THE 2011 TOHOKU EARTHQUAKE (Mw9.0): PRECEDING SEISMIC ACTIVITY AND PLATE BOUNDARY COUPLING

Fault zone heterogeneity and plate boundary coupling have been debated repeatedly to understand the mechanisms that control characteristics of earthquake occurrence. Two recent megathrust earthquakes, one in the Indian Ocean in 2004 and the other in the Japanese subduction zone in 2011, have led debates to focus on the variable natures of earthquake ruptures within the same subduction zones.

The March 11^th, 2011 Tohoku-Oki earthquake (Mw9.0) took place at the plate boundary between the subducting Pacific plate and the overriding North American plate, where a large earthquake (~Mw7.5) was anticipated to occur with a recurrence interval of 30 to 40 years. The previous event (Mw7.5) occurred in 1978 and there was an Mw7.2 event on March 9^th in the vicinity. However, the March 9^th event turned to be a mere foreshock to the March 11^th megathrust earthquake that released about 500 times as much seismic energy as that of the Mw7.2 event.

These earthquake epicenters including that of the March 11^th earthquake are located approximately along a line normal to the trench axis, the March 9^th event took place seaward, the 1978 event landward and the March 11^th epicenter in between. The 1978 earthquake showed a complex source rupture process and was followed by numerous aftershocks which expanded substantially over a 1-year period. The aftershock activity (or seismic activity after an earthquake) manifests itself the stress migration which may affect to raise the stress level beyond the ruptured fault zone. If the raised stress reaches a critical level of strength, another event would occur. If it is below the critical level, no further event should occur. The outstanding issue is how to monitor the progressing stress change in relation to the strength distribution which is heterogeneous and not known. I apply the energy contour map method which was originally developed to determine aftershock area expansion patterns (Tajima and Kanamori, 1985) to evaluate the preceding earthquake sequences in this subduction zone for the period from 1968 to present and determine if the source area of the impending megathrust earthquake could have been detected.