On 27 May 1995, a large earthquake (Ms 7.6) occurred in the northern part of Sakhalin Island near an uncertain plate boundary between the Okhotsk (or the North American) plate and the Eurasian (or the Amurian) plate. The source process of the mainshock, the detailed aftershock activity, the b-value of the frequency–magnitude relationship for aftershocks, and the 3D P-wave velocity distribution have been investigated. Thirty-one body-wave records were inverted to determine the sequence of subevents of the mainshock using an iterative deconvolution method. The source parameters obtained are the location of the initial epicenter (52.64° N, 142.83° E); the centroid depth (9 km); strike, dip, and slip, equal to (191°, 87°, – 172°) for the first subevent, (198°, 76°, – 175°) for a second subevent, and (196°, 79°, – 174°) for the combined source; and the total seismic moment M0 (4.2 × 1019 N m) and Mw (7.0). We also deployed 12 temporary seismographic stations in the focal area from 7 June to 8 July 1995. More than 2000 events were located in an aftershock area of 60 km × 10 km that trends in a N15°E direction, which is consistent with the location of the surface rupture tracing the Gyrgylan'i–Ossoy fault. The P-wave velocity patterns were compared with the aftershock seismicity and the spatial distribution of the b-values, with results suggesting that high velocity correlates with high seismicity and small b-value. Our research strongly suggests that the 1995 Sakhalin earthquake was one of the outstanding events that define the uncertain plate boundary.