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Paul Somerville
This paper reviews methods used to estimate the MCE in Australia and New Zealand. In the ICOLD (2016), NZSOLD (2015) and proposed ANCOLD (2016) guidelines, the deterministic approach is applicable only to fault sources, whereas the probabilistic approach is applicable to both fault sources and distributed earthquake sources. Although ICOLD (2016) states that the use of a deterministic approach to develop the SEE “may be more appropriate in locations with relatively frequent earthquakes that occur on well- identified sources, for example near plate boundaries,” the proposed ANCOLD (2016) guidelines retain the use of the deterministic approach for critical active faults which show evidence of movements in Holocene time (i.e. in the last 11,000 years), or large faults which show evidence of movements in Latest Pleistocene time (i.e. between 11,000 and 35,000 years ago). In Australia, active faults make a significant contribution to the probabilistic MCE only at near-fault sites, and even in those cases most of the hazard comes from distributed earthquake sources. However, some sites may be close enough to nearby or even more distant identified active faults that a Deterministic Seismic Hazard Analysis (DSHA) produces MCE ground motions that are far larger than those obtained probabilistically even for very long return periods. Conversely, the deterministically defined MCE may be lower than the probabilistically defined MCE for very long return periods at near fault sites in New Zealand, requiring the probabilistic approach.
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2016 Papers
2016 – Estimating the Maximum Credible Earthquake (MCE) and its Application in Australia and New Zealand
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Paul Somerville
This paper reviews methods used to estimate the MCE in Australia and New Zealand. In the ICOLD (2016), NZSOLD (2015) and proposed ANCOLD (2016) guidelines, the deterministic approach is applicable only to fault sources, whereas the probabilistic approach is applicable to both fault sources and distributed earthquake sources. Although ICOLD (2016) states that the use of a deterministic approach to develop the SEE “may be more appropriate in locations with relatively frequent earthquakes that occur on well- identified sources, for example near plate boundaries,” the proposed ANCOLD (2016) guidelines retain the use of the deterministic approach for critical active faults which show evidence of movements in Holocene time (i.e. in the last 11,000 years), or large faults which show evidence of movements in Latest Pleistocene time (i.e. between 11,000 and 35,000 years ago). In Australia, active faults make a significant contribution to the probabilistic MCE only at near-fault sites, and even in those cases most of the hazard comes from distributed earthquake sources. However, some sites may be close enough to nearby or even more distant identified active faults that a Deterministic Seismic Hazard Analysis (DSHA) produces MCE ground motions that are far larger than those obtained probabilistically even for very long return periods. Conversely, the deterministically defined MCE may be lower than the probabilistically defined MCE for very long return periods at near fault sites in New Zealand, requiring the probabilistic approach.
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2016 Papers
2016 – Seismic Hazard Assessment Methods including Deterministic and Probabilistic Methods and their uses in Dam Design
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Paul Somerville
The key differences between probabilistic seismic hazard analysis (PSHA) and deterministic seismic hazard analysis (DSHA, preferably referred to as a scenario-based analysis) are that, unlike DSHA, PSHA takes account of all magnitudes on all earthquake sources that may affect the site, including the frequency of occurrence of each earthquake scenario that is considered, and fully considers the random variability (epsilon) in ground motion level. The result of a DSHA is the ground motion at the site resulting from a single earthquake scenario (or a few scenarios) having a preselected value of epsilon (usually 0 or 1), and the annual frequency of exceedance (or return period) of this ground motion level is undefined. In contrast, the hazard curve produced by PSHA yields the mean annual rates of exceedance (or return period) for each ground motion level. The complementary nature of PSHA and DSHA is manifested in the fact that practical application of PSHA, especially using ground motion time histories, results in scenario earthquakes that resemble the products of DSHA. Application of the period dependence of epsilon using the conditional mean spectrum (CMS) avoids the inaccurate and overconservative representation of the hazard by the uniform hazard spectrum (UHS) obtained in PSHA.
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2017 Papers
2017 – Conditions Under Which Identified Faults Contribute Significantly to Seismic Hazard in Australia
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Paul Somerville, Andreas Skarlatoudis and Don Macfarlane
The 2017 draft ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake specify that active faults (with movement in the last 11,000 to 35,000 years) and neotectonic faults (with movement in the current crustal stress regime, in the past 5 to 10 million years) which could significantly contribute to the ground motion for the dam should be identified, and be accounted for in the seismic hazard assessment. The purpose of this paper is to provide guidance on the conditions under which these contributions could be significant in a probabilistic seismic hazard analysis (PSHA)and a deterministic seismic hazard analysis (DSHA).We consider five primary conditions under which identified faults can contribute significantly to the hazard: proximity, probability of activity, rate of activity, magnitude distribution, and return period under consideration
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