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Gary Gibson, Wayne Peck, Ian Landon-Jones and Kumara Arachchi
One of the first seismograph networks designed specifically to record local earthquakes was installed about Sydney in 1958. This network was converted to telemetry in 1983. In 1992, Sydney Water Corporation upgraded the network, integrating the functions of earthquake location and magnitude, measurement of the response of structures to earthquake motion, and provision of information for emergency response. The response function has been developed over the past six years, and is now an “Earthquake Preparation, Alarm and Response” system that provides customised information very soon after any significant event.Learn more
Vicki-Ann Dimas, Wayne Peck, Gary Gibson and Russell Cuthbertson
Globally, reservoir triggered seismicity (RTS) is a phenomenon sometimes observed in newly constructed large dams worldwide, for over 50 years now. Over 95 sites have been identified to have caused RTS by the infilling of water reservoirs upon completion of their constructions worldwide. In Australia, there are seven confirmed sites with observed RTS phenomenon that are summarized by temporal and spatial means.Learn more
With almost 40 years of seismic monitoring, primarily within eastern Australia, several of Australia’s largest dams have monitored and recorded many RTS events. At present, twelve dams are 100 metres and above in height as possible candidates, with seven of these actually causing RTS and a disputed possible eighth dam.
Important factors of RTS are reservoir characteristics (depth of the water column and reservoir volume), geological and tectonic features (how active nearby faults are and how close to the next cycle of stress release they are temporally) and ground water pore pressure (decrease in pore volume under compaction of weight of reservoir and diffusion of reservoir water through porous rock beneath). RTS is an adjustment process often delayed for several years after infilling of reservoir before eventually subsiding within 10 to 30 years, when seismic activity then returns to its prior state of stress.
Generally there are two type of RTS events, either a major fault near the reservoir most likely leading to an earthquake exceeding magnitude 5.0 to 6.0, or more commonly, a series of small shallow earthquakes.
Seismic monitoring of all dams (except for Ord River) are presented with spatial and temporal series of maps and cross sections, showing the largest earthquake, build-up and decay of RTS events.
Keywords: Seismic monitoring, reservoir triggered seismicity (RTS), earthquake cycle
Gary Gibson and Vicki-Ann Dimas
Earthquake recurrence models are based on observed seismicity, geological data and geodetic motion. They are particularly difficult to define in regions of low seismicity where the average recurrence interval between moderate to large earthquakes greatly exceeds the duration of the known earthquake catalogue.Learn more
The earthquake process may be considered as ongoing long-term deformation due to plate movement in the region about the fault, resulting in stress build-up, and a significant number of small earthquakes through the deformed region. Larger earthquakes occur at irregular intervals, with ruptures on the larger faults that release elastic strain energy from the region. Most strain energy release is during the large fault rupture.
This gives a wider range in hazard estimates compared with extrapolation methods, increasing hazard in regions of active faulting and reducing hazard where long-term geological stability can be observed. As dams are usually in regions with recent uplift, this method will tend to increase hazard estimates.
Paul Somerville, Gary Gibson
Abstract: This paper describes current methods for seismic hazard analysis and their application at Hinze Dam. Although Southeastern Queensland has experienced significant earthquakes in historical time, none of them are known to have caused surface rupture, and no active faults that could be used to represent earthquake sources have been identified in the region that surrounds the site. Under these conditions, we must estimate the seismic potential of the region using historical seismicity. Two alternative approaches to modelling future earthquake occurrence based on historical seismicity have been used. The first approach is based on the AUS5 source model of ES&S (2005), which uses geological criteria to identify zones of uniform seismic potential, and then uses historical seismicity to characterize the seismic potential of each zone. The second approach, developed by Hall et al. (2007) at Risk Frontiers, is based on the spatial smoothing of historical seismicity without identifying discrete source zones. Previous work by ES&S has shown that the attenuation of strong ground motion in Southeastern Australia is fairly well represented by ground motion models developed using strong motion data from western North America. The recently developed NGA ground motion models based mainly on data from Western North America represent the local site conditions using Vs30, the shear wave velocity averaged over the top 30 metres at the site. This provides a significant advantage over previous models, which were for broad site categories such as rock or soil, and did not provide for the use of more site-specific information. The left abutment, lower tower and valley section foundation at Hinze Dam are characterized by hard unweathered rocks with shear wave velocity of 2.0 km/sec estimated from P wave velocity measurements. The right abutment of the main embankment and the saddle embankment foundation consist of extremely weathered rock, with shear wave velocity of 0.45 km/sec estimated from P wave velocity measurements. This causes the ground motion response spectra estimated for the right abutment and neighbouring foundation components to be significantly larger than for the left abutment and neighbouring foundation components, by factors of 1.4, 2.0 and 2.3 for periods of 0 (PGA), 0.5 sec and 1 sec respectively.
Keywords: seismic hazard analysis.Learn more