4.1 Special Session: New Perspectives in Seismic Hazard
Precis of the New National Seismic Hazard Model for New Zealand
M.W. Stirling, G.H. McVerry, M.C. Gerstenberger, N. Litchfield, R. Van Dissen, K. Berryman, L. Wallace, P. Villamor, R.M. Langridge, A. Nicol, M. Reyners, D.A. Rhoades, W. Smith, K. Clark, P. Barnes, G. Lamarche, S. Nodder, B. Bradley, J. Pettinga & K. Jacobs
ABSTRACT: A team of earthquake geologists, seismologists and engineering seismologists from GNS Science, NIWA, University of Canterbury, and Victoria University of Wellington have collectively produced an update of the 2002 national probabilistic seismic hazard (PSH) model for New Zealand The new model incorporates over 200 new onshore and offshore fault sources, and utilises newly developed New Zealand-based scaling relationships and methods for the parameterisation of the fault and subduction interface sources. The background seismicity model has also been updated to include new seismicity data, a new seismicity regionalisation, and improved methodology for calculation of the seismicity parameters. Future efforts to improve the model will focus on time-dependent hazard estimation, testing and evaluation of the model, and greater use of GPS data to model potential earthquakes unaccounted for by the fault and background seismicity models.
Development of Ground Motion Time Histories for Seismic Design
P.G. Somerville & H.K. Thio
ABSTRACT: Structural engineers need ground motion time histories for the analysis of the response of structures to earthquake ground shaking. In current practice, these time histories are usually spectrally matched to a uniform hazard response spectrum. At low probabilities, this spectrum is too “broadband” (i.e. large over an unrealistically broad range of periods), and envelopes a set of more appropriate design response spectra, termed conditional mean spectra. These concepts are illustrated using a site-specific probabilistic seismic hazard analysis of ground shaking in which ground motion time histories are spectrally matched to conditional mean spectra that were derived from the uniform hazard spectrum.
Designing and Implementing a Fault Avoidance Zone Strategy for the Alpine Fault in the West Coast Region
R.M. Langridge, M. Trayes & W. Ries
ABSTRACT: GNS Science has been working closely with West Coast Regional Council to develop a Fault Avoidance Zone (FAZ) for the Alpine Fault in its region. The Alpine Fault is a Class I fault with a recurrence interval of 300-500 yr. Expected single-event displacements range from 6-9 m horizontal and 1-2 m vertical. In terms of likelihood, the Alpine Fault is the most likely fault in New Zealand to cause surface rupture within the design lifetime of built structures. Therefore, it is imperative to develop a strategy to mitigate against the next surface rupture on the fault. An initial FAZ of width 100-340 m has been presented to WCRC and has been disseminated down to District Council level where planning and building consent decisions are made. With respect to engineering outcomes, at least 5 priority areas were recognised where communities may be affected by their proximity to the zone of deformation along the fault. These include: Maruia River; Haupiri River; Inchbonnie; Toaroha River; and Franz Josef. The impacts to the town of Franz Josef particularly, have been recognised as the most serious in terms of life safety, hazard mitigation and post-event recovery. The FAZ there is currently defined to a width of 190 m and encompasses structures in the town that include hotels, houses, petrol and police stations. Franz Josef has been earmarked for further detailed mapping using LiDAR imagery, which will lead to a revised FAZ and a strategy for future town planning.
Ground-Motion Based Tests of the New Zealand National Seismic Hazard Model
M.C. Gerstenberger & M.W. Stirling
ABSTRACT: We present the results of a test of the New Zealand National Seismic Hazard model (NSHM). Our approach is to test the complete NSHM which predicts ground motion exceedances for a given return period, based on a fault source model and a distributed seismicity source model. Using up to four decades of observed ground motion data, we have tested the number of expected exceedances for specific peak ground accelerations (PGA) for 24 sites against the number observed. When testing the 2002 NSHM, the model is rejected as under-predicting the expected number of exceedances; however, when aftershock data are removed from the observations, the model is not rejected. This highlights a problem with existing probabilistic seismic hazard models, where earthquake catalogues must typically be declustered, i.e., aftershocks are removed, before the non-fault based gridded seismicity model is calculated. This problem is present not only for low levels of shaking but also for potentially damaging shaking levels of PGA ≥ 0.1g. Finally, we also present a comparison of the test results for the 2002 and 2010 NSHM.
