suggests that short-wavelength spatial heterogeneity exists in the actual ocean sound speed structure, which cannot be approximated using a simple horizontally graded sound speed structure.

The GNSS-acoustic technique is a geodetic method for oceanic areas that combines GNSS positioning of a sea-surface platform and acoustic ranging of seafloor stations. Its positioning accuracy is typically a few and several centimeters for the horizontal and vertical positions, respectively. For further accuracy enhancement, we examined the errors in travel time, the most fundamental data in acoustic ranging. The reference signal used in our observations is a series of sinusoidal waves modulated by binary phase-shift keying with a maximal length sequence whose auto-correlation has a clear main peak at zero lag. However, cross-correlation between the actual returned signal and reference signal is often accompanied by many large sidelobes and looks very different from the synthetic auto-correlation. As a practical measure, we have chosen empirically one peak among several comparable peaks in the cross-correlation, though that is likely to lead to systematic errors in travel time. In this study, we revealed that a variety of cross-correlation waveform primarily depends on the incident angle of acoustic paths and that sidelobes were significantly reduced by substituting phase-only correlation (POC) for conventional cross-correlation. We therefore developed a template-matching technique using POC for the peak detection. POC templates were prepared by stacking actual POCs with certain ranges of the incident angle for each campaign. In the application of this method to actual data, we achieved successful results of our numerous campaign data to date. We consider that POC is advantageous in identifying the main peak uniquely and performing template matching more robustly, because POC enhances short-period components and thus highlights the timing of phase changes further than conventional cross-correlation.

The Moon has an anorthositic primordial continental crust. Recently anorthosite has also been discovered on the Martian surface. Although the occurrence of anorthosite is observed to be very limited in Earth's extant geological record, both lunar and Martian surface geology suggest that anorthosite may have comprised a primordial continent on the early Earth during the first 600 million years after its formation. We hypothesized that differences in the presence of an anorthositic continent on an Earth-like planet are due to planetary size. Earth likely lost its primordial anorthositic continent by tectonic erosion through subduction associated with a kind of proto-plate tectonics (PPT). In contrast, Mars and the Moon, as much smaller planetary bodies, did not lose much of their anorthositic continental crust because mantle convection had weakened and/or largely stopped, and with time, they had appropriately cooled down. Applying this same reasoning to a super-Earth exoplanet suggests that, while a primordial anorthositic continent may briefly form on its surface, such a continent will be likely transported into the deep mantle due to intense mantle convection immediately following its formation. The presence of a primordial continent on an Earth-like planet seems to be essential to whether the planet will be habitable to Earth-like life. The key role of the primordial continent is to provide the necessary and sufficient nutrients for the emergence and evolution of life. With the appearance of a “trinity” consisting of (1) an atmosphere, (2) an ocean, and (3) the primordial continental landmass, material circulation can be maintained to enable a “Habitable Trinity” environment that will permit the emergence of Earth-like life. Thus, with little likelihood of a persistent primordial continent, a super-Earth affords very little chance for Earth-like life to emerge. Keywords: Anorthosite on Mars, Moon, Habitable trinity, Super-Earth, Plate tectonics, Origin of life

We have developed a comprehensive inversion scheme to analyze traveltime data collected through GPS-acoustic observations on a campaign basis. Our method uses a quantity called the “Nadir Total Delay” (NTD), which is analogous to the Zenith Total Delay (ZTD) in GNSS analyses, to represent the time variation of the sound speed. The observation equation using the NTD is an approximated formulation. We examine its applicable scope using numerical experiments, and demonstrate that the approximation holds well enough in practice. Traveltime data from all our observation campaigns were utilized together for inversion calculation to determine: (1) the positions of individual precision acoustic transponders (PXPs) at the time of a particular reference campaign, (2) the displacement from these positions of a rigid array at the time of each campaign, and (3) the time variation of the NTD during each campaign. We applied this method to actual data to fix the array geometry more precisely. Also, we have made a first step in detecting vertical motions as well as horizontal motions, though further enhancement of the accuracy will be required. In performing linear regression of array displacements obtained from the inversion, their error covariance has a significant effect on the resulting velocities. When the covariance is taken into account, nearly the same velocities are obtained no matter what campaign is taken as reference in the inversion.

