Asociate Professor Bogdan Enescu, Faculty of Life and Environmental Sciences, University of Tsukuba, collaborated with colleagues at the Swiss Federal Institute of Technology in Zurich (ETH Zurich), to show that the stress recovery following the 2011 M9.0 Tohoku-oki earthquake has been significantly faster than previously anticipated; specifically, the stress-state at the plate interface returned within just a few years to levels observed before the megathrust event. In addition, since there is no observable spatial difference in the stress state along the megathrust zone, it is difficult to predict the location and extent of future large ruptures. 

Constraining the recurrence of megathrust earthquakes is genuinely important for hazard assessment and mitigation. The prevailing approach to model such events worldwide relies on the segmentation of the subduction zone and quasi-periodic recurrence due to constant tectonic loading. The researchers analyzed earthquakes recorded along a 1,000-km-long section of the subducting Pacific Plate beneath Japan since 1998 to map the relative frequency of small to large earthquakes (the so-called "b-value" parameter -- which on average is close to 1.0), in both space and time. Evidence from laboratory experiments, numerical modeling and natural seismicity indicates that the b-value is negatively correlated with the differential stress. The present analysis reveals that the spatial distribution of b-values reflects well the tectonic processes accompanying plate motion. However, there is no evidence of distinct earthquake-generation regions along the megathrust, associated with the so-called "characteristic earthquakes."