Abstract
The 2011 Tohoku-Oki earthquake ruptured a large area of the megathrust east of NE Japan. The earthquake’s magnitude was 9.0, substantially larger than predicted. It is important to know what occurred in the source area prior to this great megathrust earthquake to improve understanding of the nucleation processes of large earthquakes and risk assessments in subduction zones. Seafloor observation data revealed the existence of two extremely large slip patches: one just updip of the mainshock hypocenter and the other 80–100 km to the north near the trench axis. For 70–90 years before 2003, M > 6 events and slips of M > c. 7 events on the megathrust occurred in the areas surrounding these two large slip patches. Seismic activity had increased since at least 2003 in the downdip portion of the source area of the Tohoku-Oki earthquake. In addition, long-term accelerated slow slip occurred in this downdip portion of the source area in the decades before the Tohoku-Oki earthquake. About 1 month before the earthquake, a slow slip event (SSE) took place at relatively shallow depths between the two large slip patches, accompanied by foreshock activity. Both the slow slip and foreshocks propagated from north to south toward the southern large slip patch. Two days before the earthquake, an M 7.3 foreshock and an associated postseismic slip began at relatively deep depths in the megathrust between the two large slip patches. In addition, a slow slip type event seems to have occurred approximately half a day after the M 7.3 foreshock near the mainshock hypocenter. This slow slip event and the foreshock activity again propagated from north to south toward the mainshock hypocenter. These long- and short-term preceding seismic and aseismic slip gradually reduced the interplate coupling, increased shear stresses at the two large slip patches (i.e., two strong asperity patches), and finally led to the rupture of the great Tohoku-Oki earthquake.
Highlights
The Mw 9.0 Tohoku-Oki earthquake on March 11, 2011, the largest earthquake in the modern history of Japan, occurred along the plate interface east of Tohoku, NE Japan
We have reviewed the activity of earthquakes and slow slip event (SSE) in the source area prior to the Tohoku-Oki earthquake
Seafloor observation data from directly above the large slip area of this event played an important role in constraining the coseismic slip distribution of the mainshock, showing the existence of two extremely large slip patches: one just updip of the mainshock hypocenter, 60–80 km landward of the trench axis, and the other 80–100 km to the north, near the trench axis
Summary
The Mw 9.0 Tohoku-Oki earthquake on March 11, 2011, the largest earthquake in the modern history of Japan, occurred along the plate interface east of Tohoku, NE Japan. OBP gauges installed directly above the source area detected an SSE that occurred in February 2011, just before the 2011 Tohoku-Oki earthquake [4] This SSE took place at a relatively shallow depth between the two large slip patches, as shown in Fig. 4c by a rectangle of red broken lines. Seismic and aseismic slips on the megathrust surrounding these two strong asperity patches (i.e., the two large slip patches) increased shear stress and quasi-static slips propagated toward the initial rupture point of the Tohoku-Oki earthquake within the southern strong patch, triggering the great M 9.0 megathrust earthquake This is what occurred in the source area prior to this great megathrust earthquake, which is important in relation to understanding the nucleation processes of great megathrust earthquakes
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