Additional file 1: Figure S1. Schematic image of GNSS-A positioning bias caused by a horizontally graded sound speed structure. Figure S2. Schematic image of GNSS-A positioning through moving and point surveys. Figure S3. 2-D schematic image of seafloor transponder arrangements. Figure S4. Positioning results for the actual observation data at G19 on September 6, 2012. Figure S5. Positioning results for the actual observation data at G19 on July 25, 2013. Figure S6. Positioning results for the actual observation data at G19 on February 26, 2014. Figure S7. Positioning results for the actual observation data at G19 on November 15, 2015. Figure S8. Positioning results for the actual observation data at G19 on May 2, 2016.

Figure S1. Chart table of three observation campaigns for individual PXP positioning. Each set of observations consists of moving and/or point survey data. Figure S2. Time series of round-trip traveltimes obtained during moving and point surveys in Campaign 2. The traveltimes between the transducer and the PXPs are indicated by colored dots. Figure S3. Individual PXP positioning results. The colored dots and black lines indicate the observed and modeled NTD, respectively. Figure S4. Time series of round-trip traveltimes obtained during continuous observation using a moored buoy. The dots, representing the traveltimes, are color coded in the same way as in Additional file 1 Figure S2. Figure S5. Attitude angle of the buoy from GNSS gyro during continuous observation. The plots on the left and in the center show the heading, pitch, and roll of the buoy from top to bottom during (a) the entire period until the occurrence of missing data (after mid-October 2016) and (b) a 20-minute period of a GNSS-A observation sequence on 21 March 2016. (c) Power spectrum of the attitude angle on 21 March 2016. The colored lines correspond to the same spectra of heading, pitch, and roll as in (b). The characteristic periods were 3–4 sec in pitch and roll and 30–50 sec in pitch, roll and heading. Figure S6. Map view of DOPs using our array geometry. (a) EDOP, (b) NDOP, and (c) HDOP. The large DOP region (colored in shades of blue) indicates that the poisoning accuracy suffers. The contour lines are DOP intervals of 0.5. Figure S7. Map view of DOPs using outer three PXPs. (a) EDOP, (b) NDOP, and (c) HDOP. The color scale is the same as in Fig. S6. Black indicates a DOP value greater than 5. Figure S8. Map views of components of Equation (8). (a) Dxx, (b) Dyy, and (c) Dxy. The gray circle indicates a radius of 4 km from the array center. (DOCX 2155 kb)

The subduction rate of an oceanic plate may accelerate after large earthquakes rupture the interplate coupling between the oceanic and overriding continental plates. To better understand postseismic deformation processes in an incoming oceanic plate, we directly measured the displacement rate of the Pacific Plate near the Japan Trench after the 2011 Tohoku-Oki earthquake using a GPS/acoustic technique over a period of 2 years (September 2012 to September 2014). The displacement rate was measured to be 18.0 ± 4.5 cm yr−1 (N302.0°E) relative to the North American Plate, which is almost twice as fast as the predicted interseismic plate motion. Because the sum of steady plate motion and viscoelastic response to the Tohoku-Oki earthquake roughly accounts for the observed displacement rate, we conclude that viscoelastic relaxation is the primary mechanism responsible for postseismic deformation of the Pacific Plate and that significant subduction acceleration did not occur at least not during the observation period.

