Date of Award
Doctor of Philosophy
Charles A Langston
Roy Van Arsdale
Three individual studies are performed in northern Taiwan.First, the relatively-new Strong Motion Downhole Array (SMDA) in Taipei Basin is examined. The SMDA is comprised of 32 triggered accelerometers spanning eight sites—each site hosting one at the surface and two to four additional each co-located down individual boreholes. A variety of remediable issues are demonstrated in this dataset, including: polarity reversals, swapped components, clock desynchronization, and bad components. Signal-to-noise ratios are generally excellent. However, the SMDA is plagued with orientation problems; since each instrument lies within its own individual borehole, physical orientation of each must be individually accounted for. Some orientations exhibit wander over time; polarity reversals and possibly swapped components on the horizontal further obfuscate azimuthal orientations.Second, near-surface, site-specific, and frequency-dependent amplification of seismic waves is observed on the SMDA and shown to be due to quarter-wavelength constructive interference between the surface and the base of the Sungshan silts—the topmost sedimentary formation in Taipei Basin. Geological data is used to verify a seismological relationship between dominant frequency of S-waves and the thickness of the silts in the basin. Finally, a map of Taipei Basin is produced showing contours of near-surface structure and dominant frequencies of amplification. This study empirically validates several seismological concepts, including quarter-wavelength constructive interference and downward continuation of the P–SV wavefield.Third, a tomographic study is performed in northern Taiwan, specifically targeting the Tatun Volcano Group (TVG) immediately adjacent Taipei. Thought to be extinct, the TVG has recently been subject to renewed concerns of volcanic hazard. All previous tomographic studies of Taiwan have been regional and thus used inversion parameters ill-suited for resolving small and strongly heterogeneous structure at the near-surface. A heterogeneous initial model is constructed from JHD station corrections; multiple tomography passes at several spatial resolutions show a small area of low VP, low VS, and high VP/VS extending and expanding from directly beneath Mt. Chihsing—the largest volcano in the TVG—to the south–southeast. The resultant tomographic images and patterns of seismicity are compared to recent studies hypothesizing a magma chamber at 8–10 km depth.
Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Young, Brian Anthony, "Seismological Study of North Taiwan Based on Borehole and Surface Observations" (2016). Electronic Theses and Dissertations. 1362.