Shallow S-Wave Velocity Structures in the Western Coastal Plain of Taiwan

Che-Min Lin ； Tao-Ming Chang ； Yu-Chih Huang ； Hsien-Jen Chiang ； Chun-Hsiang Kuo ； Kuo-Liang Wen

《Terrestrial, Atmospheric and Oceanic Sciences》 20卷2期 (2009/04) Pp. 299-308

並列摘要

The western coastal plain of Tai wan borders the Western Foot hills, which is a fold-and-thrust zone and one of Taiwan's major seismic zones. Earth quakes from the Western Foot hills are a frequent cause of disaster in the western coastal plain. In this study, array measurements of microtremors were con ducted 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 in version 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 in creased sharply from 0.808 to 1.289 km sec^(-1). This interface seems to be the lower boundary of the thick un consolidated sediments. The second main interfaces were between 0.9 to 1.75 km and the average velocity in creases 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 up per 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 Neo gene affects the features of these two inter faces. The depths vary apparently with the distances between the sites and the top of the Peking Basement High; i.e., Penghu Is land. 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.

並列關鍵字

Array ； Microtremor ； S-wave velocity ； Frequency-Wavenumber F-K ； Genetic algorithm ； Peikang basement high ； Western coastal plain ； Taiwan