Seafloor displacements observed after the Tohoku earthquake place large constraints on assessing the deformation process., The 2011 Tohoku-oki earthquake was the largest earthquake ever observed with seafloor geodetic techniques in and around its source region. Large crustal deformation associated with both the coseismic rupture and the rapid postseismic deformation has been reported. However, these observations are insufficient to describe the postseismic deformation processes occurring around the broad rupture area. We report the first results of seafloor Global Positioning System and acoustic ranging (GPS-A) observations based on repeated campaign surveys conducted over nearly 4 years using the extended GPS-A network deployed along the Japan Trench in September 2012. The observed postseismic displacement rates (DRs) show evident spatial variation along the trench: (i) distinct landward DRs in the large coseismic slip area [primary rupture area (PRA)], evidencing the predominance of viscoelastic relaxation; (ii) remarkable trenchward DRs in the south of the PRA, indicating rapid afterslip; and (iii) slight trenchward DRs in the north of the PRA. These features provide great insights into constructing a more complete model of viscoelastic relaxation, and they also indicate spatial variation of afterslip and fault locking along the plate interface with clear spatial resolution, providing invaluable information for the improvement of seismic hazard assessment.

We have developed a new system for real-time observation of tsunamis and crustal deformation using a seafloor pressure sensor, an array of seafloor transponders and a Precise Point Positioning (PPP ) system on a buoy. The seafloor pressure sensor and the PPP system detect tsunamis, and the pressure sensor and the transponder array measure crustal deformation. The system is designed to be capable of detecting tsunami and vertical crustal deformation of ±8 m with a resolution of less than 5 mm. A noteworthy innovation in our system is its resistance to disturbance by strong ocean currents. Seismogenic zones near Japan lie in areas of strong currents like the Kuroshio, which reaches speeds of approximately 5.5 kt (2.8 m/s) around the Nankai Trough. Our techniques include slack mooring and new acoustic transmission methods using double pulses for sending tsunami data. The slack ratio can be specified for the environment of the deployment location. We can adjust slack ratios, rope lengths, anchor weights and buoy sizes to control the ability of the buoy system to maintain freeboard. The measured pressure data is converted to time difference of a double pulse and this simple method is effective to save battery to transmit data. The time difference of the double pulse has error due to move of the buoy and fluctuation of the seawater environment. We set a wire-end station 1,000 m beneath the buoy to minimize the error. The crustal deformation data is measured by acoustic ranging between the buoy and six transponders on the seafloor. All pressure and crustal deformation data are sent to land station in real-time using iridium communication.

DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) has been developed and installed around Nankai Trough, which is motivated by the 2004 Sumatra-Andaman Earthquake. DONET contains pressure gauges as well as seismometers, which are expected to detect crustal deformations driven by peeling off subduction plate coupling process. From our simulation results, leveling changes are different sense among the DONET points even in the same science node. On the other hand, oceanic fluctuations such as melting ice masses through the global warming have so large scale as to cause ocean bottom pressure change coherently for all of DONET points especially in the same node. This difference suggests the possibility of extracting crustal deformations component from ocean bottom pressure data by differential of stacking data. However, this operation cannot be applied to local-scale fluctuations related to ocean mesoscale eddies and current fluctuations, which affect ocean bottom pressure through water density changes in the water column (from the sea surface to the bottom). Therefore, we need integral analysis by combining seismology, ocean physics and tsunami engineering so as to decompose into crustal deformation, oceanic fluctuations and instrumental drift, which will bring about high precision data enough to find geophysical phenomena.

Ocean-bottom pressure records obtained near the epicenter of the 2011 Tohoku-Oki earthquake were examined to test whether the earthquake was preceded by substantial precursory crustal deformation. The seafloor data enabled us to search for small-scale preslip near the epicenter that would be difficult to identify from terrestrial geodetic data. After treating the data to reduce nontectonic fluctuations, we obtained a time series of seafloor vertical deformation in the epicentral region with a noise level of 2–4 cm. No significant crustal deformation related to preslip was detected in the period of roughly a day before the mainshock, whereas postseismic deformation associated with the largest foreshock 2 days before the mainshock was apparent. From our quantitative estimate of the sensitivity of the seafloor network in detecting slip on the plate interface, we conclude that the Tohoku-Oki earthquake was not preceded by preslip with moment release greater than moment magnitude (Mw) 6.2 in the vicinity of the hypocenter or greater than Mw 6.0 along the subduction interface near the trench.

