Motoyuki Kido

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.

Inferences of viscosity from the oceanic geoid: Indication of a low viscosity zone below the 660-km discontinuity

Abstract We have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid 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 (l = 12–25). A genetic algorithm has been used to determine the family of viscosity models which give the best correlation 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 at a depth of about 1000 km and not at the usually stated 660-km boundary. Somewhere in the depth range of 400–1000 km, a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere. Existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle. Unfortunately, the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel.

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.

Dynamic topography compared with residual depth anomalies in oceans and implications for age-depth curves

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.

MANTLE VISCOSITY DERIVED BY GENETIC ALGORITHM USING OCEANIC GEOID AND SEISMIC TOMOGRAPHY FOR WHOLE-MANTLE VERSUS BLOCKED-FLOW SITUATIONS

Abstract 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.

Numerical simulation for the prediction of the plate motions: Effects of lateral viscosity variations in the lithosphere

A numerical simulation of Newtonian viscous flow without inertia terms in a 3-D spherical shell driven by the negative buoyancy due to the slabs has been conducted to understand the effects of weak plate margins on the plate motions. Density loads are inferred from the seismicity and the reconstruction of the subduction history. The toroidal energy of plate motion comparable to the poloidal energy appears, when γ (ratio of the viscosity at margins to that of interiors) becomes O(0.01). For the whole mantle density model, all the plates move too fast relative to the Pacific plate. The direction of major plate motions is generally improved by the inclusion of weak plate boundaries. The density loads in the upper mantle appear to explain the overall plate motions, although some of the plate motions may require hidden and/or deeper density anomalies to be consistent with the observations. As γ decreases, the geoid anomalies associated with the upper mantle slabs change their signs. This reversal affects the long-wavelength components of the geoid anomalies. A considerable part of the horizontal stress field shows a horizontal extension suggesting that another type of density anomalies is necessary to explain the general compressional field of the real Earth.

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.

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.

The role played by a low viscosity zone under a 660 km discontinuity in regional mantle layering

Abstract 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.

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.