Combined use of SPAC, FK and HVSR microtremor survey methods for site hazard study over the 2D Tamar Valley, Launceston, Tasmania

Abstract

The presence in Launceston of the ancient Tamar valley, filled with soft Tertiary and Quaternary sediments, is thought to induce a 2Dv seismic resonance pattern, amplifying the surface motion over the valley. We combine the use of spatially averaged coherency (SPAC), frequency-wavenumber (FK) and horizontal to vertical spectrum ratio (HVSR) microtremor (passive seismic) survey methods to identify and characterize possible 2D effects over the Tamar valley. Array observations are presented at two selected sites; GUNN Company (site GUN) located over an assumed 1D geology and King?s Park (site KPK) inside the valley. We obtain, by interpretation of SPAC data, a precise estimation of the shear wave velocity (or slowness) profile with depth. We calculate the dispersion curves at both sites with SPAC and FK to compare the methods. Both dispersion curves present similar features; however SPAC allows obtaining data over a wider range of frequencies. HVSR curves are compared with modelled Rayleigh wave ellipticity computed using the slowness profiles obtained by SPAC. At GUN, the peaks on HVSR and ellipticity coincide closely. However, we observe a shift to higher frequency on HVSR peak when compared to ellipticity peak at KPK. We postulate possible 2D effects might explain that shift.