A zone of anomalously low b-values within the subducting slab prior to the September 26, 2003 Tokachi-oki, Japan, earthquake (M=8.0)

The M=8.0 26 September 2003 Tokachi-oki earthquake occurred in the southern Kuril Trench southeast of Hokkaido, Japan, close to the epicentre of the another very large earthquake in 1952 (M=8.1) [Yamanaka and Kikuchi, 2003]. The coseismic rupture process during each of the two earthquakes has been analysed using seismic and geodetic data, for the 2003 event [e.g., Yamanaka and Kikuchi, 2003; Koketsu et al., 2004; Yagi, 2004], and tsunami data, for the 1952 event [e.g., Hirata et al., 2003], and the spatial distribution of asperities within the subduction zone has also been estimated. The b-value of an earthquake catalogue, defined as the slope of the Gutenberg-Richter frequency-magnitude relationship, log N = a - bM, is typically found to be ∼1 in a variety of tectonic situations. However, several factors appear to influence b locally [e.g., Mogi, 1962; Scholz, 1968; Warren and Latham, 1970; Wyss, 1973; Urbancic et al., 1992; Wiemer and Wyss, 1997; Enescu and Ito, 2002]. In the basis of the investigations of previous researchers, observations of relatively low b-values may reflect locally elevated shear or effective stresses. It is widely accepted that the bulk of the coseismic moment release during interplate earthquakes occurs recurrently near one or more large asperities at which shear stress is concentrated by incremental subduction [e.g., Tanioka and Ruff, 1996; Nagai et al., 2001; Iio et al., 2003; Igarashi et al., 2003; Uchida et al., 2003]. Our analysis of seismicity data from the subducting slab along the Kuril Trench reveals a zone of anomalously low b-values near the hypocenter of the 26 September 2003 Tokachi-oki earthquake (M=8.0). The b-value time-series shows that b-values decreased from initial values of ~0.8 to values as low as 0.4 during the three years prior to the mainshock. Here we show that the anomalously low b-value in the subducting slab prior to the mainshock provide seismological evidence for high stress concentrations associated with interseismic strain accumulation. Changes in b-value may provide a means of monitoring future stress changes at shallow depths ( ∼140 km) within the subducting slab.