Shallow Shear-Velocity Measurements and Prediction of Earthquake Shaking in the Wellington Metropolitan Area, New Zealand

Shallow Shear-Velocity Measurements and Prediction of Earthquake Shaking in the Wellington Metropolitan Area, New Zealand, Figure 1 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.
<p>
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.
</p><p>References
</p><p>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.
</p><p>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.
</p><p>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.
</p><p>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.</p>

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