Seismic Microzoning of the Ground Shaking Hazard fromSoil Geotechnical Properties, Gisborne, New Zealand

Gisborne is an urban area built upon young, soft, unconsolidated sediments. The extent of damage in the magnitude 6.3 Ormond earinquake near Giborne in August 1993, and subsequent felt intensity surveys (MM V-VI) confirmed that that the pattern of shaking and damage was not uniform. Geotechnical testing was undertaken to investigate the strength and deformation properties of the soils underlying Gisborne. Analysis and modelling of Earthquake Commission (EQC) claims from the Ormond earthquake were carried out to construct a claim density contour map. The map revealed two anomalous areas of high claim density in the northern suburbs of Mangapapa and Whataupoko compared to the rest of the city. Investigation of the underlying Neogene basement topography revealed the presence of sediment infilled valleys underneath or very close to each of the two claim density highs. The greater damage and anomalous claim density highs may have been caused by ground motion amplification at frequencies close to natural site periods, enhanced by resonance effects within the infilled sediment.