THE 1995 OFF-ETOROFU EARTHQUAKE - JOINT RELOCATION OF FORESHOCKS, THEMAINSHOCK, AND AFTERSHOCKS AND IMPLICATIONS FOR THE EARTHQUAKE NUCLEATION PROCESS

Prominent foreshock activity preceded the Mw 7.9 off-Etorofu earthquak e in the Kurile Islands on 3 December 1995. Using a modified joint hyp ocenter determination method (Hurukawa, 1995), we simultaneously reloc ated foreshocks, the mainshock, and aftershocks to study the foreshock activity in detail in view of the nucleation process of a large earth quake. The distribution of relocated earthquakes and their focal mecha nisms suggest that the 1995 off-Etorofu earthquake was an interplate e arthquake at the boundary between the North American and Pacific plate s. 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 occ urrence of the mainshock, a first immediate foreshock of magnitude 6.4 occurred at the deepest point of the foreshock area. This event was f ollowed by many foreshocks, including three additional M greater than or equal to 6 events, which occurred east and southeast of the first f oreshock. The foreshock area expanded toward the trench axis with a ve locity of several to several tens centimeters per second. The number o f events per day increased daily, and the final size of the foreshock area was about 80 x 30 km. Eventually, the rupture of the mainshock st arted at the deepest point of the foreshock area. These observational facts are consistent with recent theoretical studies and laboratory ex periments, in which foreshocks are regarded as the rupture of localize d asperities in a broad weak zone where the nucleation of the large ea rthquake started.