The western coastal plain of Taiwan borders the Western Foothills, which is a fold-and-thrust zone and one of Taiwan's major seismic zones. Earthquakes from the Western Foothills are a frequent cause of disaster in the western coastal plain. In this study, array measurements of microtremors were conducted at seven sites through out the western coastal plain. At each site, four arrays (S, M, L, and XL arrays) of different sizes were considered. The largest radius of each array ranged from 32 m to 1 km. The Maximum Likelihood Method of Frequency-Wavenumber (F-K) analyses was used to obtain phase velocity dispersion curves. After the inversion of the dispersion curves with the initial models decided by Genetic Algorithm (GA) searching, shallow S-wave velocity structures for the western coastal plain were estimated.
The S-wave velocity structures were estimated from the surface to a depth of 3 km. Above a depth of 0.4 km, they in creased gradually under all sites. The first main interfaces were between 0.4 to 0.75 km in depth, and the average velocity at all sites increased sharply from 0.808 to 1.289 km sec-1. This interface seems to be the lower boundary of the thick unconsolidated sediments. The second main interfaces were between 0.9 to 1.75 km and the average velocity increases from 1.289 to 1.703 km sec-1. The third main interfaces were between 1.15 to 2.55 km and the average velocity in creased from 1.703 to 2.45 km sec-1. The depths of the last two main interfaces corresponded to the tops of the Pliocene and upper Miocene formations, respectively, as identified by seismic surveys and exploration wells. Further, the average S-wave velocities of the Plio-Pleistocene, Pliocene, and Miocene formations were estimated in this study. The existence of the Peikang Basement High, which is a prominent barrier in the area between the pre-Miocene and Neogene affects the features of these two interfaces. The depths vary apparently with the distances between the sites and the top of the Peking Basement High; i.e., Penghu Island. Further more, the complex structures near the Yichu hinge fault make the S-wave velocities of deeper structures under the Yijhu site higher than those under other sites based on the plane-layer assumption of the F-K analysis.