Quantifying the Effect of Declustering on Probabilistic Seismic Hazard
A. Christophersen, M.C. Gerstenberger, D.A. Rhoades & M.W. Stirling
ABSTRACT: Probabilistic seismic hazard (PSH) models have three key inputs: seismicity data, fault data and attenuation relationships. The seismicity data is generally ‘declustered’, i.e. smaller earthquakes within clusters (such as aftershock sequences and swarms) are removed from the earthquake catalogue before it is processed in the PSH model. We apply different declustering methods to the New Zealand earthquake catalogue to prepare a range of seismicity data for PSH modelling. We calculate hazard consistent with the 2010 National Seismic Hazard model (NSHM) and compare the annual frequency of exceedance of peak ground acceleration (PGA) for Auckland, Taupo, Wellington and Dunedin. The different declustering methods cause differences in the annual rate of magnitude 4 and larger earthquakes and in the b-value of the magnitude-frequency relation, resulting in changes to the hazard. Differences in hazard values for PGAs above 0.4g were only a few percent compared to the method that is used in the 2010 NSHM. For smaller PGAs, the differences varied from -16% to 36% compared to the 2010 NSHM. The differences can be linked to the average rate of magnitude 4 and larger earthquakes and to the b-value.
Development of the Next Generation Australian National Earthquake Hazard Map
T.I. Allen, D.R. Burbidge, D. Clark, A.A. McPherson, C.D.N. Collins & M. Leonard
ABSTRACT: Geoscience Australia (GA) is currently undertaking a process of revising the Australian National Earthquake Hazard Map using modern methods and an updated catalogue of Australian earthquakes. This map is a key component of Australia’s earthquake loading standard, AS1170.4. Here we present an overview of work being undertaken within the GA Earthquake Hazard Project towards delivery of the next generation earthquake hazard map.
Knowledge of the recurrence and magnitude (including maximum magnitude) of historic and pre-historic earthquakes is fundamental to any Probabilistic Seismic Hazard Assessment (PSHA). Palaeoseismological investigation of neotectonic features observed in the Australian landscape has contributed to the development of a Neotectonic Domains model which describes the variation in large intraplate earthquake recurrence behaviour across the country. Analysis of fault data from each domain suggests that maximum magnitude earthquakes of MW 7.0–7.5±0.2 can occur anywhere across the continent. In addition to gathering information on the pre-historic record, more rigorous statistical analyses of the spatial distribution of the historic catalogue are also being undertaken.
Earthquake magnitudes in Australian catalogues were determined using disparate magnitude formulae, with many local magnitudes determined using Richter attenuation coefficients prior to about 1990. Consequently, efforts are underway to standardise magnitudes for specific regions and temporal periods, and to convert all earthquakes in the catalogue to moment magnitude.
Finally, we will review the general procedure for updating the national earthquake hazard map, including consideration of Australian-specific ground-motion prediction equations. We will also examine the sensitivity of hazard estimates to the assumptions of certain model components in the hazard assessment.
A Methodology for Probabilistic Post-Earthquake Risk Assessment That Accounts for Aftershocks
N. Luco, M.C. Gerstenberger, S.R. Uma, H. Ryu, A.B. Liel & M. Raghunandan
ABSTRACT: More and more probabilistic seismic risk assessment is becoming the basis for longer-term or “pre-earthquake” mitigation approaches for buildings and other structures. Such probabilistic assessments have also been proposed as bases for making shorter-term or “post-earthquake” mitigation decisions just after a mainshock occurs. This paper presents the methodology for post-earthquake probabilistic risk assessment that we propose in order to develop a computational tool for automatic (or semi-automatic) assessment. The methodology utilizes the same so-called risk integral that can be used for pre-earthquake probabilistic assessment. The risk integral couples i) ground motion hazard information for the location of a structure of interest with ii) knowledge of the fragility of the structure with respect to potential ground motion intensities. In the proposed post-mainshock methodology, the ground motion hazard component of the risk integral is adapted to account for aftershocks, which are deliberately excluded from typical pre-earthquake hazard assessments. Correspondingly, the structural fragility component is adapted to account for any damage caused by the mainshock, as well as uncertainty in the extent of this damage. The result of the adapted risk integral is a fully-probabilistic quantification of post-mainshock seismic risk that can inform emergency response mobilization, inspection prioritization, and re-occupancy decisions.