Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopof tens to hundreds of metres. Areas with soft surface sediments and steep topography have a muchhigher earthquake hazard than experienced on a horizontal surface with hard rock outcropping.Attenuation for earthquake waves travelling in the Palaeozoic bedrock beneath the Mesozoicsediments of the Sydney Basin is typical of attenuation in eastern Australia. However, the SydneyBasin sediments show higher than average attenuation, and this makes determination of localattenuation functions quite difficult, especially with limited data.Seismographs in the Sydney RegionThe first seismograph to be installed in New South Wales was a Milne instrument installed at theSydney Observatory from 1906 to 1948. The Riverview Observatory installed its first seismographin 1909, and seismic monitoring instruments are still being operated at this site. Most of the earlyseismographs were long-period instruments designed to record distant earthquakes, but with limitedcapacity for recording high frequency motion from local earthquakes.One of the first seismograph networks in the world designed specifically to record local earthquakeswas installed at five sites for the Metropolitan Water, Sewerage and Drainage Board of Sydney from1958. The network was operated by the Australian National University in Canberra. In 1970 thenetwork was reduced to four sites, and in 1983 radio telemetry was added to give central recording,although with limited dynamic range through use of a pen recorder, and very limited seismicfrequency bandwidth.Following the Newcastle earthquake in December 1989, Sydney Water upgraded the network. Itadopted an integrated monitoring network design, incorporating both sensitive seismometers andstrong motion accelerometers, fully based on digital data acquisition, with advantages as follows:• common instrumentation, reducing operation and maintenance costs.• strong motion reference sites, and even sites on structures, could act as normal seismographsites that would not exceed full scale for large nearby events.• combining strong motion bedrock sites with seismographs will eventually allowdetermination of a single consistent attenuation function for all levels of motion, and willsignificantly expedite collection of data for spectral attenuation functions. Attenuation functionsinherently must use measurements from a wide range of earthquake magnitudes and distances.• strong motion sites will be maintained regularly with seismograph maintenance, soinstruments are much more likely to be working correctly for a large nearby earthquake.In addition, high dynamic range instruments were used so that seismographs could measure largerearthquakes without going full scale, and strong motion accelerographs could trigger on smallerevents. Six-channel instruments would give an overlap in the levels of motion recorded, increasingreliability and allowing system calibration checks.The expanded network originally had instruments at 25 sites when fully installed by late 1993,operated by the Seismology Research Centre in Melbourne, and later reduced to 23 sites during1998. About half of these are six-channel instruments with a triaxial seismometer and a triaxialaccelerometer. There have been no moderate or large earthquakes in the Sydney area since thenetwork was installed. Since 1992, one moderate magnitude distant earthquake has been recorded(at Ellalong).Accelerographs in the Sydney RegionAn SMA1 accelerograph was mounted in the nuclear reactor basement at Lucas Heights in about1984. This was replaced by two digital accelerographs during 1989, one on the reactor structure andthe other on bedrock nearby. They are intended to study the dynamic properties of the reactorstructure and are in a noisy environment, so rarely trigger on small local earthquakes.The combined seismograph and accelerograph network installed by Sydney Water in 1992,described above, has significantly improved strong motion monitoring in the Sydney area. In47