Abstract

A sequence of small earthquakes in the Bay of Plenty culminated on 2 March 1987 in a Richter magnitude (ML) 6.3 event under the Rangitaiki Plains, which produced felt intensities of up to MMX in the epicentral area. However, the soft saturated sediments of the Rangitaiki plains suffered permanent inelastic deformation leading to structural and ground damage consistent with felt intensities of up to MMXI in the town of Edgecumbe. This earthquake, which resulted in ground surface rupture, provides a significant case history. Records of strong ground motion some 15 km from the epicentre show a similarity to the 1940 El Centro N-S record. Other ground damage included widespread liquefaction phenomena such as sand ‘boils’, lateral spreading of embankments, low angle slope failures, and minor slope and cutting failures across the plains and in the surrounding hills. The relationship between the sedimentary regime, topography, underlying geology and resulting ground damage in the region is discussed. The seismic responses of the soils and rock are shown to be generally consistent with their geotechnical characteristics and the magnitude of the event.

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