We have developed a seafloor acoustic ranging system to monitor local seafloor crustal deformation. In August 2007 we carried out an experiment to estimate the long-term stability of acoustic measurements collected by the system. We deployed four precision acoustic transponders with about 920 m and 590 m spacing at the depth of 2035 m in Kumano-nada, southeast coast Japan, and we collected acoustic ranges for 4 months. The observed round-trip travel times showed a variation with peak-to-peak amplitude of about 0.75 m in the range. It was confirmed that most of the variation could be explained by changes in sound speed. The standard deviation in the acoustic measurements was about 1.0 cm after the correction for sound speed, and the average value of the ranges remained constant within 1 cm.

Faults related to the tsunamigenic 2011 Tohoku-Oki Earthquake (Mw 9.0) were investigated by using multi-channel seismic reflection data acquired in 1999 and submersible seafloor observations from 2008. The location of the fault system interpreted in the seismic reflection profile is distributed around the area with largest slip and tsunami induction of the 2011 event. Cold-seep communities along the trace of the branch reverse fault and a high scarp associated with the trace of a normal fault suggest current activity on these faults. We interpret the fault system in the seismic profile as a shallow extension of the seismogenic fault that may have contributed to the resulting huge tsunami.

We have developed a seafloor acoustic ranging system as a possible future application to monitoring seafloor crustal movement with the DONET (Development of Dense Ocean-floor Network System for Earthquake and Tsunami) cabled observatory system. In 2007 we carried out an experiment for the seafloor acoustic ranging system. We deployed two precision acoustic transponders (PXPs) with about 750 m spacing in Kumano-nada at a water depth of about 2035 m. We collected 660 ranging data in this one-day experiment. The round-trip travel time shows a variation with peak-to-peak amplitude of about 25 mm in the range. It was confirmed that most of the variation could be explained by the change in sound speed estimated from measured temperature and pressure. The remaining fluctuation in the acoustic measurements is ±2 mm.

The GPS/acoustic technique applied to seafloor geodesy intrinsically measures integrated sound speed along a trajectory of an acoustic signal as well as the position of a seafloor transponder array. We present here a generalized expression of sound speed variation in terms of a traveltime residual normalized to the vertical component. With this expression, residual traveltimes to any seafloor transponders will have a same value regardless of their depths and slant angles. This is valid even for the case having horizontal gradient in sound speed structure; the gradient affects only on positioning of a transponder array and not on the estimate of sound speed just beneath the observation point. We monitored temporal variation of this quantity through a GPS/acoustic survey and compared it with in situ expendable bathythermograph (XBT) measurements periodically carried out during the survey. We found that the relative change of the two independent measurements are in good agreement within 5% of the typical amplitude of temporal variation.

The 2011 Tohoku Earthquake (M9.0), which occurred on the plate boundary between the subducting Pacific plate and continental plate has been associated with postseismic deformation, including aseismic slip at the plate interface (postseismic slip). In order to evaluate the potential for further seismic activity, we investigated the spatial and temporal evolution of the postseismic slip based not only on terrestrial GPS data but also on seafloor geodetic data. We estimated the displacements due to the postseismic slip by subtracting the displacements due to large aftershocks and viscoelastic relaxation from the original displacement time series data and used a time-dependent inversion method to estimate the postseismic slip distributions. The resultant postseismic slip distributions depend strongly on the assumed value of the viscosity. However, the following two features are independent of the viscosity assumption: (1) large postseismic slip has been occurring at a very shallow ( ≤ 20 km in depth) portion of the plate interface south of the area of huge coseismic slip and (2) significant postseismic slip has occurred at a deep (approximately 50 km in depth) portion of the plate interface. The results suggest that the elastic strain and the stress concentrated at the plate interface at a depth of approximately 30 km in the segment off the Boso Peninsula have not yet been released and continue to generate large aftershocks.

