Initial results from spatially averaged coherency, frequency-wavenumber, and horizontal to vertical spectrum ratio microtremor survey methods for site hazard study at Launceston, Tasmania

서지정보

· 발행기관 : 한국지구물리.물리탐사학회
· 수록지 정보 : 지구물리와 물리탐사 / 12권 / 1호 / 132 ~ 142페이지
· 저자명 : Maxime Claprood, Michael W. Asten

초록

The Tamar rift valley runs through the City of Launceston, Tasmania. Damage has occurred to city buildings due
to earthquake activity in Bass Strait. The presence of the ancient valley, the Tamar valley, in-filled with soft sediments that vary
rapidly in thickness from 0 to 250mover a few hundreds metres, is thought to induce a 2D resonance pattern, amplifying the
surface motions over the valley and in Launceston. Spatially averaged coherency (SPAC), frequency-wavenumber (FK) and
horizontal to vertical spectrum ratio (HVSR) microtremor survey methods are combined to identify and characterise site
effects over the Tamar valley.
Passive seismic array measurements acquired at seven selected sites were analysed with SPAC to estimate shear wave
velocity (slowness) depth profiles. SPAC was then combined with HVSR to improve the resolution of these profiles in the
sediments to an approximate depth of 125 m. Results show that sediments thicknesses vary significantly throughout
Launceston. The top layer is composed of as much as 20m of very soft Quaternary alluvial sediments with a velocity from
50 m/s to 125 m/s. Shear-wave velocities in the deeper Tertiary sediment fill of the Tamar valley, with thicknesses from 0 to
250m vary from 400 m/s to 750 m/s.
Results obtained using SPAC are presented at two selected sites (GUN and KPK) that agree well with dispersion curves
interpreted with FK analysis. FK interpretation is, however, limited to a narrower range of frequencies than SPAC and seems
to overestimate the shear wave velocity at lower frequencies. Observed HVSR are also compared with the results obtained by
SPAC, assuming a layered earth model, and provide additional constraints on the shear wave slowness profiles at these sites.
The combined SPAC and HVSR analysis confirms the hypothesis of a layered geology at the GUN site and indicates the
presence of a 2D resonance pattern across the Tamar valley at the KPK site.

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