Parameters of split transverse waves from local earthquakes along eastern Hokkaido recorded before the Tokachi-Oki September 26, 2003, earthquake (M = 8.0)

Abstract

The parameters of split S waves from local weak earthquakes along eastern Hokkaido Island are studied over the period of 2003, including the strong Tokachi-oki September 26, 2003 earthquake (M = 8.0). Earthquake records of five stations belonging to the ISV seismological network were used. The studies of the split S wave parameters showed that they vary in space and time along Hokkaido Island. The zones of the Hidaka Mountains (ERM, MYR), Tokachi Plain (IWN, URH), and Kushiro Plain (AKK) are distinguished along Hokkaido. The anisotropy coefficients beneath the ERM, MYR, IWN, URH, and AKK stations attain 10.5, 10, 5, 3.5, and 6.5%, respectively. Beneath ERM, azimuths of the fast S wave (ϕ) are predominantly in the N-S direction until July and in the E-W direction from July (parallel and normal to the Japan trench strike). By the time of the Tokachi-oki earthquake, the ϕ directions were oriented SE in agreement with the direction of the Pacific plate motion. The ϕ directions on the northern side of the Hidaka Range (MYR) are predominantly orthogonal to those beneath ERM, which can be evidence for differences in the direction of deformations on opposite sides of the range. Higher seismicity, the variation of S wave parameters, and a high anisotropy of the medium point to an intense development of deformation (dilatancy) processes in the area of the Hidaka Mountains. The fast wave azimuths beneath AKK are predominantly 50°–70°, and this orientation is consistent with the direction of migration of the Kurile arc front along the trench. Beneath IWN, the azimuths ϕ are oriented along the N-NE directions, and beneath URH, along the direction of the Pacific plate motion (100°–150°). Strengthening of mechanical properties of the medium and development and accumulation of shear deformations in a subhorizontal plane are supposed to take place in the Tokachi Plain area.