The 1989 Newcastle, Australia, earthquake: The facts and the misconceptions

Abstract

The vulnerability of urban environments in continental regions to earthquake forces was explicitly demonstrated in Australia's devastating Newcastle earthquake on December 28, 1989. This moderately-sized earthquake of Richter magnitude ML 5.6 (Moment magnitude M 5.3) claimed 13 lives, damaged more than 70,000 properties and left an estimated total loss of about $AU (1991) 4 billion. The need for an earthquake mitigation programme in Australia was thus clearly established. It is for this reason that a multidisciplinary approach involving seismology, geology, engineering, insurance, local government and emergency services is being followed to study this event and its consequences. The earthquake source is defined as being on a thrust fault trending NW-SE dipping 75° to the NE, with a depth of focus at 11.5 km, source radius of 1.86 km, stress drop of at least 24 bars and a displacement along the rupture surface of at least 310 mm. The epicentre is located at 32.95°S, 151.61°E close to Boolaroo, about 15 km SW of the City of Newcastle, and with an epicentral error of about + 15 km. More than 100,000 observations from damage and felt reports are being analysed and integrated with the wide experiences gained in the rescue, recovery and renewal phases that have extended over the two years since the event. The specific issue of the geotechnical aspects is of great importance. It is being considered from the view of urban geology (surface alluvial sediments), rather than from theoretical considerations, to explain the major extent of building damage on the alluvial areas, amplification and liquefaction. Apart from the immediate "causes" of damage, serious consideration is being given to the long-term effects which have resulted in the latent and recurrent defects to buildings. The engineering findings from the Newcastle earthquake are discussed in detail. While it is uneconomical and not necessary to design a structure to withstand the greatest likely earthquake without damage in Australia, the cost of providing strength to resist very high intensity loads must be weighed against the importance of the structure and probability of the earthquakes, particularly in areas such as this with relatively little known seismic histories. Lessons for local government authorities who had not previously considered seismic activity are addressed. Based on the response and recovery of the City of Newcastle, the lessons include the development of a recovery management plan, revision of building regulations and the requirements for hazard mitigation. Unfortunately, several misconceptions about some aspects of the consequences of this earthquake have arisen. These concern the limitations of some analyses, use of selected data sets rather than all the available data and apparent lack of understanding of complex, rather than singular, causal relationships. Implications for the engineering, insurance and possibly the legal professions need to be considered. The potential to reduce losses in future earthquakes in Australia through an earthquake mitigation programme is now an achievable goal. The scenarios of such an event occurring at a different time or in a different city can be addressed, based on the Newcastle and other international experiences. Sufficient information is available to prepare the revised Australian earthquake loading code as a reliable and practical document for use by engineers. The consequences of the 1989 Newcastle earthquake have also captured the interest of researchers from many other continental areas of the Earth who must consider preparations for similar situations. All aspects of the study ultimately lead to the preparedness of urban communities to deal with such consequences with the assistance of emergency services agencies to minimize the social and economic traumas that will inevitably occur.