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

Geophysical Journal International Geophys. J. Int. (2012) 191, 765–780 doi: 10.1111/j.1365-246X.2012.05654.x Combining HVSR microtremor observations with the SPAC method for site resonance study of the Tamar Valley in Launceston (Tasmania, Australia) M. Claprood,1, ∗ M. W. Asten1 and J. Kristek2 1 School of Geosciences, Monash University, Melbourne VIC 3800, Australia. E-mail: of Mathematics, Physics and Informatics, Comenius University, Mlynska dolina F1, 842 48 Bratislava, Slovak Republic and Geophysical Institute, Slovak Academy of Sciences, Dubravska cesta 9, 845 28 Bratislava, Slovak Republic 2 Faculty Key words: Numerical solutions; Surface waves and free oscillations; Site effects; Wave propagation; Australia. 1 I N T RO D U C T I O N The presence of low velocity sediments slows down the propagation of seismic waves generated by earthquake. It also induces amplification of the surface motion at a frequency of resonance proportional to the velocity and thickness of soft sediments above hard bedrock. Abrupt lateral variations of geology further amplify the surface motion and shift the frequency of resonance, generating a different pattern of resonance than expected above a layered earth. These local geology effects have significant importance when evaluating seismic hazard and seismic risk at specific sites (Horike 1985). ∗ Now at: Institut National de la Recherche Scientifique, Quebec City, QC, Canada.  C 2012 The Authors C 2012 RAS Geophysical Journal International  The situation of the City of Launceston (Tasmania, Australia) is an interesting example where such local geology effects are observed. While Launceston is not located in highly seismically active region (Fig. 1), damage has occurred to some buildings in the city from past earthquakes, which epicentres were located at more than 200 km from the city centre. Several hypotheses may explain the damages, including high vulnerability of the structure and complex pattern of resonance generated by abrupt changes in the near surface geology across the City of Launceston. While we do not discard the hypothesis of structure vulnerability, our study investigates the site resonance pattern expected to develop in the Tamar Valley. The recording of ambient ground vibrations, or microtremors, has proven to provide a good estimation of the frequency of resonance and shear wave velocity (SWV) structure to complete such hazard 765 GJI Seismology SUMMARY 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. Accepted 2012 August 15. Received 2012 June 7; in original form 2011 September 22 766 M. Claprood, M. W. Asten and J. Kristek zonation studies (Horike 1985; Field 1996; Kudo et al. 2002). For the purpose of this study, we use the term microtremor for ambient vibrations of any sources, from low frequency natural phenomena to high frequency human activities. Single station microtremor methods, such as the horizontal to vertical spectrum ratio (HVSR) and the standard spectral ratio (SSR), are commonly used to estimate the frequency of resonance of layered earth geology, and to generate earthquake hazard or expected ground motion zonation maps (Ibs-von Seht & Wohlenberg 1999; Parolai et al. 2002; Fäh et al. 2003; Mirzaoglu & Dýkmen 2003; Tanimoto & Alvizuri 2006). The efficiency and low cost of HVSR field survey make that method a popular choice for resonance and microzonation studies (Lachet & Bard 1994). The interpretation of HVSR observations gives an accurate estimate of the fundamental frequency of resonance of soft sediments over hard bedrock (Field & Jacob 1995). Different patterns of resonance develop above complex geologies such as 2-D and 3-D valleys in-filled with soft sediments. Several studies were completed to analyse the generation and propagation of the different components of surface waves induced in valleys of various dimensions (Bard & Bouchon 1980a,b, 1985; Kawase & Aki 1989; Frischknecht & Wagner 2004). Many authors have demonstrated the potential of single station microtremor methods to detect a 2-D pattern of resonance, and to identify the frequencies of resonance expected to develop in a valley environment. Steimen et al. (2003) used the SSR method to analyse the resonance effects from the St Jakob-Tüllingen and Vetroz valleys in Switzerland. Results from the Vetroz Valley were studied in further detail by Roten et al. (2006) to better distinguish between laterally propagating surface waves induced by a 1-D pattern of resonance and vertically propagating standing waves generated by a 2-D pattern of resonance. Uebayashi (2003) used HVSR observations to constrain the modelling of 3-D basin structures; comparing modelled HVSR, observed HVSR and theoretical Rayleigh wave ellipticity curves to analyse the complex geology across the Osaka Basin (Japan). Hinzen et al. (2004) used HVSR observations to map the changes in sediments thickness across the normal fault Lower Rhine Embayment (Germany). Cara et al. (2008) noted significant  C 2012 The Authors, GJI, 191, 765–780 C 2012 RAS Geophysical Journal Internatio (...truncated)