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Papers 2015
2015 – Estimation of Seismic Hazard for Dams with a Consistent Risk Approach
Learn moreMaz Mahzari and Chi-Fai Wan
Upgrading of an existing dam often faces challenges in both static and seismic safety assessment. The use of new hydrological and seismological data and improved design methods often mean more severe loading which outdates the original design and demands expensive upgrade works. Establishing the design criteria for checking the structural adequacy of an existing dam for multiple unusual load events occurring within a relatively short time frame presents another challenge.
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A probabilistic approach is presented to rigorously address the effects of multiple load events while maintaining a consistent risk of failure for the structure. This is based on a probabilistic conditional combination where probability of each event is defined and used to develop a joint probability distribution. For instance if an earthquake occurs following a severe flood, the seismic hazard curve of the site can be used to adjust the seismic loading with shorter average recurrence interval to be used in conjunction with the pre-earthquake flood when assessing the structural adequacy of the dam. With this method of adjustment, the design can benefit from the choice of a reduced seismic design loading and hence a more cost effective design solution.
The proposed method is straightforward and can be effectively used in most engineering practices, including the design of hydraulic structures such as dams.
Keywords: Dams, Seismic Hazard, Post-earthquake, Risk analysis -
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Papers 2016
2016 – An Earthquake Alarm and Situation Awareness Tool Developed For Areas of Low to Moderate Seismic Hazard: Recent Developments and Past Results
Learn moreWayne Peck
The SRC operated seismic network is one of the largest privately owned and operated seismic networks in the world. Importantly it bridges the situation awareness gap between the information often provided by national seismic networks, of earthquake magnitude and location, and the emergency response managers questions of “What effects will this event have on my assets?” together with “What should we now be doing to mitigate the event?”
Software development of the Quick Quake app and improved automation of PDF report generation means that detailed, bespoke client specific earthquake response reports that incorporate asset earthquake resistance and failure consequence aspects can be produced by duty seismologists within reduced timeframes.
Preliminary earthquake locations computed by the SRC operated network for the two ML 4.7 Korumburra events in March 2009 and the ML 5.6 Moe earthquake of June 2012 were significantly closer to the final computed locations than those published by any other authority. The network additionally provides bonus outcomes of highly accurate detailed seismic activity maps that reduce uncertainties for Probabilistic Seismic Hazard Assessments (PSHAs) and attenuation data that will be used to develop regional specific ground motion models.
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Papers 2016
2016 – Upcoming New National Seismic Hazard Assessment of Australia – Improvements & Novel ideas
Learn moreVicki-Ann Dimas
The National Seismic Hazard Assessment 2018 (NSHA 18) project intends to revise the existing seismic hazard map (AS1170.4 2007) for Australia. Geoscience Australia (GA) are leading the project along with a consortium of seismologists, geologists and earthquake engineers.
The NSHA 18, due to be released in 2018 is of great importance to dam owners and operators. The project intends to incorporate a comprehensive approach to seismic hazard, particularly in modelling uncertainty and variability.The Global Earthquake Model (GEM) is an international consortium of scientists, engineers and policy makers. One of the primary aims of GEM is to provide a uniform set of tools for analysis in seismic hazard and risk. GEM was established to provide a framework for global standards in comparing risk analysis, awareness and actions in an effort to increase resilience to vulnerable communities.
The NSHA 18 will use the GEM framework in order to meet its own objectives for the new upcoming hazard map. The Seismology Research Centre will contribute to the NSHA 18 in three areas. Firstly, to produce a unified earthquake catalogue where GA will homogenise magnitudes to a uniform scale. Secondly, to produce a number of applicable alternate seismotectonic models, and thirdly, through the contribution of ground motion data collected over the last forty years within Australia.
