Update of the Z-factor for Christchurch considering earthquake clustering following the Darfield earthquake

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Gerstenberger, M.C.; McVerry, G.H.; Rhoades, D.A.; Stirling, M.W.; Berryman, K.R.; Webb, T.H. 2011 Update of the Z-factor for Christchurch considering earthquake clustering following the Darfield earthquake. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/29 20 p.

Abstract: We provide preliminary updates of the NZS1170 (2004) Z- and R-factors for Christchurch based on the changes in seismicity following the recent Canterbury earthquakes. Seismicity in the region of the 4 September, 2010 and 22 February, 2011 earthquakes is presently very high relative to previous activity, requiring development of earthquake hazard estimates that model time-varying seismicity rates. We construct a composite earthquake hazard model to give the magnitude-weighted spectral acceleration (SA) level with a 10% probability of exceedance in the next 50 years for both Class C and D soils. The composite model combines several earthquake source models that are based on different concepts and cover a wide range of time, space and magnitude scales. The models are: (1) the fault source model of the national seismic hazard model (NSHM; Stirling et al, in prep), with enhancement of the earthquake probabilities for the most major active faults near to the Canterbury region (Hope, Alpine, and Porters Pass Faults) to take account of the time elapsed since last earthquake; (2) the Proximity to Past Earthquakes (PPE) smoothed seismicity model; (3) the Short Term Earthquake Probability (STEP) model (Gerstenberger et al, 2005), which targets aftershocks and; (4) the Every Earthquake a Precursor According to Scale (EEPAS) model (Rhoades and Evison, 2004) which looks at longer-term earthquake clustering. Earthquake hazard levels produced by the composite model are found to be high in the region of the city, and similar to the hazard levels close to the major active faults to the northwest of Christchurch. SA calculations are provided by the McVerry et al (2006) ground motion prediction equation (GMPE). The calculated SA estimates incorporate a magnitude-dependent scaling as required for engineering design. A critical issue is that the 22 February earthquake produced peak ground accelerations over much of the Christchurch region that were about twice the values modelled by the McVerry et al GMPE. Stress-drop scaling is included to partially account for this, however it is also likely to be due to specific characteristics of the causative fault, for example the rupture directivity of the earthquake, and is presently the focus of further investigation. Some of these characteristics are likely to occur in future earthquakes in the region, increasing the ground-motions from those given by the GMPE used in this study. It is recommended that the hazard factor Z for Christchurch in New Zealand Standard NZS1170 Structural Design Actions should be increased from 0.22 to 0.30, for application to structures with fundamental periods up to 1.5s. Spectral accelerations for periods longer than 1.5s, and peak ground accelerations for geotechnical design and assessments, should be derived from special studies. The return period factor R for Serviceability Limit State 1 associated with a return period of 25 years should be increased from 0.25 to 0.33. Return Period factors greater than 1.0 should remain unchanged from their NZS1170 values. (auth)