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

Maxime Claprood; Michael W. Asten

doi:10.1071/EG08106

RESEARCH ARTICLE

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

Maxime Claprood ¹ ² Michael W. Asten ¹

+ Author Affiliations

- Author Affiliations

¹ Centre for Environmental and Geotechnical Applications of Surface Waves (CEGAS), School of Geosciences, Monash University, Melbourne, Victoria 3800, Australia.

² Corresponding author. Email: Max.Claprood@sci.monash.edu.au

Exploration Geophysics 40(1) 132-142 https://doi.org/10.1071/EG08106
Submitted: 29 November 2007 Published: 27 February 2009

Abstract

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 250 m over 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 20 m 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 250 m 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.

Key words: frequency-wavenumber, high-resolution beamforming, microtremor survey method, passive seismic, resonance pattern, shear wave velocity profile, site effects, spatially averaged coherency.

Acknowledgements

Maxime Claprood is supported by a Monash Graduate Scholarship, an International Postgraduate Research Scholarship, and a Québec’s Funds for Nature and Technology Scholarship. Seismometers used in this project were loaned to Monash University by the Australian National Seismic Imaging Resource (ANSIR).

References

Aki, K., 1957, Space and time spectra of stationary stochastic waves, with special reference to microtremors: Bulletin Earthquake Research Institute 35, 415–456.
Asten M. W. , 1976, The use of microseisms in geophysical exploration: Ph.D. thesis, Macquarie University.

Asten M. W. , 2003, Lessons from alternative array design used for high-frequency microtremor array studies: in Earthquake Risk Mitigation, Proceedings of the 2003 Australian Earthquake and Engineering Society Conference: in Wilson, J.L., Lam, N.K., Gibson, G., and Butler, B. (eds.), Paper No. 14, Melbourne, Australia.

Asten M. W. , 2005, An assessment of information on the shear-velocity profile at Coyote Creek, San Jose, from SPAC processing of microtremor array data: in Asten, M.W. and Boore, D.M. (eds.), Blind comparisons of shear-wave velocities at closely spaced sites in San Jose, California: U.S. Geological Survey Open-File Report, 2005–1169.

Asten, M. W., 2006, On bias and noise in passive seismic data from finite circular array data processed using SPAC methods: Geophysics 71, V153–V162.
| Crossref | GoogleScholarGoogle Scholar | Asten M. , Dhu T. , and Lam N. , 2004, Optimised array design for microtremor array studies applied to site classification; comparison of results with SCPT logs: in 13th World Conference on Earthquake Engineering, Paper No. 2903, Vancouver, Canada, August 1–6, 2004.

Asten, M. W., and Henstridge, J. D., 1984, Array estimators and the use of microseisms for reconnaissance of sedimentary basins: Geophysics 49, 1828–1837.
| Crossref | GoogleScholarGoogle Scholar | Asten M. W. , Lam N. , Gibson G. , and Wilson J. , 2002, Microtremor survey design optimised for application to site amplification and resonance modelling: in Total Risk Management in the Privatised Era: in Griffith, M., Love, D., McBean, P., McDougall, A., and Butler B. (eds.), Paper 7, Proceedings of Conference, Australian Earthquake Engineering Soc., Adelaide.

Bard, P.-Y., and Bouchon, M., 1985, The two-dimensional resonance of sediment-filled valleys: Bulletin of the Seismological Society of America 75, 519–541.
Capon J. , 1969, High resolution frequency-wavenumber analysis: in Proceeding of the IEEE, 57, No. 8, 1408–1418, August, 1969.

Claprood M. , and Asten M. W. , 2007, Use of SPAC, HVSR and strong ground motion analysis for site hazard study over the 2D Tamar valley in Launceston, Tasmania: in Proceedings of the Australian Earthquake Engineering Society Conference, Wollongong, Australia, Nov. 23–25, 2007.

Field, E. H., 1996, Spectral amplification in a sediment-filled valley exhibiting clear basin-edge-induced waves: Bulletin of the Seismological Society of America 86, 991–1005.
Hermann R. , 2002, Computer programs in seismology: An overview of synthetic seismogram computation. Saint-Louis University, USA, version 3.30, 2002 edition.

Horike, M., 1985, Inversion of phase velocity of long-period microtremors to the S-wave-velocity structure down to the basement in urbanized areas: Journal of Physics of the Earth 33, 59–96.
Leaman D. , 1994, Assessment of gravity survey City of Launceston: Technical Report for Launceston City Corporation Seismic Zonation Study, Leaman Geophysics, Hobart, Tasmania, Australia.

Michael-Leiba M. , 1995, Microtremor survey and seismic microzonation Launceston, Tasmania: Technical report for Launceston City Council, Australian Geological Survey, Canberra, ACT, Australia.

Nakamura, Y., 1989, A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface: Quarterly Reports of Railway Technical Research Institute 30, 25–33.
Okada H. , 2003, The Microtremor Survey Method: Society of Exploration Geophysicists of Japan, Translated by Koya Suto, Geophysical Monograph Series No. 12, Society of Exploration Geophysicists.

Olsen, K. B., Nigbor, O. R., and Konno, T., 2000, 3D viscoelastic wave propagation in the Upper Borrego valley, California, constrained by borehole and surface data: Bulletin of the Seismological Society of America 90, 134–150.
| Crossref | GoogleScholarGoogle Scholar | Stephenson W. R. , 2003, Factors bounding prograde Rayleigh-wave particle motion in a soft-soil layer: In Proceedings of the 2003 Pacific Conference on Earthquake Engineering, Paper No. 56, Christchurch, New-Zealand, Feb. 13–15, 2003.

Tokimatsu K. , 1997, Geotechnical site characterisation using surface waves: Earthquake Geotechnical Engineering, Ed Ishihara, Balkema, Rotterdam, 1333–1368.

Toksöz, M. N., 1964, Microseisms and an attempted application to exploration: Geophysics 29, 154–177.
| Crossref | GoogleScholarGoogle Scholar |

Toksöz, M. N., and Lacoss, R. T., 1968, Microseisms: Mode structure and sources: The Sciences 159, 872–873.

Uebayashi, H., 2003, Extrapolation of irregular subsurface structures using the horizontal-to-vertical spectral ratio of long-period microtremors: Bulletin of the Seismological Society of America 93, 570–582.
| Crossref | GoogleScholarGoogle Scholar |