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Combining HVSR microtremor observations with the SPAC method for site resonance study of the Tamar Valley in Launceston (Tasmania, Australia)


Now at: Institut National de la Recherche Scientifique, Quebec City, QC, Canada.


The presence of the deep and narrow Tamar Valley in the City of Launceston (Tasmania, Australia), in-filled with soft sediments above hard dolerite bedrock, induces a complex pattern of resonance across the city. Horizontal to vertical spectrum ratio (HVSR) microtremor observations are combined with 1-D shear wave velocity (SWV) profiles evaluated from spatially averaged coherency spectra (SPAC) observations of the vertical component of the microtremor wavefield to complete a site resonance study in a valley environment such as the Tamar Valley. Using the methodology developed in a previous paper, 1-D SWV profiles are interpreted from observed coherency spectra (axial-COH) above the deepest point of the Tamar Valley, using pairs of sensors spatially separated parallel to the valley axis. The 1-D SWV profiles interpreted at five sites suggest the depth to bedrock interface varies from approximately z= 25 m north of the city, to z= 250 m above the deepest point of the valley. Numerical simulations of the propagation of surface waves in a 2-D model representation of the Tamar Valley compare well with HVSR observations recorded on two profiles transverse to the valley axis. HVSR observations can identify the in-plane shear (SV) frequency of resonance above the deepest part of the valley on two separate profiles transverse to the valley axis. By computing the ellipticity curves from the preferred SWV profiles interpreted by the SPAC method, the antiplane shear (SH) modes of resonance expected to develop in the Tamar Valley are identified; modes which HVSR observations alone fail to locate with precision. HVSR observations suggest a complex mix of 1-D and 2-D patterns of resonance develops across the valley. The results from this paper suggest that HVSR microtremor observations can be combined with SPAC microtremor method to characterize the geology and the pattern of resonance in a 2-D narrow structure such as the Tamar Valley.