Shear-wave Velocity Structure of the Ilan Basin Using the Microtremor H/V Spectral Ratio Modeling

Shear-wave Velocity Structure of the Ilan Basin Using the Microtremor H/V Spectral Ratio Modeling

The Ilan Basin located in northeastern Taiwan is a delta filled with unconsolidated quaternary alluvium with an underlying Miocene basement. Because its tectonic structure belongs to the southwestern extension of the Okinawa Trough back-arc Basin, seismicity is extremely high around this basin. The obvious seismic site-effect can amplify and extend the incident seismic waves and result in earthquake disasters. A detailed shallow velocity structure reflecting real and complex seismic site-effects is necessary for ground motion predictions of the Ilan Basin. A dense microtremor survey of 354 sites has been conducted to evaluate in-detail site responses all over the Ilan using the horizontal-to-vertical(H/V) spectral ratio analysis from a previous study. This microtremor H/V spectral ratio data is modeled to estimate the shallow shear-wave velocity profiles using a method combining the Haskell propagator matrix theoretical transfer function with a Genetic Algorithm. The numerous microtremor data provide detailed information to figure out the shear-wave velocity and the thickness of the strata, which control the seismic site-effect. Finally, a complete and detailed shear-wave velocity structure of the Ilan Basin is established and it shows good correlation with the existing geological structure map.