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Upper-plate controls on co-seismic slip in the 2011 magnitude 9.0 Tohoku-oki earthquake


The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw ≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture1, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface2. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest–northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150–200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south)3. The megathrust north of the Median Tectonic Line is interseismically locked2, has a history of large earthquakes (18 with Mw > 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake1. In contrast, the megathrust south of this boundary has higher rates of interseismic creep2, has not generated an earthquake with MJ > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 20111. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.

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Figure 1: Forearc anomalies in the northeast Japan subduction zone.
Figure 2: Simplified geology11 and major tectonic boundaries3 of Japan.
Figure 3: Slip behaviour of the northeast Japan megathrust.

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We thank S. Naif, C. Davies, C. Twardzik and S. Das for suggestions. Figures and grid processing was conducted using the Generic Mapping Tools (GMT) ( D.B. was supported by a University of Oxford Clarendon Scholarship, by a Green Foundation Postdoctoral Fellowship in the Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, by the National Geospatial Agency (HM01771310008), and by the Scripps Seafloor Electromagnetics Consortium (

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D.B. and A.B.W. conceived the study and conducted grid processing. D.B and D.T.S. calculated density anomalies. Y.F. conducted numerical simulations of earthquake cycles. D.B., D.T.S. and Y.F. wrote the initial manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Dan Bassett.

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Extended data figures and tables

Extended Data Figure 1 Rupture areas and co-seismic slip amplitudes in Earth’s largest earthquakes1,41,42,43,44,45,46,47,48.

Grey bars and black dots plot the mean and maximum amounts of co-seismic slip against the area of fault rupture. The five earthquakes with Mw ≥ 9 are labelled and numbers 6–10 refer to the location of smaller-magnitude earthquakes in the catalogue shown in Supplementary Table 1. Note the anomalously large (>70 m) amount of co-seismic slip in the 2011 Tohoku-oki event1. The black arrow shows the maximum amount of slip in the 1906 San Francisco earthquake. The rupture area in this strike-slip event was 6 × 103 km2 and two orders of magnitude smaller than the megathrust rupture areas plotted here.

Extended Data Figure 2 Grid processing methodology.

Panels illustrate the ensemble-averaging and grid-processing methodology as applied at the northeast Japan subduction zone. Regional grids of bathymetry33 and free-air/Bouguer-corrected (FA/BC) gravity anomaly32 are sampled along trench-normal profiles (a and b). The spectral average is calculated from each ensemble of profiles (shown as insets to c and d). Maintaining the geometry of the trench, grids of the average profile are constructed (c and d), and subtracted from the original data sets to reveal residual bathymetry (e) and residual gravity anomalies (f). This technique of spectral averaging is identical to that applied globally9,10.

Extended Data Figure 3 Ensemble average profiles and two-dimensional gravity model.

a, Ensemble average topographic profile. b, Ensemble average gravimetric profile (black). The red profile shows the gravity anomaly calculated for the two-dimensional density structure shown in d. c, Mean forearc density (left) and forearc crustal thickness (right) plotted against distance from the trench axis. The larger amplitudes of density anomalies near the trench predominantly reflects the reduction in forearc crustal thickness h although they may also reflect, in part, contrasting resistances to near-trench deformation. d, Model of crustal structure for the northeast Japan subduction zone. This model is constructed using the mean geometry of the trench slope (shown in a) and subducting slab, seismic constraints on forearc and subducting slab (~7 km) crustal thicknesses (Supplementary Table 2), and using reasonable values for crustal (~2,800 kg m−3) and mantle densities (~3,100 kg m−3). The good fit observed in b between the ensemble average (black) and calculated (red) gravity anomalies shows that the ensemble average gravity anomaly captures the broad crustal architecture of the subduction zone, which enables the residual anomalies revealed following the removal of this average to be interpreted. The short-wavelength nature of residuals in the northeast Japan forearc suggests that most are related to crustal structure.

