Mariko Sato

Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake

The Quake That Rocked Japan The 11 March 2011 magnitude 9.0 Tohoku-Oki megathrust earthquake just off the Eastern coast of Japan was one of the largest earthquakes in recorded history. Japans considerable investment in seismic and geodetic networks allowed for the collection of rapid and reliable data on the mechanics of the earthquake and the devastating tsunami that followed (see the Perspective by Heki). Sato et al. (p. 1395, published online 19 May) describe the huge displacements from ocean bottom transponders—previously placed directly above the earthquakes hypocenter—communicating with Global Positioning System (GPS) receivers aboard a ship. Simons et al. (p. 1421, published online 19 May) used land-based GPS receivers and tsunami gauge measurements to model the kinematics and extent of the earthquake, comparing it to past earthquakes in Japan and elsewhere. Finally, Ide et al. (p. 1426, published online 19 May) used finite-source imaging to model the evolution of the earthquakes rupture that revealed a strong depth dependence in both slip and seismic energy. These initial results provide fundamental insights into the behavior of rare, very large earthquakes that may aid in preparation and early warning efforts for future tsunamis following subduction zone earthquakes. Detailed geophysical measurements reveal features of the 2011 Tohoku-Oki megathrust earthquake. The moment magnitude (Mw) = 9.0 2011 Tohoku-Oki mega-thrust earthquake occurred off the coast of northeastern Japan. Combining Global Positioning System (GPS) and acoustic data, we detected very large sea-floor movements associated with this event directly above the focal region. An area with more than 20 meters of horizontal displacement, that is, four times larger than those detected on land, stretches several tens of kilometers long along the trench; the largest amount reaches about 24 meters toward east-southeast just above the hypocenter. Furthermore, nearly 3 meters of vertical uplift occurred, contrary to observed terrestrial subsidence.

GPS/Acoustic seafloor geodetic observation: method of data analysis and its application

We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method is adopted to determine the position of seafloor stations from coordinates of a moving survey vessel and measured travel times of acoustic waves in seawater. The positioning accuracy is substantially improved by estimating the temporal variation of the acoustic velocity structure. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a huge earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from seven campaign observations for the period 2002–2005 exhibits a linear trend with a scattering rms of about 2 cm. A linear fit to the time series gives an intraplate crustal velocity of more than several centimeters per year towards the WNW, which implies strong interplate coupling around this region. The precision of each campaign solution was examined at MYGI and other seafloor reference points along the Nankai Trough through comparison of independent one-day subset solutions within the campaign. The resultant repeatability looks to be well-correlated with the temporal and spatial stability of the acoustic velocity structure in the seawater depending on the region as well as the season.

Evidence of viscoelastic deformation following the 2011 Tohoku‐Oki earthquake revealed from seafloor geodetic observation

The GPS/acoustic seafloor positioning has detected significant postseismic movements after the 2011 Tohoku-Oki earthquake (M9.0), just above the source region off the Pacific coast of eastern Japan. In contrast to the coastal Global Navigation Satellite Systems sites where trenchward-upward movements were reported, the offshore sites above the main rupture zone in the northern part of the source region exhibit landward displacements of tens of centimeters with significant subsidence from almost 3 years of repeated observations. At the sites above around the edge of the main rupture zone, smaller amount of trench-normal movements was found. Although the terrestrial movements were reasonably interpreted by afterslip beneath the coastal area, these offshore results are rather consistent with effects predicted from viscoelastic relaxation in the upper mantle, providing definitive evidence of its occurrence. On the other hand, the results in the southern part of the source region imply superposition of effects from viscoelastic relaxation and afterslip.

Weak interplate coupling beneath the subduction zone off Fukushima, NE Japan, inferred from GPS/acoustic seafloor geodetic observation

We have been carrying out GPS/acoustic seafloor geodetic observations at several reference points situated along the Japan Trench, a major plate boundary of subduction. A time series of horizontal coordinates of one of the seafloor reference points, located off Fukushima, obtained from seven campaign observations for the period 2002-2008, exhibits a linear trend with a scattering root mean square of about 3 cm. A linear fit to the time series gives an intraplate crustal movement velocity of 3.1 cm per year in a westerly direction, which is significantly smaller than that at the other seafloor reference point 120 km away along the trench axis. This result implies weak interplate coupling in this region.