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Papers 2017
2017 – Detailed Seismic Hazard Assessment of Mt Bold Area: Comprehensive Site-Specific Investigations on Willunga Fault
Learn moreZivko R. Terzic, Mark C. Quigley, Francisco Lopez
The Mt Bold Dam, located in the Mt Lofty Ranges in South Australia, is a 54m high concrete arch-gravity dam that impounds Adelaide’s largest reservoir. The dam site is located less than 500m from a suspected surface rupture trace of the Willunga fault.Preliminary assessments indicate that Mt Bold Dam is likely to be the dam with the highest seismic hazard in Australia, with the Flinders Ranges-Mt Lofty region experiencing earthquakes of sufficient magnitude to generate shaking damage every 8-10 years on average. Prior evidence suggests that the Willunga Fault is likely capable of generating M 7-7.2 earthquakes.As part of the South Australia Water Corporation (SA Water) portfolio of dams, Mt Bold Dam is regularly reviewed against the up-to-date dam safety guidelines and standards. SA Water commissioned GHD to undertake detailed site-specific geophysics, geotechnical and geomorphological investigations, and a detailed site-specific Seismic Hazard Assessment (SHA) of the Mt Bold Dam area. The results of this investigation will be used to inform decisions related to planned upgrade works of the dam.Geomorphological mapping of Willunga Fault, detailed geological mapping, analysis of airborne lidar data, geophysical seismic refraction tomography and seismic reflection surveys,and paleoseismic trenching and luminescence dating of faulted sediments was conducted to obtain input parameters for the site-specific SHA.Discrete single-event surface rupture displacements were estimated at ~60 cm at dam-proximal sites. The mean long-term recurrence interval (~37,000 yrs) is exceeded by the quiescent period since the most recent earthquake (~71,000 yrs ago) suggesting long-term variations in rupture frequency and slip rates and/or that the fault is in the late stage of a seismic cycle. The length-averaged slip rate for the entire Willunga Fault is estimated at 38 ± 13 m / Myr. Shear wave velocity (Vs30) of the dam foundations was estimated based on geotechnical data and geological models developed from geophysical surveys and boreholes drilled through the dam and into the foundation rock. The nearest seismic refraction tomography (SRT) lines were correlated with the boreholes and those velocity values used in the Vs30 parameter determination. All relevant input parameters were included into seismic hazard analysis with comprehensive treatment of epistemic uncertainties using logic trees for all inputs.Deterministic Seismic Hazard Analysis (DSHA) confirmed that the controlling fault source for the Mt Bold Dam site is Willunga Fault, which is located very close to main dam site (420m to the West).For more frequent seismic events (1 in 150, 1 in 500 and 1 in 1,000 AEP), the probabilistic analysis indicates that the main seismic hazard on the dam originates from the area seismic sources (background seismicity).Based on deaggregation analysis from the site specific Probabilistic Seismic Hazard (PSHA), the earthquakes capable of generating level of ground motion for the 1 in 10,000 AEP event can be expected to occur at mean distances of approximately 22km from the dam site(with the mean expected magnitude atMt Bold Damsite estimated at Mw >6).For less frequent (larger) seismic events, the contribution of the Willunga Fault to the seismic hazard of Mt Bold Dam can be clearly noted with Mode distance in the 0-5 km range, which indicates that most of the seismic hazard events larger than the 1 in 10,000 AEP comes from the Willunga Fault. The Mode magnitudes of the events are expected to be Mode Magnitude at Mw= 6.6 for a segmented Willunga Fault scenario, and Mw= 7.2 for a non-segmented fault scenario.Consideration was also given to the upcoming update of the ANCOLD Guidelines for Earthquake, which calls for the determination of the Maximum Credible Earthquake (MCE) on known faults for the Safety Evaluation Earthquake (SEE) of “Extreme” consequence category dams. The MCE for Mt Bold Dam was estimated from the DSHA; in terms of acceleration amplitude, the MCE event approximately equals the 1 in 50,000AEP seismic events.
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Papers 2018
2018 – Evaluating potentially active tectonic faults for probabilistic seismic hazard assessments in Australia
Learn moreDee Ninis, Elodie Borleis & Wayne Peck
For intraplate regions such as Australia, identifying and quantifying activity on tectonic faults for inclusion in probabilistic seismic hazard assessments can be challenging due to the typically long return period for ground-rupturing earthquakes associated with these structures. Return periods of 10,000’s to 1,000,000’s of years mean that surface displacement evidence is prone to degradation through erosion and burial, and paleoseismological ‘trench’ excavations may not uncover geology old enough to reveal previous events. As a consequence, there is often little or no preserved evidence of past ground rupturing events on these structures. Rather than ignoring faults which show no evidence of neotectonic displacement, we present an alternative approach; in addition to considering active faults (movement in the last 35,000 years) and neotectonic faults (movement in the last 10 Myr) in seismic hazard assessments, we also consider faults which otherwise show no evidence of neotectonic activity but which are aligned favourably with the current stress regime and are therefore potential sources of earthquakes and accompanying strong ground motion.
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Papers 2018
2018 – Potential Geologic Sources of Seismic Hazard in Australia’s South-Eastern Highlands: What Do We Know?
Learn moreDan Clark, Andrew McPherson, Brad Pillans, Duanne White, Don MacFarlane
Many mapped faults in the south-eastern highlands of New South Wales and Victoria are associated with apparently youthful topography, suggesting that faulting may have played a role in shaping the modern landscape. This has been demonstrated to be the case for the Lake George Fault, and may reasonably be inferred for the poorly characterised Murrumbidgee, Khancoban, Tantangara, Berridale Wrench and Tawonga faults. More than a dozen nearby major faults with similarly youthful topography are uncharacterised. In general, fault locations and extents are inconsistent across different scales of geologic mapping, and rupture lengths, slip rates and other fault behaviours remain largely unquantified. A more comprehensive understanding of these faults is required to support safety assessments for communities and large infrastructure in the region.
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Papers 2018
2018 – Probability of Surface Faulting from an Earthquake Affecting a Vulnerable Structure
Learn moreRussell Cuthbertson and Leanne Capewell
While structures such as a dam walls, pipelines, gas storage tanks, and nuclear facilities are vulnerable to the shaking from earthquakes, they are even more susceptible to differential movement on faults passing beneath their foundations.