In this study, we investigated the state of interplate coupling between the subducting Pacific plate and continental plate around the rupture area of the 2011 Tohoku Earthquake (M9.0) based on seafloor geodetic data as well as terrestrial GPS data obtained before the M9 earthquake in order to evaluate the potential for further seismic activity from the viewpoint of the slip deficit budget balance. Due to the difference between the site spatial densities on land and the seafloor observation networks, it was necessary to utilize a covariance matrix with non-zero off-diagonal components in the inversion to estimate the interplate coupling distribution on the plate interface and to account for the correlations among the observation errors for components and stations. We validated the implementation of the assumed covariance matrix by performing a computational test and we then applied the method to actual geodetic data obtained before the 2011 Tohoku Earthquake. The results demonstrate the persistence of the interplate coupling in the area of a very large coseismic slip near the Japan Trench.

We constructed a buoy system for real-time observations of tsunamis and crustal deformation in collaboration with the Japan Agency for Marine-Earth Science and Technology, Tohoku University, and the Japan Aerospace Exploration Agency. The most important characteristics of our system are resistance to the strong sea currents in the large-earthquake rupture zone around Japan (e.g., the Kuroshio maximum speed > 5 knots), and the capability to transmit data in real-time. Our system has four units: (1) a buoy station with a GPS/Acoustic station serving as a central base, (2) a wire-end station (WES) 1,000 m below the sea surface that serves as a staging base, (3) a pressure seafloor unit (PSU) comprising a pressure sensor, and (4) six GPS/Acoustic transponders to measure crustal deformation. The pressure data used to detect tsunamis and the vertical component of crustal deformation are sent to the land station via the wire-end and buoy stations at intervals of 1 h in normal mode and 15 s in tsunami mode. The data measured between the buoy and six transponders are also sent to the land station at 1-week intervals. The Iridium satellite is used for data transmission of all data to land station. The dynamic range for pressure observations is + ∕− 8 m with a fine resolution of 2 mm, and the accuracy of the crustal deformation measurements is less than 1 m. We tuned the system for an observation period of 5 months and carried out a sea trial. The length of the observation period influences the total system due to the weight of the battery. We rearranged the geometry of the total system to new one with heavier weight and a lot of batteries on the buoy considering long period observation and decided upon a slack ratio of 1.6. In addition, it is important for a long observation period to minimize electrical consumption. We used double pulses for acoustic data transmission between the PSU and WES. The time difference between two pulses indicates the observed pressure value. For the PSU, we designed a tsunami mode on the basis of data from the tsunami generated by the 2011 earthquake off Tohoku, which were recorded by cabled network system data and offline bottom pressure data. The results confirmed that a tsunami can be detected even if the first tsunami signals include strong-motion signals. In this case, the tsunami was detected 10–20 s after the first seismic arrival. During sea trials, we successfully tested the tsunami mode we designed. We succeeded real-time observation of pressure and crustal deformation using buoy system in strong sea current speed area for 5 months. However, there are some issues to be resolved at this moment. For acoustic data transmission, 1 ms step difference of the detection of acoustic signals at the WES, wrong detection of the multiple phases are issues to be resolved. We will consider assigned mapping of transmitted data to the time difference of the double pulses and take measures on the PSU and WES. In addition, we consider strategy to reduce slack ratio in the future. For data transmission from the WES to the buoy station, we experienced electrical unhealthy of the wire rope due to damages by the fisheries activities and the torsion brought by rotation of the buoy. We consider the countermeasure to reduce the rotation.

A component of the multi-purpose moored buoy has been improved for the instantaneous detection of seafloor displacement and possible tsunami generation upon the occurrence of large earthquakes. Here, improvements to the acoustic ranging component of this buoy, which is a key element of on-demand GPS/acoustic (GPS/A) measurements, are demonstrated. A 1-m positioning accuracy is required in GPS/A measurements using the buoy system for the detection of the horizontal seafloor crustal deformation associated with large earthquakes. Owing to the limitation of collecting data on only a limited range of sub-surface depth from a single point located far from the optimal location, resulting from the slack mooring system, obtaining the positioning accuracy is challenging. To overcome this challenge, we developed an automatic travel-time algorithm that reliably excludes the multipath from acoustic waves. Applying the algorithm to the data from 4 months of sea trial, we revealed that the short-period repeatability of the positioning improved from 4 to 0.5 m, while the long-term repeatability improved from 8 to 4 m, which is still beyond the required accuracy of 1 m. Because acoustic ranging under sub-optimal conditions will propagate any error in the data at intermediate steps into the uncertainty of the final positioning, approaches to reduce the errors at each step must be undertaken, such as determination of the pre-defined geometry of seafloor transponders more precisely.

