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Shallow Shear-Velocity Measurements and Prediction of
Earthquake Shaking in the Wellington Metropolitan Area,
New Zealand
John N. Louie (University of Nevada, Reno)
The city of Wellington, New Zealand’s capital, sits astride the Australia-Pacific plate boundary at a transition from strike slip
to subduction motion. The resulting high earthquake hazard and risk motivate multiple research efforts to better understand the
potential for seismic shaking. Physics-based modeling of a Landers-type M7.2 rupture on the Wellington fault, which transects
the city, by Benites and Olsen [2005] showed potential for peak ground velocities as high as 1.5 m/s. Such a high hazard demands
a thorough understanding of the setting, and few measurements of ground-stiffness parameters such as the average shear velocity
from the surface to 30 m depth (Vs30) existed in Wellington prior to 1996. That year Kaiser and Louie (2006 and not yet published)
made refraction microtremor measurements of Vs30 at 46 sites in Wellington and Lower Hutt cities (fig. 1). Benites and
Olsen’s (2005) geotechnical model included velocities for “rock” sites that were a factor of two higher than the measurements, so
we developed a revised model from the measurements. We then used the E3D physics-based modeling code of Larsen et al. [2001]
to predict ground motions for a M3.2 event 8 km below the city that year, using both the original and revised models (fig. 2). The
revised model is not quite as efficient at trapping wave energy in basins, as was the original model. Most of the Vs30 measurements
were made at strong-motion recording stations, so the resulting seismometer data are now better calibrated for site conditions.
References
Benites, Rafael and Kim B. Olsen, 2005, Modeling strong ground motion in the Wellington metropolitan area, New Zealand: Bull. Seismol.
Soc. Amer., 95, 2180–2196.
Kaiser, A. E., and J. N. Louie, 2006, Shear-wave velocities in Parkway basin, Wainuiomata, from refraction microtremor surface wave dispersion:
GNS Science Report 2006/024, July, Lower Hutt, New Zealand, 16 pp.
Larsen, S., Wiley, R., Roberts, P., and House, L., 2001, Next-generation numerical modeling: incorporating elasticity, anisotropy and attenuation:
Society of Exploration Geophysicists Annual International Meeting, Expanded Abstracts, 1218-1221.
Acknowledgements: Research supported by a 2006 Fulbright Senior Scholar award to Louie for work in New Zealand, and by GNS Science. Instruments
used in the field program were provided courtesy of M. Savage of the Victoria University of Wellington, and S. Harder of the University of Texas El Paso.
Figure 1: Map of average shear velocity from the surface to 30 m depth assembled for the Wellington – Lower Hutt region of New Zealand, with the 1500 m/s velocity
isosurface in shaded relief to show bedrock and basin-floor topography from Benites and Olsen (2005). Locations of 27 of 46 sites measured in 2006 for shallow shear
velocity are marked with dashed circles, labeled with the measured Vs30 in m/s. Kaiser and Louie (2006) made an additional 19 measurements in one neighborhood
in the lower center of the map, with only two results shown here. The measurements allowed a revision of the shallow velocity model, with Vs30 not exceeding 800
m/s. The basin-bounding Wellington fault runs along the northwest side of the basin.
Figure 2: Map of peak ground velocity (PGV) computed for the Wellington – Lower Hutt region of New Zealand for a M3.2 event 8 km below the city, with the 1500 m/s
velocity isosurface in shaded relief to show bedrock and basin-floor topography from Benites and Olsen (2005). The yellow color indicates PGV as high as 2.27 cm/s.
3D effects of complex basin geometry and soft soils are evident.
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