Geological variation in S-wave velocity structures in Northern Taiwan and implications for seismic hazards based on ambient noise analysis

Ambient noise analysis in Northern Taiwan revealed obvious lateral variations related to major geological units. The empirical Green's functions extracted from interstation ambient noise were regarded as Rayleigh waves, from which we analyzed the group velocities for period from 3 to 6 s. According to geological features, we divided Northern Taiwan into seven subregions, for which regionalized group velocities were derived by using the pure-path method. On average, the group velocities in mountain areas were higher than those in the plain areas. We subsequently inverted the S-wave velocity structure for each subregion down to 6 km in depth. Following the analysis, we proposed the first models of geology-dependent shallow S-wave structures in Northern Taiwan. Overall, the velocity increased substantially from west to east; specifically, the mountain areas, composed of metamorphic rocks, exhibited higher velocities than did the coastal plain and basin, which consist of soft sediment. At a shallow depth, the Western Coastal Plain, Taipei Basin, and Ilan Plain displayed a larger velocity gradient than did other regions. At the top 3 km of the model, the average velocity gradient was 0.39 km/s per km for the Western Coastal Plain and 0.15 km/s per km for the Central Range. These S-wave velocity models with large velocity gradients caused the seismic waves to become trapped easily in strata and, thus, the ground motion was amplified. The regionalized S-wave velocity models derived from ambient noises can provide useful information regarding seismic wave propagation and for assessing seismic hazards in Northern Taiwan.