Coupling coefficient, hierarchical structure, and earthquake cycle for the source area of the 2011 off the Pacific coast of Tohoku earthquake inferred from small repeating earthquake data

We have estimated the spatial distribution of interplate coupling in and around the source area for the 2011 Tohoku earthquake from small repeating earthquake data. The source area of the 2011 Tohoku earthquake exhibited a relatively high coupling coefficient (>0.5) for the period from 1993 to 2007 and was surrounded by areas of low coupling (<0.5) at its western (down-dip), northern and southern extents. These low-coupling areas probably prevented further propagation of the mainshock rupture. A high coupling coefficient is estimated even near the trench that can act as the source of the large tsunami of the present earthquake. The averaged seismic coupling of 0.5-0.8 in the M 9 earthquake's source area and the seismic moment of the earthquake suggest that the slip deficit for the M 9 earthquake was accumulated over a period of 260-880 years, consistent with the recurrence interval of such great earthquakes from tsunami deposit data. It also suggests that the several M ∼ 7 earthquakes which have occurred in the source area since 1926 were minor events that released only a part of the accumulated strain energy. The hierarchical structure of asperities, as in the case of the Kamaishi sequence, may be the key to understanding huge earthquakes that encompass several smaller asperities.