Using Sp converted phases to study variation of sediment thicknesses beneath Chia-Nan area of Taiwan
Date Issued
2011
Date
2011
Author(s)
Wei, Jia-Yi
Abstract
The Chia-Nan Plain is a long and narrow alluvial plain, located in the west of Central Mountain Range and the east of Taiwan Strait. Accelerograms recorded from 1992 to 2010 May by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations are used to study the thickness variations of the Quaternary alluviums beneath the Chia-Nan Plain, the biggest alluvial plain in Taiwan, by examining Sp converted phase. The thicknesses are estimated by the travel-time difference of S and Sp waves. This technique of using P-S converted phases had a quite success in the previous study of Lan-Yang Plain.
The records of seismic events that were located in the southwestern Taiwan region and recorded by 142 TSMIP stations of Chianan Plain are used. After choosing local earthquakes that have a takeoff angle greater than 150° and a focus depth deeper than 6 km to better conform vertical incidence of local propagating body waves when crossing the sediment–basement interface, there are 1,200 manually picked time differences of S-Sp arrivals from 109 TSMIP stations. With this time differences and the average P waves velocity from seismic reflection profiles and boreholes information, the thickness of the sedimentary in this area could be estimated.
By the high seismic activity and dense of TSMIP stations in this study area, we used this technique to depict the variations of sediments thicknesses in Chia-Nan Plain. In general, the thicknesses become thicker toward south and east, and the boundary between sediments and basement which generates the Sp converted waves might be located in Miocene strata. This approach is simple and time-effective and can be used to preliminarily define the general characteristics of thickness structure for a region with available converted phases before conducting comprehensive onsite geophysical surveys.
Subjects
Sp converted phase
Taiwan
Chia-Nan Plain
Type
thesis
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