Yukihito Osada

Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake

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.

Seafloor displacement at Kumano-nada caused by the 2004 off Kii Peninsula earthquakes, detected through repeated GPS/Acoustic surveys

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.

Estimation and correction for the effect of sound velocity variation on GPS/Acoustic seafloor positioning : An experiment off Hawaii Island

A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5—4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4—7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.

First measurement of the displacement rate of the Pacific Plate near the Japan Trench after the 2011 Tohoku‐Oki earthquake using GPS/acoustic technique

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.

Development of instruments for seafloor geodesy

We have developed systems for measuring differential displacements across a fault zone, and examined their resolutions through seafloor experiments at relatively short baselines. A system for a seafloor extensometer makes use of precise acoustic ranging with a linear pulse compression technique. The system has a resolution better than 1 cm in acoustic ranging over a baseline of at least 1 km. The most critical problem is correction for temperature variations, and we estimate that the effect can be corrected with cm-order accuracy in the case of a deep-sea experiment. We have also examined a leveling system on the seafloor using an array of ocean bottom pressure gauges and an ocean bottom gravimeter to detect differential vertical motion. The system is estimated to have a resolution of several centimeters in vertical displacement. These system will be useful for triangulation and leveling on the seafloor, but we need further studies over a longer baseline and to achieve better long-term stability.

Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements

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.

Seafloor observations indicate spatial separation of coseismic and postseismic slips in the 2011 Tohoku earthquake.

Large interplate earthquakes are often followed by postseismic slip that is considered to occur in areas surrounding the coseismic ruptures. Such spatial separation is expected from the difference in frictional and material properties in and around the faults. However, even though the 2011 Tohoku Earthquake ruptured a vast area on the plate interface, the estimation of high-resolution slip is usually difficult because of the lack of seafloor geodetic data. Here using the seafloor and terrestrial geodetic data, we investigated the postseismic slip to examine whether it was spatially separated with the coseismic slip by applying a comprehensive finite-element method model to subtract the viscoelastic components from the observed postseismic displacements. The high-resolution co- and postseismic slip distributions clarified the spatial separation, which also agreed with the activities of interplate and repeating earthquakes. These findings suggest that the conventional frictional property model is valid for the source region of gigantic earthquakes.

Progress in the Project for Development of GPS/Acoustic Technique Over the Last 4 Years

GPS/Acoustic (GPS/A) survey is the most promising way to detect crustal deformation in the ocean far from the coast, where a dense onshore GPS network is not available. Monitoring seafloor deformation is crucial to understand the tectonic state in regions of geophysical significance such as subduction zones. We, Tohoku University, together with Nagoya University and Japan Coast Guard have been dedicated to GPS/A survey around the Japanese Islands and developing its instruments for more than a decade. Especially in 2010, a new project for the development of the GPS/A technique commenced, and since 2012 following the Tohoku earthquake, further acceleration of the project has been taken place. Tohoku and Nagoya Universities have been working on this project for 4 years. In the project, Tohoku University worked on several topics, such as realtime/continuous monitoring of crustal deformation using a moored buoy, automatic survey using an Autonomous Surface Vehicle (ASV), which makes the survey as efficient as possible, and constructing a new GPS/acoustic survey network along the Japan Trench and their intensive survey using a chartered ship. In this paper, we summarize the achievements in each of the topics above.

Investigation on the Postseismic Deformation Associated with the 2011 Tohoku Earthquake Based on Terrestrial and Seafloor Geodetic Observations: To Evaluate the Further Seismic Hazard Potential on the Plate Interface Beneath the Northeastern Japanese Islands

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.

Extended GPS/Acoustic geodetic observation near the Japan trench axis for the study of the giant 2011 Tohoku-oki earthquake

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.