Microtremor Observations in Tamar Valley, Launceston, Tasmania: Evidence of 2D Resonance from Observed Microtremor and Numerical Modelling

Maxime Claprood; Michael W. Asten

doi:10.1071/ASEG2009ab139

ISSN: 2202-0586
E-ISSN:

oa Microtremor Observations in Tamar Valley, Launceston, Tasmania: Evidence of 2D Resonance from Observed Microtremor and Numerical Modelling
Authors Maxime Claprood^a and Michael W. Asten^b
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^a CEGAS – Centre for Environmental and Geotechnical Applications of Surface Waves, Monash University, , Melbourne, , Maxime, . [email protected] ^b CEGAS – Centre for Environmental and Geotechnical Applications of Surface Waves, Monash University, , Melbourne, , Michael, . [email protected]
Publisher: Australian Society of Exploration Geophysicists
Source: ASEG Extended Abstracts, Volume 2009, Issue ASEG2009 - 20th Geophysical Conference, Dec 2009, p. 1 - 5
DOI: https://doi.org/10.1071/ASEG2009ab139
- Published online: 01 Dec 2009

Abstract

We use the microtremor survey method to record ambient ground vibrations in Launceston, Tasmania. The presence of the ancient Tamar Valley, in-filled with soft sediments that vary rapidly in thickness from 0 to 250m over a few hundreds meters, is thought to induce a 2D resonance pattern, amplifying the surface motions over the valley and in Launceston. We use the spatially averaged coherency (SPAC) microtremor survey method to study its potential for characterising site effects across the Tamar Valley.

We present SPAC observations, recorded with increasing array size at 3 sites KPK (r1=28m), DBL (r1=50m) and RGB (r1=140m) in Launceston. Replacing the traditional spatial averaging of coherencies by temporal averaging, we compute coherencies with single pairs of sensors, oriented parallel (axial-coherency) and perpendicular (transverse-coherency) to the valley axis. Using multi-radii arrays, we analyse the impact of increasing radius on the spatial and temporal averaged coherencies recorded at all sites, to study SPAC capability to detect a 2D resonance pattern induced from the Tamar valley in Launceston.

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References

Asten, M., 2006, On bias and noise in passive seismic data from finite circular array data processed using SPAC methods: Geophysics, 71, no.6, V153-V162.
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.
Chavez-Garcia, F.J., Rodriguez, M. and Stephenson, W.R., 2005, An alternative approach to the SPAC analysis of microtremors: Exploiting stationarity of noise: Bulletin of the Seismological Society of America, 95, no.l, 277-293, doi: 10.1785/0120030179.
Claprood, M. and Asten, M.W., 2008, Microtremor survey methods in the Tamar Valley, Launceston, Tasmania: Evidence of 2D resonance from microtremor observations: Submitted for publication in Proceedings of Conference, Australian Earthquake Engineering Society, Ballarat.
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, no.4, 991-1005.
Leaman, D., 1994, Assessment of gravity survey City of Launceston: Technical Report, Leaman Geophysics, Hobart, Tasmania, Australia, for Launceston City Corporation Seismic Zonation Study.
Okada, H., 2006, Theory of efficient array observations of microtremors with special reference to the SPAC method: Exploration Geophysics, 37, 73-85.
Roten, D., Fah, D., Comou, C. and Giardini, D., 2006, Two-dimensional resonances in Alpine valleys identified from ambient vibration wavefields: Geophysical Journal International, 165, 889-905, doi: 10.11 ll/j.l365-246X.2006.02935.x.
Steimen, S., Fah, D., Kind, F., Schmid, C., and Giardini, D., 2003, Identifying 2D resonance in microtremor wave fields: Bulletin of the Seismological Society of America: 93, no.2, 583-599.
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, no.2, 570-582.

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Article Type: Research Article

Keyword(s): 2D resonance pattern; microtremor survey method; shear wave velocity; SPAC

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oa Microtremor Observations in Tamar Valley, Launceston, Tasmania: Evidence of 2D Resonance from Observed Microtremor and Numerical Modelling

Abstract

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