Recent progress of seafloor observations for earthquake and crustal deformation, such as deployme... more Recent progress of seafloor observations for earthquake and crustal deformation, such as deployment of submarine cable networks of S-net and DONET, and repeated observations of GNSS/A and acoustic extensometer (direct path acoustic ranging), enable us to evaluate ongoing crustal activities in the megathrust regions along the Japan trench and the Nankai trough. We review the present status and the future plans of such seafloor observations, and discuss the future directions of seafloor observation networks, especially for real-time monitoring of crustal activities. Toward these directions, we welcome papers introducing the present status of novel approaches and systems such as optical fiber, laser ranging or seafloor SAR and real-time geodetic observations using mooring buoys or wave glider, and so on. We also welcome future plans to integrate observation for the crustal activity under the seafloor with observation for ocean and climate changes.
ABSTRACT Hydrothermal vent fields are known to exist on the spreading axis, where sea water penet... more ABSTRACT Hydrothermal vent fields are known to exist on the spreading axis, where sea water penetrates into the crust and upwells through the hydrothermal vents. Understanding of the hydrothermal circulation system is extremely important to reveal the cooling process of the oceanic crust. The thermal structure beneath the hydrothermal vent reflects the extent of underground activity and the convection scale of the hot water. Temperature in the crust can be estimated from the electrical conductivity because the conductivity depends on the water volume, the salinity concentration and the temperature of the sea water in the crust. The Alice Spring Field (18 o12.9'N, 144 o42.5'E and 3600m deep), on the spreading axis in the central Mariana Back-Arc Basin, is a suitable site for this purpose. Hydrothermal vent in this field was firstly discovered by Alvin in 1987 (Hawkins et al., 1990). Shinkai6500 also confirmed the hydrothermal activity in 1992 and 1996 (Gamou et al., 1994; Fujikura et al., 1997). In November, 2002, we conducted Magnetometric Resistivity (MMR) survey using R/V Kairei, JAMSTEC in this field. In the MMR method, controlled electric current was applied from a pair of electrodes; one is just beneath the sea surface and the other is close to the seafloor. To record electoromagnetic responses of the crust to the inputed current, we deployed six ocean bottom electromagnetometers (OBEMs), which can measure 3-components of magnetic and electric fields simultaneously. Measurements were conducted at 34 sites around the field, each of which consists of 30 minutes stacking for repeated current signals to keep better S/N ratio. Apparent resistivity is given by a function of amplitudes of magnetic field variation and source-receiver distance. We recovered the data from four OBEMs (two were on the spreading axis and other two were off axis). The plot of magnetic amplitudes to source-receiver distances shows different trend between OBEMs on-axis and off-axis. Therefore, we applied the Occam's inversions (Constable et al., 1987) to estimate 1D conductivity structure separately in these two areas. We obtained similar resistivity at upper part (0--100m) in both the two area, however, the resistivity at 100--300m in the on-axis is lower than that in the off-axis by roughly two orders of magnitude. This indicates that the temperature of crust under 300m depth on-axis is higher than that of off-axis. We will present further discussion on this meeting.
In 2004, we started monitoring crustal deformation at Kumano-nada in the Nankai trough using the ... more In 2004, we started monitoring crustal deformation at Kumano-nada in the Nankai trough using the GPS/Acoustic technique. We observed a large southward seafloor displacement of ∼30 cm associated with the off Kii Peninsula earthquake, which occurred in September 2004, between our two survey campaigns in August and November 2004. The observed seafloor displacement is larger than that predicted from a slip model derived solely from GPS measurements on land. This may indicate the earthquake fault is slightly shallower and extends move to the NW than previously estimated.
As a countermeasure against M8-9 class Nankai trough earthquakes, the Japan Meteorological Agency... more As a countermeasure against M8-9 class Nankai trough earthquakes, the Japan Meteorological Agency started a service to release “Nankai Trough Earthquake Extra Information (Megathrust Earthquake Alert).” This alert is released after an M8.0 or higher earthquake occurs, and the possibility of having a subsequent earthquake is evaluated to be higher than usual. This is an innovative attempt at disaster mitigation in Japan as it encourages residents in the predefined area to pre-evacuate for a one week period when tsuanmi risk is higher. However, the factors influencing the evacuation behavior of residents are unknown. In this study, we investigated factors that contribute to residents’ pre-event evacuation intentions using the hierarchical multiple regression analysis method. We focused on the extent to which the recognition of the hazard and risk of the Nankai trough earthquake and the response to the Extra Information, which are changeable by the local governments’ public relations activities, contributed to pre-event evacuation intentions after controlling for disaster-related general attitude and socio-demographic factors. Further, we paid special attention to residents’ degree of recognition of this information by checking the accuracy of their understanding of whether they lived within the pre-event evacuation area. The results showed that the recognition factors were relevant, but less so than the general attitude toward disaster and more so than the sociodemographic factors. In addition, residents’ recognition accuracy was found to be low. Our results suggest that it is important for local governments to make adequate efforts to encourage residents to evacuate.
GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monito... more GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monitoring of crustal activities near plate boundaries. There remain, however, substantial differences from GPS observation on land. Our group in Tohoku University has been working to cope with the problems under the program of the DONET, JAMSTEC.One of critical problems regarding the present GPS/A observation lies in the campaign
ABSTRACT The M9.0 earthquake in March 2011 along the Japan Trench off Tohoku has elucidated that ... more ABSTRACT The M9.0 earthquake in March 2011 along the Japan Trench off Tohoku has elucidated that it is crucial to geodetically observe seismic coupling on the subduction plate boundary, and that the most important observation that should be strengthened is seafloor geodetic observation, especially on the deep seafloor near the trench axis. Repeated observation of GPS/Acoustic (GPS/A) seafloor positioning is the most probable way to cope with the requirement. The observation is urgent along the Japan Trench to analyze how the stress was accumulated that caused the giant earthquake. It is also important along the Nankai Trough off southwestern Japan where another giant earthquake can be being prepared. These surveys will aim at mapping of seismic coupling, and GPS/A observation points should be extended to be arrays in focused areas. On the other hand, ship time for the surveys will remain limited. Extensive geodetic observation along the subduction zones around Japan will not be attained without improvement in GPS/A observation systems. The first step that we are planning is common use of seafloor instruments. In Japan, mainly three groups are engaged in seafloor geodetic measurement by means of GPS/Acoustic technique with their own type of instruments. This has prevented us to conduct mutual surveys to each other, which would become crucial when number of survey sites increases in the future. A precision acoustic transponder (PXP) is a mirror transponder, which returns a received signal without any change after a fixed delay. Considering that different PXPs have intrinsically similar operational principle, we plan to develop PXPs which can be used in common. Then there will be more chance of repeated observations and evaluation of the observed results. A joint observation will An AUV (autonomous underwater vehicle) or an auto-navigation buoy can be an alternative of a survey ship. Asada and Ura (2005) have investigated a GPS/A observation system by using an AUV. Extending surveys into the deeper ocean to cover the wide range of possible rupture area is another important item to be carried out. Our current sites lie on the seafloor shallower than 3500 m. However, the 2011 Tohoku-oki earthquake revealed that the rupture area extended close to the trench, where depth ranges 5000 to 7000 m. Observation of tectonic motion of the incoming plate which is under deep sea is another target of seafloor geodesy. GPS/A observation on such a deep seafloor was technically confirmed (Osada et al., 2003, EPS), but there has been no report on repeated seafloor positioning. We plan to extend the depth range of GPS/A observation by optimizing threshold of responding level of the PXPs as well as increasing its power. Assessment of the application needs careful offshore test.
ABSTRACT Seafloor geodesy, measuring crustal deformation or displacement beneath the ocean, relie... more ABSTRACT Seafloor geodesy, measuring crustal deformation or displacement beneath the ocean, relies on acoustic ranging principal. Traveltime of an acoustic signal between two sites is proportional to averaged slowness or reciprocal of sound speed along the acoustic ray path. A typical sound speed variation produces ~1m of apparent distance change for sea surface-seafloor ranging. Most of such variation can be explained with temporal variation of sound speed depth profile for laterally stratified structure, which have been properly estimated and corrected for using ranging procedure alone with multiple transponders in our past survey strategy. However there still remains 10--30cm of inexplicable apparent fluctuation in position of transponders. These fluctuations can be evened out within 5cm precision by taking 24-hours average. For achievement of higher precision or shorter survey time, the fluctuation must be explicitly corrected without averaging. The fluctuation is interpreted as violation of stratified condition, i.e., existence of lateral variation. Our recent data along Nankai trough and Japan trench indicate that the horizontal scale of the lateral variation is not always large enough ( ˜10km) to be approximated as gradient and that the time scale extends from 1-hour to DC. The DC component is associated with ocean current while the short-time periodic undulation might be due to higher-order internal waves, which are exited by semi-diurnal tidal wave. The tidal wave is large enough ( ˜100km) to generate only stratified temporal variation, which is clearly observed in our surveys. In this study, we summarize the knowledge obtained through the fact addressed above and interpret them in the oceanographic point of view, and further propose a new generation of seafloor geodetic survey strategy to control the variations along with instrumental development for efficient observation.