We have developed a 2D isotropic continuous wavelet-like transform for a spherical surface. The transform is simply defined as the surface convolution between the original field and a kernel, based on the zeroth-order Bessel function with a spherical correction. This spherical correction violates the geometric similarity for the various scales of the kernels, which becomes more apparent at longer wavelengths. We found numerically that this transform is practically equivalent to a Gaussian bandpass filter in the spherical harmonic domain. We have applied this wavelet-like transform on the recently acquired Martian gravity and topography fields. Using a ratio constructed locally from these two fields, we have constructed a map describing the lateral variations of the localized admittance function on Mars.

We present imaging results in geophysics based on using multidimensional Gaussian wavelets as a filter in a 2-D Cartesian domain. Besides decomposing the field into various distinct lengthscales, we have also constructed the 2-D maps describing the spatial distributions of the maximum of the wavelet-transformed L2-norm Emax (x,y) and its corresponding local wavenumber k max (x,y),where x and y are the Cartesian coordinates. For geoid anomalies, using a wavelet filter extending to 90 degrees, we have discerned the distinct outlines of convergent and divergent tectonic zones and have conducted a quantitative comparison of the short-wavelength gravitational anomalies at those wavelengths between two different geographical locations. We have also compared the wavelet results with a nonlinear bandpass filter in the spectral domain where a Gaussian filter with the logarithm of the degree/acting as the argument has been employed. A wavelet solution, with a length-scale corresponding to 256 degrees, would need a filter with over 400 spherical harmonics centering around l = 157 for an optimal spatial fit. The computational effort with the bandpass filter technique greatly exceeds those associated with wavelets. We have also shown the ability of the wavelets to analyze the vastly different scales present in high Rayleigh number convection and the mixing of passive heterogeneities driven by thermal convection. Wavelets will be a useful tool for rapid analyzing of the large multidimensional fields to be captured in many other geophysical endeavors, such as the upcoming gravity satellite missions and satellite radar interferometry images.

After a large subduction earthquake, crustal deformation continues to occur, with a complex pattern of evolution. This postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip (afterslip) or relocking of different parts of the fault. When postseismic geodetic observations are used to study Earth's rheology and fault behaviour, it is commonly assumed that short-term (a few years) deformation near the rupture zone is caused mainly by afterslip, and that viscoelasticity is important only for longer-term deformation. However, it is difficult to test the validity of this assumption against conventional geodetic data. Here we show that new seafloor GPS (Global Positioning System) observations immediately after the great Tohoku-oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short-term postseismic deformation. These data reveal fast landward motion of the trench area, opposing the seaward motion of GPS sites on land. Using numerical models of transient viscoelastic mantle rheology, we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake, a process previously unknown because of the lack of near-field observations. Our findings indicate that previous models assuming an elastic Earth will have substantially overestimated afterslip downdip of the rupture zone, and underestimated afterslip updip of the rupture zone; our knowledge of fault friction based on these estimates therefore needs to be revised.

Our previous work from regional geoid inversion over oceans has shown that there exists a low viscosity zone (LVZ) under the 660 km boundary. Nonlinear inversion has been applied for fitting to the intermediate-wavelength oceanic geoid without invoking a priori any impermeable boundary. In this study we have conducted a systematic search of the dependence of the depth of this regional boundary from 660 to 1000 km. We found that the use of this boundary at the depth of the LVZ exerts little influence on the inversion. The LVZ can act regionally as an impermeable boundary, and enhances very fast lateral flow at the uppermost lower mantle. On the other hand, impermeable boundary alone is not caused by a LVZ, as shown also by analysis of the 2D flow patterns, driven by internal density anomalies inferred from tomography. From the intermediate-wavelength geoid, we can not resolve the existence of layering, when the LVZ exists beneath the 660 km boundary. However, a LVZ is needed to fit the geoid. This LVZ can have profound influence on the geodynamical behavior of mantle circulation and the dispersal of geochemical anomalies and the generation of seismic anisotropy.

