|Session 2.3 - Hazard Assessment: Methodology and Examples|
The Porters Pass fault (PPF) has a 40 km-long active trace and is a prominent element of the Porters Pass-Amberley Fault Zone (PPAFZ) which forms a broad zone of active earth deformation ca 100 km long, 60-90 km west and north of Christchurch. The fault has produced repeated large magnitude earthquakes over the last 10,000 years (i.e. the Holocene) and could contribute significantly to seismic hazard in the Canterbury region.
The amount of slip/event and the timing of paleo-earthquakes are crucial components needed to estimate the earthquake potential of a fault. Clustering of offset geomorphic features suggests that four to five earthquakes of ca 5-7 m/event occurred on the PPF during the Holocene. Timing information obtained from radiocarbon dated organic samples from excavations across the fault suggests the occurrence of at least four Holocene earthquakes, producing recurrence intervals of ~2000-2500 years.
The combination of geometric, slip rate and timing data has enabled an estimated magnitude range of prehistoric earthquakes on the PPF/PPAFZ of between 6.9 and 7.4. Using these data, the earthquake shaking likely to be experienced in Christchurch due to a PPF rupture has been calculated using a recent seismic hazard assessment and attenuation model (Stirling et al., 1999).
Paper 067: [Read]
Keywords: Porters Pass Fault, paleoearthquake, displacement, Holocene, trench, radiocarbon date
An integrated approach for evaluating site-dependent seismic hazard (SH) is presented. A probabilistic SH analysis in terms of Fourier amplitude spectra (FAS) of ground acceleration is performed to calculate Uniform Hazard Fourier (UHF) spectra for a given soil condition using spectral amplification functions. The spectral source scaling and attenuation models are developed on the basis of regional ground-motion data. The site amplification functions may be calculated using earthquake recordings and "hard rock" spectral models. The "dominant earthquakes" contributing to the hazards are determined for a given return period in considered frequency range. The UHF spectra combined with strong-motion durations are used for generation of strong ground motion time series, so-called "Uniform Hazard Accelerograms", which allow determining "Site&Region&Return period-dependent" ground motion parameters. When using FAS as an input parameter, it is possible to obtain the results in terms of seismic intensity (MMI or MSK scales), peak acceleration, response spectra, and characteristic accelerograms. The described approach was applied for evaluating design input ground motion parameters and probabilistic microzonation for territories, which are characterized by different seismicity and tectonic settings: Taiwan, Central Asia, Caucasus, and the Far East region in Russia. Selected results are described in the paper.
Keywords: fourier spectra, seismic hazard, design parameters
M.W. Stirling, D. Rhoades, W. Smith, J. Beavan, B. Pace, M. Petersen, A. Frankel and I. Wong
In recent years, the Institute of Geological and Nuclear Sciences (GNS) has applied the methods of probabilistic seismic hazard analysis to develop a new seismic hazard model for New Zealand. GNS is now undertaking collaborative work with organisations from the USA and Italy to improve the New Zealand model by incorporating new input data, and by developing methods to validate the model. The initial part of this work includes: (1) critically reviewing the input parameters responsible for anomalously high seismic hazard in the present probabilistic seismic hazard maps for New Zealand; (2) introducing new active fault parameters and earthquake scaling relations to the model that come from recently-completed major seismic hazard assessments; (3) introducing variable site geology into the model; (4) comparing the methodology of probabilistic seismic hazard analysis embodied in the New Zealand and USA models against geodetic data and historical MM Intensity data, and; (5) engaging in a project to cross-validate a number of probabilistic seismic hazard source codes. Experiences gained from this work may eventually be applied to seismic hazard models outside of New Zealand and the USA.
A detailed seismic assessment has been carried out for an industrial site in Dunedin. Initial geotechnical assessment, using published seismic data, suggested that the site was prone to liquefaction. As the site is near to the harbour, lateral spreading was also considered to be a risk. Mitigation measures for the lateral spreading were likely to be extremely expensive. A site-specific seismic assessment incorporating direct assessment of the liquefaction and lateral spreading risks was carried out to enable a more realistic assessment of these risks to be obtained, than is available from more commonly used procedures.
Keywords: liquefaction, lateral spreading, seismic assessment,