B747 Captain reported receiving a TCAS RA on an oceanic flight when his aircraft experienced altitude deviations due to turbulence; possibly wake-induced in RJJJ airspace.

Narrative

'I signed in as PIC to relieve [the] Captain. We were cruising at FL410 southbound on R220 with strong quartering headwinds (~220/184) and occasional chop. There was an aircraft at FL400 approximately 20 NM ahead on R220 southbound. When it became apparent that we would be overtaking the aircraft ahead; we discussed offsetting to the right to prevent wake turbulence encounter for the aircraft below us but considering that the winds were from the right; we concluded it would be best to maintain centerline. Approximately 3NM from the aircraft below; we noticed the aircraft was experiencing altitude oscillations of 200-300 feet due to turbulence. We also began to experience some turbulence with slightly lower oscillations. I requested 2NM offset to the right and the First Officer executed the offset. While overtaking the aircraft past position NOGAL; we received a TCAS 'Traffic; Traffic' alert followed by an RA 'Maintain Level'. I disconnected the autopilot and complied with the RA. Once cleared of conflict; I requested for left autopilot and continued to complete the offset. In hindsight; one option I neglected was an attempt to coordinate on 123.45 with the aircraft ahead to see if he would be willing to offset to the right so as to avoid our wake and prevent a possible TCAS event due to turbulence. Another option could have been to perhaps attempt to coordinate with RJJJ via CPDLC. Although; the decision to maintain centerline was made based on safety concerns for the other aircraft; I now see that it would have been much safer to offset 2 miles and avoid the RA and possibly a worse case of altitude separation loss due to worse turbulence than we encountered.