We revisited the resolving power of viscosity inversion in terms of geoid misfit in a 2-D Cartesian geometry under the assumption that the mantle viscosity is laterally stratified. Firstly, we considered a simple case of two viscosity layers only, which is described by two parameters of the amount and the depth of the viscosity jump. The uniqueness of the inversion was examined by evaluating misfits between the reference geoid for a reference viscosity and that for a viscosity described by the changing two parameters. The misfits are mapped into 2-D model space as a function of the two parameters. Three types of density distribution are tested; they are vertically constant (1), taken from a tomographic model (2), and the same but includes artificial noise (3). We found that, at least for this simple case, the viscosity solution keeps unique in the entire 2-D model space using whole degree band (1–8) of geoid. This holds even if the artificial noise is rather large (70%), though the solution is slightly different from the reference viscosity. However, we also observed non-uniqueness, such as trade-off between the two parameters, when individual degree components of geoid are concerned. In the next, we employed a more realistic viscosity structure, having seven iso-viscous layers. It is no longer possible to describe 6-D model space easily. Therefore we tried to reconstruct a reference viscosity from the reference geoid using genetic algorithm search. According to this analysis, nearly the same solution with the reference viscosity can be reconstructed, while solutions apart from the reference viscosity with increase of noise in the density distribution.

We have applied the genetic algorithm (GA) technique, a nonlinear global optimization method, to determine the radial viscosity structure of the mantle from regional geoidal patterns. From numerical simulations of 2-D mantle convection, we examine the horizontal spectra of the vertical mass flux at 660 km depth and find that for long wavelengths there are minor differences between partially layered convection induced by the phase transitions and mantle convection without any phase transition. The differences in the spectra of the vertical mass flux become more prominent at shorter wavelengths. This result has led us to study mantle viscosity for the intermediate wavelength geoid from the whole-mantle and blocked-flow situations, in which the appropriate boundary condition is imposed on the radial velocity at 660 km depth. In order to confirm the robustness of this study, two different density models have been used, which were constructed from three tomographic models and appropriate velocity-to-density scaling relations based on recent results from mineral physics. We have analyzed only oceanic geoid spanning between spherical harmonic degree l=12–25. The correlation of the predicted geoid with the observations over the Atlantic, Indian, and Pacific Oceans have been employed as the fitting function in our GA approach, which has been modified from the common algorithm. In constructing the families of suitable viscosity profiles, we have used 100 parents, which have been iterated for 100 generations, and have been started with 10 different sets of initial parents. Convergence to acceptable viscosity solutions is obtained for all the three oceans and for both the whole-mantle and layered models. In some cases multiple viscosity solutions are found acceptable by using the correlation criteria. The outstanding feature of these models is the nearly ubiquitous presence of two low viscosity zones, one lying under the lithosphere, the other right under the bottom of the spinel to perovskite phase change. The solutions for the whole-mantle model can fit better and are preferred over the solutions with the layered boundary condition, which generally result in unrealistic viscosity profiles. Our results would suggest a more complex mantle viscosity structure, which has not been detected previously from geoid signals with longer wavelengths, and also reveal the potential difficulties in treating the dynamical boundary condition at the 660 km discontinuity.

GPS/Acoustic seafloor geodetic observation played a key role to estimate the slip distribution of the 2011 giant Tohoku-oki earthquake. After the event, seismic coupling near the trench axis has been a focus to understand how such a huge earthquake occurs in a subduction zone. Observation of postseimic deformation is another and urgent task required in the Japan Trench. In order to realize these observations, we have newly installed 20 GPS/A observation sites extending some 400 km along the Japan Trench in September 2012. Precision acoustic transponders were newly developed to cope with the following three requirements: (1) observation on the deep seafloor, (2) compatibility among three institutions in Japan, and (3) observation for ten years.

