Abstract
A clear aftershock gap was found in the central portion of the belt shaped aftershock area of the 1995 Hyogo-ken Nanbu earthquake. We term this gap an "earthquake bright spot." In this paper, the rupture growth process of the main shock is discussed in the framework of the earthquake bright spot model. The resultant process is as follows: The rupture initiated at the depth of 14 km, which was the bottom of the seismogenic zone, and it propagated in a semicircular direction. The early stage of rupture was very slow with small dislocation. After about 4 s, it accelerated by successive fault step-over and some subfaults were produced in the confined region, the "bright spot." The preliminary duration of 4s satisfies Umeda's empirical formula, which expresses the relationship between the nucleation time and earthquake magnitude. Powerful high-frequency waves of 0.5-1.5 Hz were concentrically radiated from the bright spot within a short time. After about 9s, just beneath Kobe City, predominant long-period waves of 5-6 s were produced by moment release from smooth dislocation of a fault.
