Prominent foreshock activity preceded the Mw 7.9 off-Etorofu earthquake in the Kurile Islands on 3 December 1995. Using a modified joint hypocenter determination method (Hurukawa, 1995), we simultaneously relocated foreshocks, the mainshock, and aftershocks to study the foreshock activity in detail in view of the nucleation process of a large earthquake. The distribution of relocated earthquakes and their focal mechanisms suggest that the 1995 off-Etorofu earthquake was an interplate earthquake at the boundary between the North American and Pacific plates. Its source area overlaps with those of the 1958 off-Etorofu (Mw 8.3) and the 1963 off-Urup (Mw 8.5) earthquakes.
Nine days before the occurrence of the mainshock, a first immediate foreshock of magnitude 6.4 occurred at the deepest point of the foreshock area. This event was followed by many foreshocks, including three additional M ≧ 6 events, which occurred east and southeast of the first foreshock. The foreshock area expanded toward the trench axis with a velocity of several to several tens centimeters per second. The number of events per day increased daily, and the final size of the foreshock area was about 80 × 30 km. Eventually, the rupture of the mainshock started at the deepest point of the foreshock area. These observational facts are consistent with recent theoretical studies and laboratory experiments, in which foreshocks are regarded as the rupture of localized asperities in a broad weak zone where the nucleation of the large earthquake started.