Extended Data Figure 4 Subducting slab geometry and forearc crustal thickness.

a, Geometry of the subducting Pacific Plate as constrained by linearly interpolating along-strike between active-source wide-angle seismic profiles49,50,51,52,53,54,55,56. Profiles are numbered as listed in Supplementary Table 2 and plotted in Extended Data Fig. 5. Red triangles show arc volcanoes. b, Forearc crustal thickness as constrained by the wide-angle profiles shown in a. The dotted line marks the intersection of the subducting slab with the forearc Moho. Crustal thickness is calculated by subtracting the observed bathymetry from the seismically constrained base of the forearc crust. c, As in b, but with forearc Moho depth constrained by the tomographic model of Katsumata37.

Extended Data Figure 5 Wide-angle seismic models.

Profiles are numbered as in Extended Data Fig. 4a and Supplementary Table 249,50,51,52,53,54,55,56. See legend for figure nomenclature. Dots show slab and Moho positions at profile intersections. Horizontal axes show model kilometres. The slab and Moho geometries shown for profiles 5 and 6 are from reflector distributions imaged by travel-time mapping54. Intersecting profile 3 suggests that Moho reflectors interpreted south of 120 km on profile 6 may originate from the top of the subducting Pacific plate and forearc Moho constraints are only incorporated north of model kilometre 150.

Extended Data Figure 6 Calculation of forearc density anomalies.

a, Observed residual gravity anomalies. Black contours show forearc crustal thickness (5 km increment). b, Synthetic gravity anomalies calculated from the distribution of forearc density anomalies shown in c. c, Distribution of density anomalies. Density contrasts are constant within 10 km × 10 km vertical prisms extending between the seabed and either the top of the subducting slab or the forearc Moho (whichever is shallower). Initial density contrasts for each prism are estimated directly from residual gravity anomalies using the known thicknesses of each prism, with the difference between observed and synthetic residual gravity anomalies similarly applied to update model parameters. The inset to b shows the reduction in root-mean-square misfit with each update of model parameters. After 12 iterations the root-mean-square misfit <1 mGal. d, Difference between observed and synthetic residual gravity anomalies.

Extended Data Figure 7 Density anomalies calculated using different constraints on forearc crustal thickness.

a, Density anomalies calculated using active-source seismic constraints on the slab and forearc Moho (Extended Data Fig. 4a and b). b, Density anomalies calculated using active-source seismic constraints on the geometry of the subducting Pacific Plate, but using the tomographic model of Katsumata37 to constrain the forearc Moho (Extended Data Fig. 4c). c, Density anomalies calculated using SLAB1.036 for the subducting Pacific Plate and assuming a planar forearc Moho at the mean depth (25 km) determined by active-source seismic models. The difference in density models calculated using these different model parameterizations are of the order of 10–20 kg m−3. All panels show a clear north-to-south reduction in density anomalies across the forearc segment boundary (red dashed line), and our interpretation of this contrast is not dependent on the observations used to constrain forearc crustal thickness. A comparison between forearc structure inferred from residual gravity anomalies and the seismic velocity structure of the forearc57 is shown in Supplementary Fig. 1.

Extended Data Figure 8 Co-seismic slip models for the March 2011 Tohoku-oki earthquake.

Plots showing the correlation between overthrusting plate structure as constrained by residual gravity anomalies and the distribution of slip in the Tohoku-oki earthquake. These models have been constructed using different data types and inversion strategies. In all plots, grey and red dashed lines mark the trench axis and the forearc segment boundary respectively. Contour intervals are labelled and the outermost contour is 0. a, Minson et al.1; b, Simons, et al.58; c, Ammon et al.59; d, Yue and Lay60; e, Melgar and Bock61; f, Sato et al.62; g, Ozawa et al.63 (allowing slip at trench); h, Ozawa et al.63 (imposing no-slip condition at trench); i, Fujii et al.64. In all plots, large co-seismic slip is focused north of the forearc segment boundary in regions characterized by positive residual gravity anomalies. Most models also show a sharp reduction in the magnitude of slip from north to south across the forearc segment boundary. The wide range of data types and inversion strategies represented by this ensemble of models suggests that the common features identified above are probably robust characteristics of the Tohoku-oki earthquake rupture.