Compared with land-based GPS observation, GPS/Acoustic seafloor positioning has critical problems... more Compared with land-based GPS observation, GPS/Acoustic seafloor positioning has critical problems against on-line, continuous, and long-term observations. Since GPS/A observation on a moored buoy can be a solution to cope with these problems, an autonomous GPS/A observation system on a buoy is under development in preparation for a possible future application to the cabled ocean floor observation project DONET funded by the MEXT, Japan. A key point is reduction of power consumption. A low-power GPS receiver will take the place of three GPS receivers, and the acoustic ranging system will be replaced with a battery-driven compact system.
ABSTRACT GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for ... more ABSTRACT GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monitoring of crustal activities in subduction zones. There remain, however, some key problems to be settled. Among them is long-term stability of acoustic benchmarks deployed on the seafloor: long-term attitude stability, especially against ground motions of earthquakes, and their electrical/mechanical durability for long-term geodetic observation. M7-class earthquakes occurred in 2004 off Kii Peninsula, Central Japan, and then coseismic seafloor crustal movements were detected by using the GPS/Acoustic (GPS/A) observations. This event gave us a unique opportunity to test the stability. We carried out diving surveys in 2006 and visually inspected the benchmarks that detected the crustal movements. All of them stood stably on the flat sediment. No effects of the earthquakes were observed. In case that the slight tilts of the instruments were caused by the earthquakes, the effect on the observed crustal movements is estimated to be within 1 cm. One of old benchmarks deployed 6 years before was recovered and inspected. There was no problem on the battery, and no damages on the outer frame of the instrument. The diving surveys have settled a basic problem in seafloor geodesy and paved the way for long-term geodetic monitoring on the seafloor. KeywordsSeafloor benchmark-Long-term stability-GPS/Acoustic-Seafloor geodesy
We have attempted to infer details of the viscosity structure in the top 1000 km of the mantle fr... more We have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the igeoid and tomographic structure beneath the oceans. In order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents, we have considered only the intermediate degrees of the oceanic geoid (/= 12-25). A genetic algorithm has been used to determine the family of viscosity models which give the best correlatiofi with the observed geoid. Our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms tHe viscosity increase in the lower mantle predicted by previous inferences, but suggests that the main viscosity jump occurs ad a depth of about 1000 km and not at the usually stated 660-km boundary. Somewhere in the depth range of 400-1000 lkm, a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere. Existenc& of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern wifh a strong horizontal component in the top part of the lower mantle. Unfortunately, the resolution of the inversion as well aslthe quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accudacy for the viscosity of this low viscosity channel. 0 1997 Elsevier Science B.V.
One of the important issues on the GPS-acoustic (GPS-A) observation for sea bottom positioning is... more One of the important issues on the GPS-acoustic (GPS-A) observation for sea bottom positioning is how to address the horizontal heterogeneity of the sound speed in oceans. This study presents an analysis method of GPS-A data in the presence of a sloping sound speed structure. By applying this method and revising the analysis scheme to make full use of existing data, we reevaluated the horizontal postseismic deformations occurring~1.5-5 years after the 2011 Tohoku earthquake. The revised horizontal movements have more uniform directions and rates between neighboring sites, suggesting enhancement of the array positioning accuracy. The revised displacement rate of the site on the incoming Pacific plate, located~100 km northeast of the main rupture zone, was decreased significantly; it was only slightly, by 1.4 cm/year larger than the global motion of the Pacific plate, suggesting a relatively small effect of viscoelastic relaxation. The horizontal movements of the near-trench sites above the main rupture zone were generally landward and were significantly faster than the Pacific plate motion, indicating a viscoelastic relaxation of 5-10 cm/year. The distribution of the fast landward movements peaked near 38°N at an updip of the mainshock hypocenter and extended significantly farther to the north than to the south. This implies the existence of a secondary coseismic slip patch in the northern area in addition to a primary slip patch at~38°N. The occurrence of episodic slow slip in early 2015 to the north of the main rupture zone was also verified from the GPS-A analyses.
