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Paul Somerville

This paper describes the unique characteristics of near-fault ground motions for use in developing ground motions for the design and evaluation of dams that are located close to identified active faults. These characteristics include near-fault rupture directivity effects, permanent ground displacements, and hanging wall effects. In Australia, active faults make a significant contribution to the Maximum Credible Earthquake (MCE) only at near-fault sites when Probabilistic Seismic Hazard Analysis (PSHA) is used. 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 for larger than those obtained using a probabilistic approach even for very long return periods. Knowledge of the unique characteristics of near-fault ground motions should be applied to the development of ground motions for the design and evaluation of dams that are located close to identified active faults.

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2016 Papers

2016 – Characteristics of Near Fault Ground Motions for Seismic Design
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Paul Somerville

This paper describes the unique characteristics of near-fault ground motions for use in developing ground motions for the design and evaluation of dams that are located close to identified active faults. These characteristics include near-fault rupture directivity effects, permanent ground displacements, and hanging wall effects. In Australia, active faults make a significant contribution to the Maximum Credible Earthquake (MCE) only at near-fault sites when Probabilistic Seismic Hazard Analysis (PSHA) is used. 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 for larger than those obtained using a probabilistic approach even for very long return periods. Knowledge of the unique characteristics of near-fault ground motions should be applied to the development of ground motions for the design and evaluation of dams that are located close to identified active faults.
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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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2018 Papers

2018 – Impacts of New Knowledge and the 2018 Draft ANCOLD Guidelines on Ground Motion Levels at Dam Sites in Australia
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Paul Somerville, Andreas Skarlatoudis, Jeff Bayless and Don Macfarlane

The revised magnitudes of the Geoscience Australia’s NSHA18 earthquake catalogue approximately halve the rate of occurrence of earthquakes of a given Mw magnitude in Australia. This yields probabilistic ground motion levels that are significantly lower than the present design levels at dam sites in Australia that are not near faults, and is expected to result in a general reduction in ground motion levels at dams not near faults estimated for all Risk Assessments, and for Deterministic Assessments for all consequence levels except Extreme Consequence. For the latter, the ANCOLD (2018) guidelines will tend to increase existing SEE ground motion estimates for both of the methods used to estimate the safety evaluation earthquake (SEE). By requiring the use of the Deterministic SEE if it is larger than the probabilistic SEE, and by requiring use of the 85th fractile of the Probabilistic SEE if it is larger than the Deterministic SEE, the ANCOLD (2018) guidelines for Deterministic Assessments are much more conservative than the ICOLD and NZSOLD guidelines for Extreme Consequence dams, especially at those located near faults.
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2019 Papers

2019 – Probabilistic Fault Displacement Hazard Analysis for Dams in Australia
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Hong Kie Thio, Paul Somerville, Don Macfarlane

Fault displacement can occur due to primary faulting on a main fault intersecting a dam foundation or rim, as well as by secondary faulting. This secondary faulting may be triggered locally by the occurrence of primary faulting on a main fault; its occurrence is conditional on the occurrence of an earthquake on the main fault. A probabilistic approach is most viable for fault displacement hazard analysis. Unlike the case of probabilistic ground motion hazard, which is nonzero even for short return periods due to the occurrence of a broad range of earthquake magnitudes in a wide region around the site, probabilistic fault displacement hazard is zero for return periods less than the recurrence interval of surface faulting earthquakes on the fault. In Australia, these recurrence intervals typically lie in the range of 10,000 to 100,000 years.

