Analysis of Sources The earthquake source greatly affects seismic waves and ground motions as their origin. In the case of the 1995 Kobe earthquake, we were surprised at the long-period and short-duration features of the ground motion causing the Hanshin-Awaji earthquake disaster (Fig. 2). We have identified the directivity effect as the cause of these features. According to this effect, seismic waves from various parts of the fault form a large long-period puls in the forward direction of fault rupture (Fig. 3, Koketsu, 1996; Yoshida et al., 1996).

Fig. 2. The ground motions at the Kobe university (upper) and the Kobe JMA station (middle) during the 1995 Kobe earthquake are compared with the one observed at the Kushiro JMA station during the 1993 Kushiro-oki earthquake (lower). Fig. 3. (a) The shakings from a point source show the four-lobe pattern and the two diagonals partition them into the four lobes. (b) However, an earthquake source is a fault with some extent. Its rupture is initiated at a point and propagates at a speed of a few km/s over the fault plane. (c) The seismic waves from the parts of the fault then overlap each other with time lags. The way of overlapping varies with the direction, so that the magnitude of the shaking becomes directive. The overlapping provides it with a longer-period waveform.

Since the complex velocity structure (Fig. 4) affected seismic ground motions during this earthquake, we have developed a method for simulating ground motions in such a 3-D structure. If we use Green's functions calculated in the 3-D structure, the result of fault modeling will change significantly and we can obtain a more accurate model (Fig. 5). The city center of Kobe is locate at around x = 20km, where a slip larger than 2m is recovered in the accurate 3-D model. This result suggests positive correlation between the seismological slip distribution and the damage distribution in the Hanshin-Awaji earthquake disaster (Koketsu and Kikuchi, 2003).

Fig. 4. 3-D velocity structure in the Osaka basin (topography of the basement surface). Fig. 5. Fault models in the 3-D (upper) and 1-D (lower) structures.

Analysis of Velocity Structures It is important for these kinds of studies to reveal the structures of the crust, the subducting slab and sedimentary layers overlying them, that greatly affect seismic waves and ground motions. For this purpose, we have developed a method of ray tracing (Koketsu and Sekine, 1998), and carried out seismic tomography for the S-wave velocities and Q-values beneath the Japan islands (Fig. 6). In addition, we also developed a joint tomographic inversion of seismic and gravity data and obtain the accurate velocity structures in the Osaka and Kanto basins (Figs. 4 and 7. Koketsu and Higashi, 1992; Afnimar et al., 2002).

Fig. 6. Distribution of Q-values at a depth of 40km (reds indicate lower Q-values and higher attenuation). We can see low Q along the volcanic front, and high Q at the subducting slabs. Fig. 7.3-D velocity structure in the Kanto basin (topography of the basement surface).

Strong Motion Observation and Simulation Observations promote researches on seismic waves and ground motions as in other disciplines of seismology. In order to illustrate how seismic strong motions are built up in a source region, we are conducting strong motion observations in the Izu-Suruga Bay area, the southern part of the Tokyo metropolitan area and so on. We also carried out temporary strong-motion observations for the earthquake swarm associated with the 2000 Miyakejima eruption (Kikuchi et al., 2001) and aftershocks of the 2003 Tokachi-oki earthquake. There are six hundred strong-motion observation stations in the Kanto basin. We have developed a system widely collecting their data (SK-net, http://www.sknet.eri.u-tokyo.ac.jp/) in cooperation with the earthquake data center, and investigated how the seismic ground motion propagated within the basin. The surface wave inside the basin propagates more slowly than that outside, so that their wavefronts separate from each other and the refracted wave heals the discrepancy in the speed of advance of the wavefronts inside and outside the basin. This refracted arrival forms a new oblique wavefont traversing the basin (Fig. 8, Koketsu and Kikuchi, 2000). We also confirmed that a strong-motion simulation (Furumura et al., 2003; Koketsu et al., 2004) in the 3-D velocity structure of Fig. 6 can recover these observations (Fig. 9).

Fig. 8. Propagation of ground motions from the 1998 Izu-hanto Toho-oki earthquake (star symbol) in the Kanto basin. Fig. 9. Comparison of the observation (bs) and simulation of the ground motion propagation.

Jul. 03, 2004
Jul. 23, 2006
Aug. 27, 2010
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