Episodic slow slip events in the Japan subduction zone before the 2011 Tohoku-Oki earthquake

We describe two transient slow slip events that occurred before the 2011 Tohoku-Oki earthquake. The first transient crustal deformation, which occurred over a period of a week in November 2008, was recorded simultaneously using ocean-bottom pressure gauges and an on-shore volumetric strainmeter; this deformation has been interpreted as being an M6.8 episodic slow slip event. The second had a duration exceeding 1 month and was observed in February 2011, just before the 2011 Tohoku-Oki earthquake; the moment magnitude of this event reached 7.0. The two events preceded interplate earthquakes of magnitudes M6.1 (December 2008) and M7.3 (March 9, 2011), respectively; the latter is the largest foreshock of the 2011 Tohoku-Oki earthquake. Our findings indicate that these slow slip events induced increases in shear stress, which in turn triggered the interplate earthquakes. The slow slip event source area on the fault is also located within the downdip portion of the huge-coseismic-slip area of the 2011 earthquake. This demonstrates episodic slow slip and seismic behavior occurring on the same portions of the megathrust fault, suggesting that the faults undergo slip in slow slip events can also rupture seismically.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Transient crustal deformations were seen in 2008 and 2011 near the Japan Trench. ► The observed deformations are explained by M6.8 and M7.0 slow slip events. ► The slow slip events preceded moderate or large interplate earthquakes. ► The fault is located in the coseismic slip area of the 2011 Tohoku-Oki earthquake.