Earthquakes in the Canberra Region - Geoscience Australia

Earthquakes in the Canberra RegionFigure 16. Seismograms of the magnitude ML 3.3 Oolong earthquake of 13 August 2006,recorded by Geoscience Australia’s digital seismographs at Canberra, Young, Riverview(Sydney) and Mangrove Creek.In the Dalton-Gunning zone, the average time interval between large, potentiallydamaging magnitude ML 6.0 earthquakes is of the order of 120 years (McCue and others,1989). Events of this magnitude occurring in the part of the zone closest to Canberrawould be expected to produce intensities in the city ranging from MMIV to MMVII,depending on the foundation conditions. Thus, these earthquakes may cause some damagein Canberra.CANBERRA EARTHQUAKE HAZARDLet us assume that the frequency of occurrence of earthquakes is the same for Canberra asfor the rest of eastern New South Wales and northern Victoria (excluding the Dalton-Gunning Zone). Then we can calculate the average time interval which would beexpected between earthquakes within 20 km of the GPO with magnitudes equalling orexceeding a given value. A magnitude ML 3.0 or greater would occur on average onceevery 15 years, magnitude ML 4.0 once every 170 years, and magnitude ML 5.5 onceevery 6000 years. A Richter magnitude 5.6 or greater event, the size of the December1989 Newcastle earthquake, would be expected to occur on average once every 8000years. Remember that earthquakes are not evenly spaced in time, and these time intervalsare only very approximate average values. For example, the return period for a magnitudeML 5.5 or greater event, given as 6000 years above, could actually be anything from 3000to 9000 years. The uncertainty comes from the best estimate of 6000 years being based ona very small amount of data, as earthquakes of this size are rare in southeastern Australia.For magnitude ML 4.0 or greater earthquakes, the return period could range from 150 to26