Recent progress of seafloor observations for earthquake and crustal deformation, such as deployme... more Recent progress of seafloor observations for earthquake and crustal deformation, such as deployment of submarine cable networks of S-net and DONET, and repeated observations of GNSS/A and acoustic extensometer (direct path acoustic ranging), enable us to evaluate ongoing crustal activities in the megathrust regions along the Japan trench and the Nankai trough. We review the present status and the future plans of such seafloor observations, and discuss the future directions of seafloor observation networks, especially for real-time monitoring of crustal activities. Toward these directions, we welcome papers introducing the present status of novel approaches and systems such as optical fiber, laser ranging or seafloor SAR and real-time geodetic observations using mooring buoys or wave glider, and so on. We also welcome future plans to integrate observation for the crustal activity under the seafloor with observation for ocean and climate changes.
ABSTRACT Hydrothermal vent fields are known to exist on the spreading axis, where sea water penet... more ABSTRACT Hydrothermal vent fields are known to exist on the spreading axis, where sea water penetrates into the crust and upwells through the hydrothermal vents. Understanding of the hydrothermal circulation system is extremely important to reveal the cooling process of the oceanic crust. The thermal structure beneath the hydrothermal vent reflects the extent of underground activity and the convection scale of the hot water. Temperature in the crust can be estimated from the electrical conductivity because the conductivity depends on the water volume, the salinity concentration and the temperature of the sea water in the crust. The Alice Spring Field (18 o12.9'N, 144 o42.5'E and 3600m deep), on the spreading axis in the central Mariana Back-Arc Basin, is a suitable site for this purpose. Hydrothermal vent in this field was firstly discovered by Alvin in 1987 (Hawkins et al., 1990). Shinkai6500 also confirmed the hydrothermal activity in 1992 and 1996 (Gamou et al., 1994; Fujikura et al., 1997). In November, 2002, we conducted Magnetometric Resistivity (MMR) survey using R/V Kairei, JAMSTEC in this field. In the MMR method, controlled electric current was applied from a pair of electrodes; one is just beneath the sea surface and the other is close to the seafloor. To record electoromagnetic responses of the crust to the inputed current, we deployed six ocean bottom electromagnetometers (OBEMs), which can measure 3-components of magnetic and electric fields simultaneously. Measurements were conducted at 34 sites around the field, each of which consists of 30 minutes stacking for repeated current signals to keep better S/N ratio. Apparent resistivity is given by a function of amplitudes of magnetic field variation and source-receiver distance. We recovered the data from four OBEMs (two were on the spreading axis and other two were off axis). The plot of magnetic amplitudes to source-receiver distances shows different trend between OBEMs on-axis and off-axis. Therefore, we applied the Occam's inversions (Constable et al., 1987) to estimate 1D conductivity structure separately in these two areas. We obtained similar resistivity at upper part (0--100m) in both the two area, however, the resistivity at 100--300m in the on-axis is lower than that in the off-axis by roughly two orders of magnitude. This indicates that the temperature of crust under 300m depth on-axis is higher than that of off-axis. We will present further discussion on this meeting.
In 2004, we started monitoring crustal deformation at Kumano-nada in the Nankai trough using the ... more In 2004, we started monitoring crustal deformation at Kumano-nada in the Nankai trough using the GPS/Acoustic technique. We observed a large southward seafloor displacement of ∼30 cm associated with the off Kii Peninsula earthquake, which occurred in September 2004, between our two survey campaigns in August and November 2004. The observed seafloor displacement is larger than that predicted from a slip model derived solely from GPS measurements on land. This may indicate the earthquake fault is slightly shallower and extends move to the NW than previously estimated.
As a countermeasure against M8-9 class Nankai trough earthquakes, the Japan Meteorological Agency... more As a countermeasure against M8-9 class Nankai trough earthquakes, the Japan Meteorological Agency started a service to release “Nankai Trough Earthquake Extra Information (Megathrust Earthquake Alert).” This alert is released after an M8.0 or higher earthquake occurs, and the possibility of having a subsequent earthquake is evaluated to be higher than usual. This is an innovative attempt at disaster mitigation in Japan as it encourages residents in the predefined area to pre-evacuate for a one week period when tsuanmi risk is higher. However, the factors influencing the evacuation behavior of residents are unknown. In this study, we investigated factors that contribute to residents’ pre-event evacuation intentions using the hierarchical multiple regression analysis method. We focused on the extent to which the recognition of the hazard and risk of the Nankai trough earthquake and the response to the Extra Information, which are changeable by the local governments’ public relations activities, contributed to pre-event evacuation intentions after controlling for disaster-related general attitude and socio-demographic factors. Further, we paid special attention to residents’ degree of recognition of this information by checking the accuracy of their understanding of whether they lived within the pre-event evacuation area. The results showed that the recognition factors were relevant, but less so than the general attitude toward disaster and more so than the sociodemographic factors. In addition, residents’ recognition accuracy was found to be low. Our results suggest that it is important for local governments to make adequate efforts to encourage residents to evacuate.
GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monito... more GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monitoring of crustal activities near plate boundaries. There remain, however, substantial differences from GPS observation on land. Our group in Tohoku University has been working to cope with the problems under the program of the DONET, JAMSTEC.One of critical problems regarding the present GPS/A observation lies in the campaign
ABSTRACT The M9.0 earthquake in March 2011 along the Japan Trench off Tohoku has elucidated that ... more ABSTRACT The M9.0 earthquake in March 2011 along the Japan Trench off Tohoku has elucidated that it is crucial to geodetically observe seismic coupling on the subduction plate boundary, and that the most important observation that should be strengthened is seafloor geodetic observation, especially on the deep seafloor near the trench axis. Repeated observation of GPS/Acoustic (GPS/A) seafloor positioning is the most probable way to cope with the requirement. The observation is urgent along the Japan Trench to analyze how the stress was accumulated that caused the giant earthquake. It is also important along the Nankai Trough off southwestern Japan where another giant earthquake can be being prepared. These surveys will aim at mapping of seismic coupling, and GPS/A observation points should be extended to be arrays in focused areas. On the other hand, ship time for the surveys will remain limited. Extensive geodetic observation along the subduction zones around Japan will not be attained without improvement in GPS/A observation systems. The first step that we are planning is common use of seafloor instruments. In Japan, mainly three groups are engaged in seafloor geodetic measurement by means of GPS/Acoustic technique with their own type of instruments. This has prevented us to conduct mutual surveys to each other, which would become crucial when number of survey sites increases in the future. A precision acoustic transponder (PXP) is a mirror transponder, which returns a received signal without any change after a fixed delay. Considering that different PXPs have intrinsically similar operational principle, we plan to develop PXPs which can be used in common. Then there will be more chance of repeated observations and evaluation of the observed results. A joint observation will An AUV (autonomous underwater vehicle) or an auto-navigation buoy can be an alternative of a survey ship. Asada and Ura (2005) have investigated a GPS/A observation system by using an AUV. Extending surveys into the deeper ocean to cover the wide range of possible rupture area is another important item to be carried out. Our current sites lie on the seafloor shallower than 3500 m. However, the 2011 Tohoku-oki earthquake revealed that the rupture area extended close to the trench, where depth ranges 5000 to 7000 m. Observation of tectonic motion of the incoming plate which is under deep sea is another target of seafloor geodesy. GPS/A observation on such a deep seafloor was technically confirmed (Osada et al., 2003, EPS), but there has been no report on repeated seafloor positioning. We plan to extend the depth range of GPS/A observation by optimizing threshold of responding level of the PXPs as well as increasing its power. Assessment of the application needs careful offshore test.
ABSTRACT Seafloor geodesy, measuring crustal deformation or displacement beneath the ocean, relie... more ABSTRACT Seafloor geodesy, measuring crustal deformation or displacement beneath the ocean, relies on acoustic ranging principal. Traveltime of an acoustic signal between two sites is proportional to averaged slowness or reciprocal of sound speed along the acoustic ray path. A typical sound speed variation produces ~1m of apparent distance change for sea surface-seafloor ranging. Most of such variation can be explained with temporal variation of sound speed depth profile for laterally stratified structure, which have been properly estimated and corrected for using ranging procedure alone with multiple transponders in our past survey strategy. However there still remains 10--30cm of inexplicable apparent fluctuation in position of transponders. These fluctuations can be evened out within 5cm precision by taking 24-hours average. For achievement of higher precision or shorter survey time, the fluctuation must be explicitly corrected without averaging. The fluctuation is interpreted as violation of stratified condition, i.e., existence of lateral variation. Our recent data along Nankai trough and Japan trench indicate that the horizontal scale of the lateral variation is not always large enough ( ˜10km) to be approximated as gradient and that the time scale extends from 1-hour to DC. The DC component is associated with ocean current while the short-time periodic undulation might be due to higher-order internal waves, which are exited by semi-diurnal tidal wave. The tidal wave is large enough ( ˜100km) to generate only stratified temporal variation, which is clearly observed in our surveys. In this study, we summarize the knowledge obtained through the fact addressed above and interpret them in the oceanographic point of view, and further propose a new generation of seafloor geodetic survey strategy to control the variations along with instrumental development for efficient observation.