The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow-spreading lid-ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the avcrage length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30'N, the opening of the trough started about 25 Ma with an average half-spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active lid-ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active lid-ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma.

[1] On 11 March 2011, the devastating M9.0 Tohoku Earthquake occurred on the interface of the subducting Pacific plate, and was followed by a huge tsunami that killed about 20,000 people. Several geophysical studies have already suggested that the very shallow portion of the plate interface might have played an important role in producing such a large earthquake and tsunami. However, the sparsity of seafloor observations leads to insufficient spatial resolution of the fault slip on such a shallow plate interface. For this reason, the location and degree of the slip has not yet been estimated accurately enough to assess future seismic risks. Thus, we estimated the coseismic slip distribution based on terrestrial GPS observations and all available seafloor geodetic data that significantly improve the spatial resolution at the shallow portion of the plate interface. The results reveal that an extremely large (greater than 50 m) slip occurred in a small (about 40 km in width and 120 km in length) area near the Japan Trench and generated the huge tsunami. The estimated slip distribution and a comparison of it with the coupling coefficient distribution deduced from the analysis of the small repeating earthquakes suggest that the 2011 Tohoku Earthquake released strain energy that had accumulated over the past 1000 years, probably since the Jogan Earthquake in 869. The accurate assessments of seismic risks on very shallow plate interfaces in subduction zones throughout the world can be obtained by improving the quality and quantity of seafloor geodetic observations.

Ocean depth is affected by dynamic topography caused by mantle flow. Regional bathymetric deviations from the cooling plate age-depth curve, called residual depth anomalies, can therefore be used as indicators of dynamic topography in oceans. In this work, we first evaluated the oceanic residual depth anomalies. We then compute the dynamic topography for the flow induced by density perturbations converted from seismic tomography models by assuming δρ∝δν. We found that the predicted dynamic topography correlates with the depth anomalies when the density perturbations in the shallow part of the upper mantle were inferred from slab densities, not from tomographic models. We estimated a new age-depth curve based on depths corrected for dynamic topography. This corrected age-depth curve shows that the corrected depths for old seafloor (70∼110 Ma) are a few hundred meters deeper than those uncorrected.

[1] Seafloor geodetic measurements using the GPS/acoustic technique just above the rupture area of the 2011 Tohoku-Oki earthquake revealed strong trench-normal variation in the horizontal crustal displacement associated with the earthquake. One set of observations, at a site only 50 km away from the trench, yielded the most trench-ward data ever reported with an accuracy better than 1 m. It showed a coseismic displacement of up to 31 m, whereas another set of observations 100 km away from the trench showed a 15-m displacement. These horizontal data, as well as vertical data from pressure gauges and tsunami observations, strongly indicate that the seismic slip reached the trench. They are also likely to contribute quantitatively to further investigations that reveal the coseismic behavior of the frontal wedge of the overriding plate in the source region along the Japan Trench.

Acoustic signal processing is crucial for precise positioning of a seafloor transponder in GPS/Acoustic seafloor geodesy, which aims at 1 cm-precision. For this purpose, pseudorandom signals are usually employed for acoustic ranging. Considering the characteristics of the signal and the typical frequency of 10 kHz in carrier wave, detecting an arrival-time with 0.01 ms (∼0.75 cm) resolution in round trip can be readily achieved if the correct peak in correlogram is identified. However, due to sidelobes around the peak and pseudo peaks associated with possible multipath, a simple algorithm detecting the maximum peak often results in a wrong peak, which has a offset with a wavelength or larger from the correct peak. In our numerical study, behavior of the pseudo peak is found being quite complex having a no-integer wavelength offset when a short-range multipath is superimposed. Our observed data clearly show nadir-angle dependence in the appearance of received signals. This is attributed to significant change in the short-range multipath with angle within a sound-hood of a transducer as well as transducer characteristics itself. In this study, firstly we demonstrate the complexity in the behavior of the sidelobes with varying offset of multipath. Then, we analyze observed signal to reveal the angle-dependency of our transducer with different types of sound-hoods.