Engineers Australia

Geotechnical aspects of earthquake hazard assessment for Brisbane

Graham G. Shorten and Ken Granger

Abstract

An effective method was needed to resolve earthquake hazard at the city-scale during the development of an earthquake risk assessment for Brisbane. While an assessment of regional seismicity is a fundamental input to the process, it can only provide a broad idea of earthquake hazard. Two of the major determinants in magnifying local earthquake shaking above regional levels are (i) the presence of significant thickness of weak foundation sediments with high void ratios and, (ii) the effects of ridge-tops and steep slopes. Soft, Recent sediments, deposited in low-lying areas around the Brisbane River and Moreton Bay coastal plain will tend to magnify the effects of earthquakes. Steep, narrow ridges are a common feature in the central and western districts of the city. Recent acquisition by Brisbane City Council (BCC) of a high-resolution digital elevation model (DEM) of the city area and a broader range of sub-surface geotechnical information has enabled an enhancement of earlier hazard analyses. The geotechnical borehole dataset available to BCC, although now broader, is nevertheless limited as a much larger database remains for potential exploitation. The new DEM enabled refinement of geological boundaries to match the scale of assets at risk, and provided additional visual clues as to subsurface conditions and indications of sediment thickness, as well as providing accurate delineation of slope and slope change. The existing coverage of geological mapping is comprehensive but mapping is at a coarse scale compared to that at which city assets are described. Available microtremor recordings are available but too restricted to be sufficiently representative for a whole-of-city assessment. Taken together, however, the DEM, available geotechnical borehole data, geological mapping and microtremor recordings are able to provide a reasonable delineation of potential variations in the magnitude of earthquake shaking, within the constraints of the given regional seismic setting. While such studies provide an assessment of hazard, further combination with information on the demographic setting, vulnerability and assets at risk through risk-GIS was necessary to quantify risk. The study shows that while nowhere is entirely immune to earthquake effects, the chance of an earthquake causing significant damage in Brisbane city is unlikely. The earthquake hazard to Brisbane is low by global standards. Less than ten minor earthquakes have been felt in the city since its founding, some 180 years ago. The largest of these, the Richter magnitude ML 4.4 Mt Glorious earthquake of 1960 caused minor damage but was felt throughout Brisbane with intensities as high as Modified Mercalli intensity MM IV. A maximum Richter magnitude of ML 6.5 is theoretically possible within the region which includes Brisbane. However, concentrations of vulnerable populations, housing and infrastructure largely lie outside of the zones of highest earthquake hazard, reducing the overall risk to Brisbane considerably.