We discovered a clear positive dependence of oceanic intraplate earthquake (OCEQ) b‐values on the age of the oceanic lithosphere. OCEQ b‐values in the youngest (<10 Ma) oceanic lithosphere are around 1.0, while those in middle to old (>20 Ma) oceanic lithosphere exceed 1.5, which is significantly higher than the average worldwide earthquake b‐value (around 1.0). On the other hand, the b‐value of intraplate earthquakes in the Ninety East‐Sumatra orogen, where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere, is 0.93, which is significantly lower than the OCEQ b‐value (about 1.9) with the same age (50–110 Ma). Thus, the variation in b‐values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure. We revealed a negative strain rate dependency of the b‐value at extremely low strain rates (<2 × 10−10/year), which can clearly explain the above b‐values. We propose that the OCEQ b‐value depends strongly on strain rate (either directly or indirectly) at extremely low strain rates. The high OCEQ b‐values (>1.5) in oceanic lithosphere >20 Ma old imply that future improvement in seismic observation will capture many smaller magnitude OCEQs, which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics.

The forearc in Northeast Japan subsided (3–4 mm/year) in the interseismic ~100 years before the 2011 Tohoku earthquake (M W 9.1) just like it did during this event. This study attempts to understand the mechanism of the vertical displacement of the forearc during gigantic earthquake cycles via numerical modeling. The results suggest that the interseismic subsidence rate in the forearc increases with the duration of the locking of the asperity of the gigantic earthquake over several hundred years, due to the increasing slip deficit rate on the deeper parts of the plate interface. The increasing slip deficit rate is caused by both the decreasing the shear stress in the shear zone owing to the continuous locking of the asperity and the increasing the mobility of the continental lithosphere owing to the viscoelastic relaxation in the mantle wedge. The deep slip deficit rate extending to ~100 km depth of the plate interface is necessary to explain the observed interseismic forearc subsidence rate. The results also suggest hundreds of years of continuous locking of the asperities of a gigantic earthquake in the western Kuril subduction zone, where fast forearc subsidence has been observed as well.

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How freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information, because it reflects the boundary condition of the oceanic lithosphere, which includes information regarding the magnitude of driving/resisting forces of plate tectonics. We investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes (OCEQs) and numerical simulations. The stress release due to OCEQs in young oceanic lithosphere (5–15 Ma) shows significant differences between the spreading directional component and the ridge‐parallel component. The extensional stress release of the ridge‐parallel component is 6 times as large as that of the spreading directional component, while the compressional stress release of the ridge‐parallel component is one seventh that of the spreading directional component. We conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge‐parallel direction can explain the observed anisotropic stress release. The result indicates that young oceanic lithosphere (5–15 Ma) barely contracts in the ridge‐parallel direction (only 0–30% of the spreading directional contraction rate), while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low‐viscosity asthenosphere. From the results, we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0.44 MPa.