Consequently, the fault displacement hazard due to primary faulting may be zero or negligible for return periods shorter than 10,000 or 100,000 years. For longer return periods, the hazard is best evaluated using a risk-based approach, as recommended by ANCOLD (2018); the alternative of using a deterministic approach, which disregards return period, could potentially yield a large fault displacement. The probability of triggered secondary faulting, conditional on the occurrence of a large earthquake on the main fault, is typically one or two orders of magnitude lower than that on the main fault, and so is even more likely to be zero or negligible for return periods shorter than 10,000 to 100,000 years
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2020 Papers

2020 – Use of Conditional Mean Spectra with Minimum Magnitude less than 5 in Seismic Hazard Analysis
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Paul Somerville, Andreas Skarlatoudis, Jeff Bayless, Polly Guan

The 2019 ANCOLD seismic guidelines state that “A hazard assessment should be conducted for earthquake magnitudes Mw 5 and above. However, under certain circumstances, smaller magnitude earthquakes may form the lower limit. With masonry dams, slab and buttress dams, older concrete dams, and structural concrete components of dams, Mw 4 earthquake magnitudes should form the lower limit.” However, when using probabilistic Uniform Hazard Spectra (UHS) with Mmin less than 5.0 per the 2019 ANCOLD Guidelines, the hazard will be overestimated unless Conditional Mean Spectra (CMS) are used to represent the ground motions. As described by Somerville et al. (2015), use of the UHS can significantly overestimate the seismic hazard levels presented by individual earthquake scenarios because the UHS envelopes the ground motions from multiple earthquake scenarios in one spectrum. This overestimation is especially true of the ground motions from small magnitude earthquake scenarios. The probabilistic UHS may have large short period ground motions with contributions from a range of scenario earthquakes, but if the UHS is used as the design spectrum, these ground motions will often be represented by earthquake scenarios having inappropriately large magnitudes, long durations, and high long period ground motion levels. As a result, these design ground motions have the potential to overestimate the response of the structure under consideration. By using CMS spectra and time histories, the large probabilistic peak accelerations, predominantly from small earthquakes, are better represented by earthquakes having appropriately small magnitudes, short durations, and lower long period ground motion levels, yielding more realistic estimates of the response of the structure.
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2013 Papers

2013 – Development of design ground motions for the Tekapo Canal Project
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Paul Somerville

Earthquake ground motions were developed for the Tekapo Canal Remediation Project, including both Canal and Bridge sites. This work involved the specifications of the parameters of active faults and seismic source zones, the development of an aftershock sequence, and the review and selection of suitable ground motion prediction equations. The seismic hazard at the project sites is dominated by earthquakes occurring on the Irishman Creek fault. The characteristics of an inferred active (unnamed) fault shown crossing the Tekapo Canal near Forks Stream and the hazard it poses to the canal were also assessed, and it was concluded that there was no need to further investigate it as part of the canal upgrade project. A probabilistic seismic hazard analysis was used to develop response spectra for mainshock events for the various return periods relevant to components of the canal system having different PIC categories. A deterministic seismic hazard analysis methodology was used to estimate the aftershock spectra. Depending on the PIC category, time histories were developed to represent the response spectrum for both mainshock and aftershock events at some canal sites.
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2008 Papers

2008 – Seismic hazard assessment at Hinze Dam
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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.
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2011 Papers

2011 – Recent Developments in Seismic Hazard Analysis
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Paul Somerville, Hong Kie Thio

There is a large degree of uncertainty as to the true state of nature (i.e. epistemic uncertainty) regarding many aspects of seismic hazard analysis. Such differences are often highlighted by differences between alternative models put forth by different model proponents. This epistemic uncertainty is treated by giving weight to all viable alternative models through the use of logic trees in probabilistic seismic hazard analysis, rather than just using a preferred model. This paper reviews epistemic uncertainties that arise from alternative distributed earthquake source models; alternative models for the recurrence of earthquakes on those sources; alternative approaches to including active faults; alternative models for the recurrence of earthquakes on active faults; alternative ground motion prediction models for Australia; and alternative methods for incorporating site response. It also reviews alternative representations of the design response spectrum for the development of ground motion time histories.

2011 – Recent Developments in Seismic Hazard Analysis

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Search Results: “” — Page 1

Paul Somerville, Andreas Skarlatoudis, Jeff Bayless and Don Macfarlane

Hong Kie Thio, Paul Somerville, Don Macfarlane