Source process of the Mw 8.3, 2003 Tokachi-Oki, Japan earthquake and its aftershocks

We analyse both the broad-band SH wave and long-period mantle wave seismograms from the 2003 September 25, Tokachi-Oki earthquake, to investigate its source process. We find that the previously published long-period solution is likely to represent an overestimation of the size of the earthquake and that the earthquake is more likely to have had a moment of 2.18 x 10(21) N m (M-w 8.2) rather than the previously published 3.05 x 10(21) N m (M-w 8.3). A thorough investigation of the SH wave seismograms indicates that rupture occurred on a shallowly dipping fault, propagating in all directions from the hypocentre. The average rupture velocity in the downdip and updip directions are similar to 2.0-3 and similar to 1.0 kms(-1), respectively. The maximum slip is found to be similar to 12 m and is located downdip of the earthquake's hypocentre. Taken over the fault as a whole, the average stress drop is similar to 0.55 MPa. Used in conjunction with relocated aftershocks, we find that two barriers constrain the faulting extent of slip, creating a rupture area of similar to 165 km x 140 km. These barriers coincide with submarine topographic features, are domain boundaries for the seismic gaps along the arc, and have high concentrations of large aftershocks near them. The high slip regions align with the orientation of the subducting plate, and resemble in size and shape the many seamounts about to subduct.