Regional estimation of electric fields and currents in the polar ionosphere

A new technique is presented to estimate electric fields and currents in a localized region of the high-latitude ionosphere by combining two magnetogram-inversion algorithms. This paper describes the concept and practical procedures of the method as well as the first results of our efforts in which this new scheme is applied to northern Scandinavia, computing the ionospheric parameters on a small scale. Examining latitudinal profiles of these parameters and precipitating particles, it is found that the region of the most intense precipitation in the morning sector is located equatorward of the region of the strongest electric field. To evaluate the relative importance of ionospheric and magnetospheric effects, the field-aligned current is divided into two components: (∇Σ) · E and Σ∇ · E. These two components give often the opposite directions in the resultant field-aligned currents. The relative strength of the two components appears to vary considerably with latitude.

Heterogeneous interplate coupling along the Nankai Trough, Japan, detected by GPS-acoustic seafloor geodetic observation

The recurring devastating earthquake that occurs in the Nankai Trough subduction zone between the Philippine Sea plate and the Eurasian plate has the potential to cause an extremely dangerous natural disaster in the foreseeable future. Many previous studies have assumed interplate-coupling ratios for this region along the trench axis using onshore geodetic data in order to understand this recursive event. However, the offshore region that has the potential to drive a devastating tsunami cannot be resolved sufficiently because the observation network is biased to the land area. Therefore, the Hydrographic and Oceanographic Department of Japan constructed a geodetic observation network on the seafloor along the Nankai Trough using a GPS-acoustic combination technique and has used it to observe seafloor crustal movements directly above the Nankai Trough subduction zone. We have set six seafloor sites and cumulated enough data to determine the displacement rate from 2006 to January 2011. Our seafloor geodetic observations at these sites revealed a heterogeneous interplate coupling that has three particular features. The fast displacement rates observed in the easternmost area indicate strong interplate coupling (>75%) around not only the future Tokai earthquake source region but also the Paleo-Zenisu ridge. The slow displacement rates near the trench axis in the Kumano-nada Sea, a shallow part of the 1944 Tonankai earthquake source region, show a lower coupling ratio (50% to 75%). The slow displacement rate observed in the area shallower than the 1946 Nankaido earthquake source region off Cape Muroto-zaki reflects weakening interplate coupling (about 50%) probably due to a subducting seamount. Our observations above the subducting ridge and seamount indicate that the effect of a subducting seamount on an interplate-coupling region depends on various conditions such as the geometry of the seamount and the friction parameters on the plate boundary.

Fundamental Developments of New Generation Seafloor Geodetic Observation System Based on AUV Technology

Institute of Industrial Science, University of Tokyo (IIS) has started a project to develop the fundamental technologies for constructing new-generation seafloor geodetic observation system. The current observational method using research vessel cannot help being subjected to annual cruise schedule of research vessels. It has been difficult for us to change the cruise schedule as appropriate according to weather and sea condition, GPS satellite distribution and so on. The new system, which we are developing, based on AUV technology will give us opportunities for observation with choosing favorable conditions of sea and GPS satellite distribution, much more frequent observations and flexible planning of observation in response to sudden geodetic events. Trial models of the sea surface and seafloor units were finished. We conducted several performance evaluation experiments in the sea and dam site, in order to bring the observation system to completion.

Trials of seafloor geodetic monitoring around Japan

Institute of Industrial Science, University of Tokyo and the Hydrographic and Oceanographic Department, Japan Coast Guard have been constructing a seafloor geodetic observation network around Japan in order to detect and monitor the crustal deformation of the seafloor directly. The observation network had been constructed along the major trenches, such as the Japan trench and the Nankai trough. Observations to measure the positions of the seafloor stations have been conducted repeatedly. Sufficient observational results have been established as we accumulate the experience of the observations to discus the system and the observational method. Although we reached certain level with the existing system, we come up with possible improvements of the system. Trials to improve the system are always done in order to achieve a fully centimeter level geodesy on the seafloor

Development of seafloor geodetic observation system based on AUV and submarine cable technologies

Institute of Industrial Science, University of Tokyo (IIS) has launched a five-year long project to develop next-generation seafloor geodetic observation system in 2005. Principal idea of the observation system is to utilize AUV, seafloor platform and submarine cable in place of using a research vessel and a stand-alone type seafloor benchmark transponder. This observation system aims at promoting efficiency of the observation for maintaining the observation system over a long period in response to geodetic events that have long time scales. Trial model of the surface system that should be installed on an AUV observation platform was finished. And then, based on this technological development, we have just started to develop the AUV specially design for seafloor geodetic observation. The seafloor transponder was also finished and installed on the existing submarine cable system. We are in the final stage of the development of the new observation system for seafloor geodesy.