Compared with land-based GPS observation, GPS/Acoustic seafloor positioning has critical problems... more Compared with land-based GPS observation, GPS/Acoustic seafloor positioning has critical problems against on-line, continuous, and long-term observations. Since GPS/A observation on a moored buoy can be a solution to cope with these problems, an autonomous GPS/A observation system on a buoy is under development in preparation for a possible future application to the cabled ocean floor observation project DONET funded by the MEXT, Japan. A key point is reduction of power consumption. A low-power GPS receiver will take the place of three GPS receivers, and the acoustic ranging system will be replaced with a battery-driven compact system.
ABSTRACT GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for ... more ABSTRACT GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monitoring of crustal activities in subduction zones. There remain, however, some key problems to be settled. Among them is long-term stability of acoustic benchmarks deployed on the seafloor: long-term attitude stability, especially against ground motions of earthquakes, and their electrical/mechanical durability for long-term geodetic observation. M7-class earthquakes occurred in 2004 off Kii Peninsula, Central Japan, and then coseismic seafloor crustal movements were detected by using the GPS/Acoustic (GPS/A) observations. This event gave us a unique opportunity to test the stability. We carried out diving surveys in 2006 and visually inspected the benchmarks that detected the crustal movements. All of them stood stably on the flat sediment. No effects of the earthquakes were observed. In case that the slight tilts of the instruments were caused by the earthquakes, the effect on the observed crustal movements is estimated to be within 1 cm. One of old benchmarks deployed 6 years before was recovered and inspected. There was no problem on the battery, and no damages on the outer frame of the instrument. The diving surveys have settled a basic problem in seafloor geodesy and paved the way for long-term geodetic monitoring on the seafloor. KeywordsSeafloor benchmark-Long-term stability-GPS/Acoustic-Seafloor geodesy
We have attempted to infer details of the viscosity structure in the top 1000 km of the mantle fr... more We have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the igeoid and tomographic structure beneath the oceans. In order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents, we have considered only the intermediate degrees of the oceanic geoid (/= 12-25). A genetic algorithm has been used to determine the family of viscosity models which give the best correlatiofi with the observed geoid. Our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms tHe viscosity increase in the lower mantle predicted by previous inferences, but suggests that the main viscosity jump occurs ad a depth of about 1000 km and not at the usually stated 660-km boundary. Somewhere in the depth range of 400-1000 lkm, a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere. Existenc& of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern wifh a strong horizontal component in the top part of the lower mantle. Unfortunately, the resolution of the inversion as well aslthe quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accudacy for the viscosity of this low viscosity channel. 0 1997 Elsevier Science B.V.
One of the important issues on the GPS-acoustic (GPS-A) observation for sea bottom positioning is... more One of the important issues on the GPS-acoustic (GPS-A) observation for sea bottom positioning is how to address the horizontal heterogeneity of the sound speed in oceans. This study presents an analysis method of GPS-A data in the presence of a sloping sound speed structure. By applying this method and revising the analysis scheme to make full use of existing data, we reevaluated the horizontal postseismic deformations occurring~1.5-5 years after the 2011 Tohoku earthquake. The revised horizontal movements have more uniform directions and rates between neighboring sites, suggesting enhancement of the array positioning accuracy. The revised displacement rate of the site on the incoming Pacific plate, located~100 km northeast of the main rupture zone, was decreased significantly; it was only slightly, by 1.4 cm/year larger than the global motion of the Pacific plate, suggesting a relatively small effect of viscoelastic relaxation. The horizontal movements of the near-trench sites above the main rupture zone were generally landward and were significantly faster than the Pacific plate motion, indicating a viscoelastic relaxation of 5-10 cm/year. The distribution of the fast landward movements peaked near 38°N at an updip of the mainshock hypocenter and extended significantly farther to the north than to the south. This implies the existence of a secondary coseismic slip patch in the northern area in addition to a primary slip patch at~38°N. The occurrence of episodic slow slip in early 2015 to the north of the main rupture zone was also verified from the GPS-A analyses.
Uploads
Papers by Motoyuki Kido