6.a Engineering Seismology
Observed Ground Motions in the 4 September 2010 Darfield and 22 February 2011 Christchurch Earthquakes
B.A. Bradley
This paper provides an overview of the salient aspects of the dense array of ground motions observed in the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes. Particular attention is given to inferred physical reasons for the observed ground motions, which include: (i) source features such as forward directivity effects; (ii) The effects of the Canterbury Plains sedimentary basin on basin-generated surface waves, and waveguide effects through the region; and (iii) the importance of local site response as evidenced by observations of large long period amplification and liquefaction. The significance of vertical ground motion intensity is also examined.
Lessons from the Canterbury events: preliminary improvements to the online felt reports
T. Goded, K.F. Fenaughty & R.J. Michell
In 2004, the GeoNet project operated by GNS Science implemented an internet-based questionnaire. Its aim was to provide an automatic intensity assignment in New Zealand's Modified Mercalli (MM) intensity scale based on answers to a set of standardized questions. The devastating Darfield (4 September 2010, Mw=7.1) earthquake and its two main aftershocks (22 February 2011, Mw=6.2 and 13 June 2011, Mw=6.0) have shown how well-known these reports have been to the public, with nearly 15,000 reports completed. For these shocks, nearly 600 reports have been assigned with MM intensity values of 8 or above, indicating major earthquake damage and the need for a detailed analysis of the damage, building by building. This huge amount of invaluable information has served as a way of testing the methodology and provided a check for any further required information. The usability of the web interface is also being reviewed to remedy common mistakes and to adopt concepts common in other online questionnaires. New improvements are proposed for the algorithm, including new questions to take account of all types of damage and other relevant issues. These changes could provide an improved algorithm to assign intensities in a more reliable way in future New Zealand events.
Kinematic source studies of the ongoing (2010-2011) sequence of recent large earthquakes in Canterbury
C. Holden & J. Beavan
On 4 September 2010, a surface rupturing earthquake (Mw 7.1) struck the Canterbury Plains region in New Zealand's South Island. The Canterbury Plains is a region of relatively low seismicity, and the structure that ruptured was a previously unmapped fault. The earthquake has been followed by more than 10,000 catalogued aftershocks, including two of magnitude 6.0 and one of magnitude 6.2. On 22 February 2011 a destructive Mw 6.2 aftershock with shallow depth struck approximately 8 km southeast of downtown Christchurch, causing extensive damage in the central city and its eastern suburbs and 185 fatalities. This earthquake was very energetic, with recorded maximum vertical accelerations of 2.2 g near the epicentre. On 13 June 2011 a further aftershock of magnitude (Mw) 6.0 struck Christchurch. It was located only a few km east of the previous event, and again caused extensive damage, landslides, rock falls and liquefaction. Accelerations over 2g were also recorded. On 23 December 2011 Christchurch was again struck by two large aftershocks, 2 hours apart, of magnitude (Ml) 5.8 and 6.0. These were located offshore, about 10 km east of the central city. Unlike the Mw 7.1 event no surface rupture has been found for any of the large aftershocks. The source process of all of these events have been well constrained by geodetic and seismological data. We present a preliminary earthquake sequence based on kinematic source models of the earthquakes.
Spectra and PGAs for the Assessment and Reconstruction of Christchurch
G.H. McVerry, M.C. Gerstenberger, D.A. Rhoades & M.W. Stirling
Spectra and peak ground accelerations for the assessment and rebuilding of structures in Christchurch were revised rapidly following the magnitude 6.2 earthquake on 22 February 2011. The NZS1170.5:2004 hazard factor Z for Christchurch was quickly raised from 0.22 to 0.3. There have been different requirements for different purposes: spectra for the design of new structures and the assessment of existing ones, and peak ground accelerations for liquefaction assessments and for evaluating the probabilities of rock falls in the Port Hills. Particularly challenging has been the need to take into account the time-varying nature of a productive on-going earthquake sequence. The increased design levels recognise that the motions in Christchurch were abnormally strong, the strongest that have been recorded in any New Zealand earthquake. Ground motions around the CBD were enhanced by factors of about 2 or more from median values given by local and US ground-motion prediction equations for the magnitude and distance of the earthquake. The recorded CBD motions were generally about double the 500-year design motions for Christchurch. Systematic effects such as enhanced stress-drop, rupture-directivity, site-effects and basin effects are being modelled.
New National Seismic Hazard Model for New Zealand: Changes to Estimated Long-Term Hazard
M.W. Stirling, G.H. McVerry & M.C. Gerstenberger
We compare site-specific response spectra and hazard maps from the recently-updated national seismic hazard model (2010 NSHM) and predecessor NSHM of 2002. 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 distributed seismicity model has also been updated to include new seismicity data, a new seismicity regionalisation, and improved methodology for calculation of the seismicity parameters. The new spectra and maps show some significant reductions to estimated hazard in areas dominated by the distributed seismicity model (e.g. Auckland and Northland), increases in the Wellington region due to the new Hikurangi subduction zone model, and some reductions along some major faults in the South Island. Future improvements to the NSHM will include the treatment of epistemic uncertainties (source models and ground motion prediction equations), incorporation of GPS data into the distributed seismicity model, nationally-based integration of time-dependent seismicity modelling methodologies, and inclusion of formal testing methodologies in the model.
Construction Cost Implications of the Increased Seismic Coefficient Z for Christchurch: A Case Study
A.D. Amaris & K. Hoglund
Following the magnitude 7.1 Greendale earthquake on 4 September, 2010 and the magnitude 6.3 Lyttelton aftershock on 22 February, 2011 (which caused severe damage to the Christchurch CBD, the Eastern and Southern suburbs, the Port Hills and Lyttelton), the Department of Building and Housing (DBH) have imposed modifications to increase the seismic coefficient from Z=0.22 to Z=0.3 and the Return Period Factor at Serviceability Limit State (SLS) from Rs = 0.21 to Rs=0.33.
While these changes to the seismic coefficient were put into effect the Christchurch City Council (CCC), Capital Programme Group (CPG) were in the process of tendering a new Council facility; the Aranui Library. This change led to a redesign of the foundations under the different performance criteria and a review of the structural capacities with consideration to the additional seismic forces.
This paper will present the Aranui Library as a case study showing changes to the construction methodology to meet the new DBH modifications and identify the increase in cost that these changes imply. It also investigates different foundation options designed with performance levels to maintain the life cost of a building through high performance structural design for specific seismic conditions.