The b value of an earthquake catalogue, defined as the slope of the Gutenberg-Richter frequency-magnitude relationship (logN = a - bM), is typically close to unity in many tectonic settings. However, analysis of seismicity data for the subducting slab along the Kuril Trench reveals a zone of anomalously low b values near the hypocenter of the 2003 Tokachi-oki earthquake (M 8.0). The zone of low b value (0.50--0.65) at depths of less than 80 km in the subducting slab changes in location with time in 1990--2003 and intensifies leading up to the 2003 earthquake. This transient low b value zone corresponds roughly with the location of an asperity inferred from waveform inversion in recent research. In the shallow zone corresponding to the immediate region (<125 km along the subducting slab) of the 2003 event, the b value is shown to have decreased in the year prior to the main shock from initial values of ~0.65 to values as low as 0.5. The occurrence of the 2003 earthquake, a large thrust event on the subducting plate interface, in this zone of anomalously low b values suggests that low b values in the subducting slab prior to the main shock and during the aftershock series are concordant with high stress concentrations associated with interseismic strain accumulation and redistribution in the vicinity of the rupture.