Extended Data Figure 9 Postseismic observations.

a, Aftershocks between March 11 and May 24 occurring on the subduction interface65. All plots show the trench axis (grey dashed line), forearc segment boundary (red dashed line) and contours (10 m) of co-seismic slip1. b, All aftershocks (variable location/mechanism) occurring within seven months of the Tohoku mainshock with Mw ≥ 5 (ref. 63). Panels a and b show that interplate aftershocks for the Tohoko-oki earthquake did not occur in areas that experienced large co-seismic displacements. The vast majority occur in regions surrounding the mainshock rupture area, the distribution of which supports the Bayesian slip distribution of ref. 1, and provides a useful constraint on the along-strike extent of co-seismic rupture. The negative correlation between co-seismic slip and aftershock locations is strongest for the aftershock locations of ref. 65, because they have a lower magnitude cut-off (and hence more events) and because they evaluate Kagans angles to isolate interplate aftershocks from those occurring within either the subducting or overthrusting crust, both of which show no correlation with the co-seismic rupture area (see figure 3b and c of ref. 65). These aftershock locations and the distributed slip models shown in Extended Data Fig. 8 suggest that large co-seismic displacements (>20 m) in the Tohoku-oki earthquake did not continue >50 km southeast of the MTL. c, Afterslip. Red arrows show one-year postseismic displacements of seafloor GPS sites. Blue arrows show predicted GPS vectors from the viscoelastic model of Sun and Wang24. Thick grey contours (numbers are in metres) show the distribution of afterslip24. In the dip direction, shallow afterslip is constrained to occur predominantly seaward of site FUKU and thus south of the forearc segment boundary. In the along-strike direction, the northern termination of the afterslip patch is expected to be south of the main rupture area. To the south, the afterslip may extend much farther than depicted by the slip patch shown in Extended Data Fig. 9c and may extend as far south as the Joban seamount chain24. The occurrence of rapid afterslip is exactly what would be expected if the region southeast of the forearc segment boundary displayed rate-strengthening behaviour during the Tohoku-oki earthquake.

Extended Data Figure 10 Numerical model of earthquake cycles on a fault obeying rate-state friction in the presence of spatially heterogeneous frictional properties.

a, Assumed distribution of the rate-dependence parameter a  −  b. b, Assumed distribution of the coefficient of friction. c, Evolution of fault slip in space and time. Black lines denote interseismic fault slip every 5 years, and red dashed lines denote coseismic slip with a time interval of 2 s. To illustrate the effects of spatial variations in the coefficient of friction and the rate-dependence parameter a  −  b on the patterns of seismicity, we performed simulations of earthquake cycles of a fault governed by rate-state friction. We assumed a relatively simple case of two asperities (high stress, strong velocity-weakening fault sections) separated by a weak (low coefficient of friction, weak velocity-weakening fault section. The computational domain is 100 km long, and the characteristic size of asperities is 20 km. Simulations were performed using a boundary integral method66,67. The fault is driven at a fault distance of 100 km by prescribing a constant velocity of 100 mm yr−1. We assumed a constant normal stress of 50 MPa, and slip-weakening displacement of 10 mm. Unlike the case of a single velocity-weakening asperity that evolves to a sequence of characteristic earthquakes, the modelled earthquake sequence reveals a rich complexity and resembles many features of seismicity in the Tohoku area. There are a number of sub-events of variable size that nucleate predominantly at the boundaries of high-strength asperities, but are arrested before they grow into system-size earthquakes. These sub-events may be analogous to large (Mw 7) earthquakes that occurred in the Tohoku area before and after the great 2011 earthquake. Occasionally, the entire area breaks in a mega-event (for example, between 60 m and 70 m of cumulative slip). The slip magnitude in these events is determined by the prior slip history and pre-stress. The model also predicts episodic creep in the middle of the velocity-weakening patch (for example, at 38 m of cumulative slip between 45 km and 60 km) that may be relevant to inferences of low seismic coupling of certain parts of the megathrust. Additional complexity in slip behaviour is likely to be introduced by variations in frictional properties at different spatial wavelengths, and in both along-strike and down-dip directions.

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Bassett, D., Sandwell, D., Fialko, Y. et al. Upper-plate controls on co-seismic slip in the 2011 magnitude 9.0 Tohoku-oki earthquake. Nature 531, 92–96 (2016).

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