Most large earthquakes occur along an oceanic trench, where an oceanic plate subducts beneath a continental plate. Massive earthquakes with a moment magnitude, M(w), of nine have been known to occur in only a few areas, including Chile, Alaska, Kamchatka and Sumatra. No historical records exist of a M(w) = 9 earthquake along the Japan trench, where the Pacific plate subducts beneath the Okhotsk plate, with the possible exception of the ad 869 Jogan earthquake, the magnitude of which has not been well constrained. However, the strain accumulation rate estimated there from recent geodetic observations is much higher than the average strain rate released in previous interplate earthquakes. This finding raises the question of how such areas release the accumulated strain. A megathrust earthquake with M(w) = 9.0 (hereafter referred to as the Tohoku-Oki earthquake) occurred on 11 March 2011, rupturing the plate boundary off the Pacific coast of northeastern Japan. Here we report the distributions of the coseismic slip and postseismic slip as determined from ground displacement detected using a network based on the Global Positioning System. The coseismic slip area extends approximately 400 km along the Japan trench, matching the area of the pre-seismic locked zone. The afterslip has begun to overlap the coseismic slip area and extends into the surrounding region. In particular, the afterslip area reached a depth of approximately 100 km, with M(w) = 8.3, on 25 March 2011. Because the Tohoku-Oki earthquake released the strain accumulated for several hundred years, the paradox of the strain budget imbalance may be partly resolved. This earthquake reminds us of the potential for M(w) ≈ 9 earthquakes to occur along other trench systems, even if no past evidence of such events exists. Therefore, it is imperative that strain accumulation be monitored using a space geodetic technique to assess earthquake potential.

We estimated the spatial and temporal evolution of the preceding aseismic slip from January 2003 to January 2011, the coseismic slip of the Tohoku earthquake, and the postseismic slip after the earthquake based on global positioning system (GPS) data. Time‐dependent analysis indicates aseismic slip offshore of Miyagi and Fukushima prefectures from 2004 associated with a series of subduction earthquakes that overlap the aseismic slip area. These preceding aseismic and coseismic slip areas are centered between the centers of the coseismic and afterslip areas of the Tohoku earthquake offshore of Miyagi prefecture, while they overlap the coseismic and afterslip areas of the Tohoku earthquake off Fukushima prefecture. The timing of moment magnitude nine (Mw9) ‐class earthquakes appears to be controlled by multiple preceding slip events, smaller earthquakes and their afterslip. The preceding aseismic and coseismic slip decreased the coupling rate off the Tohoku coast, suggesting the possibility that the preceding slip represented a precursive stage of the Tohoku earthquake. The afterslip of the Tohoku earthquake occurred in an area where the coseismic slip was not large, complementing the large coseismic slip zone. The afterslip along Iwate‐Miyagi extends up to 80 km in depth and is currently the sole mechanism of strain release in this depth range. The source region of the anticipated Miyagi‐Oki earthquake shows small postseismic slip after the Tohoku earthquake, reflecting the energy release at the time of the earthquake. Aftershock activity is roughly governed by an afterslip process.

Analysis of global positioning system data shows that the rate of crustal deformations in the Tokai region of Japan, a seismic gap area, changed over the past 18 months. Kalman filtering analysis shows aseismic slip on the plate boundary in the western Tokai region centered on Lake Hamana, adjacent to the anticipated Tokai earthquake source area. The cumulative moment magnitude reaches 6.7 in June 2002 with a relative slip increase northeast of Lake Haman from January 2002. An existence of aseismic slip in the western Tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 Tonankai earthquake.

Investigating slow earthquake activity in subduction zones provides insight into the slip behavior of megathrusts, which can provide important clues about the rupture extent of future great earthquakes. Using the S-net ocean-bottom seismograph network along the Japan Trench, we mapped a detailed distribution of tectonic tremors, which coincided with very-low-frequency earthquakes and a slow slip event. Compiling these and other related observations, including repeating earthquakes and earthquake swarms, we found that the slow earthquake distribution is complementary to the Tohoku-Oki earthquake rupture. We used our observations to divide the megathrust in the Japan Trench into three along-strike segments characterized by different slip behaviors. We found that the rupture of the Tohoku-Oki earthquake, which nucleated in the central segment, was terminated by the two adjacent segments.

We detected short‐term slow slip events (SSEs) previously observable only with tilt and strain data along the Nankai Trough, southwest Japan, using GNSS (Global Navigation Satellite System) data. Offsets detected in GNSS time series using Akaike's information criterion helped automatically identify 207 episodes with a motion direction opposite to that of the relative plate motion from June 1996 to January 2012. By nonlinear inversion of the detected displacement, we estimated rectangular fault models for 133 probable and 25 possible short‐term SSEs over 15 years. The SSE moment magnitudes range from 5.5 to 6.3. Most SSE fault models are located in a narrow band of non‐volcanic tremors on the interfaces of the subducting Philippine Sea Plate. Large SSEs (moment magnitude, Mw, ≥6) often occur in western and central Shikoku. The cumulative slip is distributed heterogeneously along the strike, generally decreasing eastward with the maximum slip (~50 cm) in western Shikoku. No definite short‐term SSEs were found in the Kii Channel, but several short‐term SSEs occurred in Ise Bay. Both regions are known as tremor gaps. The local maximum of the cumulative slip fills in the tremor gap located in Ise Bay. The long‐term rate of short‐term SSE cumulative moment increased by threefold around 2003 in eastern Shikoku, whereas it was almost constant in other regions. Comparison with short‐term SSE catalogues using tilt data suggests that both this study and previous studies missed some SSEs along the Nankai Trough. A combination of geodetic data is important in the monitoring of the spatiotemporal distribution of short‐term SSEs.