Long-Period Volume-Strain Seismogram of the 8 August 1993 Esashi-Oki Earthquake, off Southwest of Hokkaido, Japan and its Source Mechanism

We present an analysis of the volume-strain data for source parameters of the 8 August 1993 Esashi-Oki earthquake (MJMA =6.3) which may have been induced by the 12 July 1993 Hokkaido-Nansei-Oki earthquake (MJMA=7.8). Several numerical experiments were carried out to test the hypothesis that the data dependency is attributed to the effect of the source mechanism on the volume-strain seismograms. We used the long-period volume-strain seismogram (DCs; T s; 30 Hz) recorded at the Urakawa Seismological Observatory (KMU) of Hokkaido University, about 230 km south-east of the epicenter of the mainshock. The synthetic volume-strain seismo· grams were calculated using the normal mode theory. The experiments provided the best fitted source parameters: (strike, dip, rake) = (196°, 5T, 90°) with the seismic moment Mo=2.8 x 10 18 Nm (Mw=6.2). This solution is similar to the CMT solution obtained by using the world-wide long-period seismograms, of which mechanism is characterized by steeper-dipping and north-south-directing nodal planes. This result is supported by the distribution of aftershoks. Such a volume-strain seismo· gram may provide not only an estimation of the seismic moment but also a refinement • Present affiliation: UrakawaTown Office, Urakawa 057-8511 524 T. Takanami, T. Ogawa, 1.S. Sacks, A.T. Linde and 1. Nakanishi of the earthquake mechanism solution.