Taipei, the capital of Taiwan, suffered from destructive earthquakes four times during the 20th century (M (L) = 7.3 on April 15, 1909; M (L) = 6.8 on November 15, 1986; the Chi-Chi M (L) = 7.3 earthquake on September 21, 1999; and M (L) = 6.8 on March 31, 2002). Analysis of recorded data shows a strong dependence of spectral amplification in the Taipei Basin on earthquake depth and azimuth. At low frequencies (f < 3 Hz) significant larger amplifications are observed for shallow earthquakes as compared to intermediate depth events. The former ones also display strong azimuthal dependence. As structures with large response periods such as bridges and tall buildings are sensitive to these low frequencies the understanding of the associated wave effects within the basin and their role for site effect amplification is critical. The tool we employ is 3D finite-difference modeling of wave propagation of incident wave fronts. The available detailed model of the basin allows studying the wave effects. Modeling clearly reveals that basin edge effects as observed in data are related to surface wave generation at the basin edges with a high degree of azimuthal dependency. The reproduced site amplification effects are in qualitative agreement with the observations from strong motion data.
