Modeling strong ground motion in the Wellington Metropolitan Area, New Zealand

The Wellington Metropolitan Area, New Zealand, about 35 km by 10 km, is crossed by the southernmost segment of the Wellington fault, striking roughly in the northeast-southwest direction. Numerical modeling of M 6.7 earthquakes due to ruptures on this fault, intended to characterize the 3D effects of the region's main geological features on the ground motion, are performed by using a finite-differences scheme with staggered grid, for frequencies up to 1.5 Hz. We consider the part of the Wellington fault that crosses the Wellington Metropolitan Region as an almost vertical strike-slip fault, 30 km long, and with slip distribution history taken from the slip history of the 1992 M 7.3 Landers, California, earthquake. The 3D stratigraphy of the region has been built by integrating all available geological and geophysical data. Results of the modeling show several dominant features. When the fault ruptures from south to north, resonance occurs within a small area close to the harbor, where the depth of the basin is largest. This resonance produces a train of seismic waves traveling northward with dominant frequency of about 0.5 Hz. As the rupture propagates into the harbor, the radiated wavefield focuses on the Lower Hutt Valley, to the north, due to the presence of a ridge that rises up from the bottom of the harbor to emerge as Somes Island. This effect considerably enhances the incident seismic energy in the valley, particularly the horizontal motions. Further focusing occurs due to the triangular shape of the Lower Hutt Valley, whose alluvial deposits exhibit large impedance contrasts with the surrounding bedrock. The combined focusing effects produce amplification factors in the valley of about 5 between 0.5 Hz and 0.7 Hz. In Wellington City the amplification factors are between 0.5 (deamplification) and 2, except for an area on reclaimed land in the harbor, which shows an amplification factor as large as 9. After the rupture has stopped, higher frequency waves, between 1 and 1.2 Hz, appear trapped (reverberating) in the harbor for the remaining duration of the seismograms, up to 60 see. On the other hand, when the rupture is from north to south, none of the focusing effects occur. Only the long-lasting reverberations in the harbor seem a characteristic of the wave propagation common to both rupture directions. Nevertheless, the amplification factors in this north-to-south scenario are about the same as for the south-to-north rupture.