In the past, the probability of surface rupture of a fault was calculated by making some simplistic assumptions about the distribution of earthquake magnitudes. Improved databases of earthquake ground faulting now allow the probability of surface rupture to be estimated in a more realistic fashion. Computing software that uses a Monte Carlo approach has been developed to allow the effect of various scenario choices on rupture probability to be investigated.
Using this software, it is found that the most significant influence on rupture probability is the long-term fault slip-rate. Other assumptions about the faulting style, maximum magnitude and conversion parameters have only a moderate influence on the results.There have been several instances in recent history in Australia of surface faulting due to earthquakes, but there has been only limited damage to infrastructure due to the remoteness of these earthquakes. The software that has been developed will allow a considered assessment and comparison of the hazard and risk due to both ground shaking from earthquakes and from surface rupture.
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Papers 2014
2014 – Clyde Dam and Lake Dunstan; The First 21 Years
Learn morePeter F Foster and Peter K Silvester
Clyde Dam, the largest concrete gravity dam in New Zealand, was constructed in the 1980’s on the Clutha River in New Zealand. Lake Dunstan, which is the reservoir formed by the dam, reached its full operating level in 1993, some 21 years ago.
This paper summarises the performance of the dam over this period, the changes in operations that have been undertaken and looks to future challenges. The performance and management of the landslides around Lake Dunstan that were remediated prior to lake filling is outlined. The large floods experienced in the Clutha River in the 1990’s highlighted aspects of the flood management procedures that needed amending to capture lessons learned and some modifications to appurtenant structures have been completed. Changes to the environmental management in moving from water rights to consent conditions under the Resource Management Act are addressed.
Over the last 21 years a sediment delta has progressed down Lake Dunstan, as expected, and a long term sediment management plan has been developed for both Lake Dunstan and Lake Roxburgh which is downstream of Clyde Dam. A summary of the plan is discussed. The seismic hazard at the dam site is currently under study to update the seismic assessment parameters for the dam.
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Papers 2014
2014 – Earthquake recurrence models for seismic hazard assessments
Learn moreGary 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.
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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. -
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Papers 2014
2014 – Comparing Probabilistic and Historical Seismic Hazard Estimates in Southeast Australia
Learn moreRussell Cuthbertson
Two techniques were used to calculate seismic hazard at a number of locations in southeast Australia. To simplify matters only Peak Ground Accelerations were compared.
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The first technique used a seismological model of areal source zones that was based on the recorded seismicity as well as geological and tectonic inputs. Each zone was assigned a rate of earthquake activity that had been calculated from the recorded seismicity and a magnitude completeness function. Known geological faults that are also part of the model had to be excluded to allow a direct comparison with the second technique. A standard probabilistic seismic hazard analysis then gave PGA values versus return periods. This is the approach that has been used for the current Australian earthquake loading code (AS1170.4).
The second technique used a simple historical approach whereby recorded earthquakes were combined with an attenuation function to directly give the estimated return periods. This approach takes no account of tectonics, geological terranes or faulting – it simply uses the known, recorded earthquake catalogue. This is the technique used in the original Australian earthquake loading code (AS 2121).
The same ground motion attenuation function was used in both techniques but for a direct comparison the aleatory variability was set to zero in the probabilistic case because the historical approach did not include this effect.
In the historical approach the variability in completeness of the recorded catalogue was not considered. It was simply assumed that all earthquakes producing accelerations greater than a given value would be recorded over the last 100 years.
The comparisons were made for minimum considered magnitudes of 4 and 5.
There was general agreement between the two approaches especially at shorter return periods (lower PGA amplitudes). At longer return periods (higher PGA amplitudes) where there were higher uncertainties, the results at some sites diverged.
This simple comparison of two approaches to the same problem of estimating earthquake hazard is shown to be of value in ensuring that the AUS5 model used by SRC is producing results that are consistent with the historically recorded data. -
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Papers 2013
2013 – Fault rupture hazard assessments for dam foundations: examples from contrasting tectonic environments
Learn moreTim McMorran and Alan Hull
Accurate assessment of potential fault rupture hazard in dam sites is a critical factor in managing dam safety. Assessment of the location and activity of a surface fault within or near an existing or proposed dam can be technically challenging, expensive and affect design and construction schedules.
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Three examples from regions of relatively high, moderate and low tectonic activity are used to illustrate that fault rupture hazard assessment is generally feasible in regions with high rates of tectonic activity, historic earthquake occurrence and the presence of Quaternary and Holocene-age landforms and sediments. In regions with relatively low rates of tectonic activity and landscape development, the fault rupture hazard assessment is more challenging.
The examples illustrate that robust geologic and geomorphic analysis provides critical information on the fault rupture hazard at existing and proposed dams. These analyses assist dam owners to obtain a more complete understanding of the fault rupture hazard at their facility, and support their longer term risk assessments.