The Basin-Edge Effect from Weak Ground Motions Across the Fault-Bounded Edge of the Lower Hutt Valley, New Zealand

Abstract

Basin-edge effects are investigated in the Lower Hutt Valley, Wellington, New Zealand, using recordings of weak-motion earthquakes. Twelve seismographs were deployed in a closely spaced array along the fault-bounded edge of the valley. Peak ground motions appear to be locally amplified at 2.0-2.5 Hz in the edge-parallel component at around 116-172 m from the fault and in the edge-normal component more than 295 m from the fault. Group velocity observations made of edge-generated Love waves show the fundamental-mode Airy phase frequency at 2.0-2.5 Hz. This frequency corresponds to the fundamental resonance mode of the Holocene sediments as well as the third-mode resonance of the whole basin. Large differential particle motions are observed at neighboring stations less than 50 m apart. The most plausible explanation to account for these observations is constructive interference between edge-generated surface waves and the delayed direct arrival, commonly referred to as the basin-edge effect. Manuscript received 23 October 2001.