Earthquakes in the Canberra Region - Geoscience Australia

Earthquakes in the Canberra Region210 years, with 170 years being the best estimate, as mentioned above. However, thereturn periods give a rough idea of the expected frequency of earthquakes within 20 km ofthe GPO.For building design, disaster planning and insurance purposes, it is useful to have anassessment of the probability of experiencing a certain intensity of ground shaking fromearthquakes in a given period of time. This was done for the whole of Australia by Gaulland others (1990). They produced maps showing contours of the ground motion whichhas a 10% probability of being exceeded in a 50 year period. For Canberra, they assessedthis Modified Mercalli intensity to be MM VI-VII. As mentioned previously, shallowearthquakes are usually felt more strongly than deeper ones. Gaull and others (1990)adopted an average depth for eastern Australian earthquakes of 10 km. However, inCanberra and the Dalton region, earthquakes are commonly at a depth of 5 km or less. Ifthis depth is used, the intensity with a 10% chance of being equalled or exceeded in 50years may be as high as MMVII for the Canberra GPO and other localities in the northernpart of the ACT.An earthquake action guide, outlining what to do before, during and after anearthquake, is given in Appendix 2.EARTHQUAKE FREQUENCY IN THE CANBERRA 1:100 000 MAP SHEETThe Canberra 1:100 000 map sheet (Figure 2) has a much bigger area than the circle ofradius 20 km centred on the Canberra GPO discussed in the previous section.Consequently, earthquakes would be expected to be more frequent in the region coveredby Figure 2 and the average expected time intervals between them will be less. Amagnitude ML 3.0 or greater would occur on average once every 7 years and a potentiallydamaging ML 4.0 or greater once every 85 years. An event the size of the December 1989Newcastle earthquake (ML 5.6) or greater would be expected to happen on average onceevery 4000 years. Again it must be remembered that earthquakes are not evenly spaced intime, and that these calculated average intervals between events are only very approximateestimates. For example, the best estimate for the average time interval between magnitudeML 5.5 or greater earthquakes is 3000 years, but it could be anything from 1000 to 5000years. The return period for a magnitude ML 4.0 or greater earthquake could lie between75 and 105 years, with the best estimate being 85 years, as mentioned above.REFERENCESAbell, R.S., 1992 - Geology of the Canberra 1:100000 sheet area. BMR Bulletin 233.Cotton, L.A., 1925 - Note on the earthquakes at Murrumbateman during March and April,1924, and January to April, 1925. Journal of the Proceedings of the Royal Society of NewSouth Wales, 59, 325-328.Eiby, G.A., 1966 - The Modified Mercalli Scale of earthquake intensity and its use in NewZealand. New Zealand Journal of Geology and Geophysics, 9, 1-2.27