This paper presents site-specific and spatially-distributed ground-motion intensity estimates which have been utilized in the aftermath of the 2010-2011 Canterbury, New Zealand earthquakes. The methodology underpinning the ground motion intensity estimation makes use of both prediction models for ground motion intensity and its within-event spatial correlation. A key benefit of the methodology is that the estimated ground motion intensity at a given location is not a single value but a distribution of values. The distribution is comprised of both a mean and standard deviation, with the standard deviation being a function of the distance to nearby observations at strong motion stations. The methodology is illustrated for two applications. Firstly, maps of conditional peak ground acceleration (PGA) have been developed for the major events in the Canterbury earthquake sequence, which among other things, have been utilized for assessing liquefaction triggering susceptibility of land in residential areas. Secondly, the conditional distribution of response spectral ordinates is obtained at the location of the Canterbury Television building (CTV), which catastrophically collapsed in the 22 February 2011 earthquake. The conditional response spectra provide insight for the selection of ground motion records for use in forensic seismic response analyses of important structures at locations where direct recordings are absent.
Research Report No.2010-03Ground motion prediction equations (GMPEs) for geometric-mean pseudo-spectral acceleration amplitudes from New Zealand (NZ) earthquakes are developed. A database of 2437 three-component ground motion records is developed by applying stringent quality criteria to the historically recorded events in NZ. Despite the large number of records, the database is deficient in empirical records from large magnitude events recorded at close distances to the fault rupture plane. As a result, the basis for the NZ-specific GMPE development is to examine the applicability of foreign GMPEs for similar tectonic regions and then modify the most applicable GMPEs based on both theoretical and statistically significant empirically-driven arguments. For active shallow crustal events, five different GMPEs are considered. It was found that the McVerry et al. (2006) model, which is the current model upon which seismic design guidelines and site-specific seismic hazard analyses in NZ are based, provided the worst fit to the NZ database, and that the Chiou et al. (2010) (C10) modification of the Chiou and Youngs (2008) model was the most applicable. Discrepancies between the C10 model and the NZ database that were empirically identified and theoretically justified were used to modify the C10 model for: (i) small magnitude scaling; (ii) scaling of short period ground motion from normal faulting events in volcanic crust; (iii) scaling of ground motions on very hard rock sites; (iv) anelastic attenuation in the NZ crust; and (v) consideration of the increased anelastic attenuation in the Taupo Volcanic Zone (TVZ). For subduction slab events, initially three models were considered. It was found that all of the models had some significant biases with respect to applicability for NZ. The Zhao et al. (2006) (Z06) model was selected because of the rigorous database upon which it was developed and modified by: (i) NZ-specific scaling at small magnitudes; (ii) path scaling at large distances; (iii) consideration of the increased TVZ attenuation; and (iv) revision of the standard deviation model. Based on these modifications the developed model showed no bias of the inter- and intra-event residuals as a function of various predictor variables. The standard deviation of the residuals using the revised standard deviation model also indicated that the model has an adequate precision. Three GMPEs were considered for subduction interface events. The Zhao et al. (2006) (Z06) model was the best performing model with only bias exhibited in the site response model, and possible over-prediction of large magnitude events. The Z06 interface model was modified to account for site response and magnitude scaling using the same functional forms as those of the developed active shallow crustal and subduction slab models. The developed model showed no bias of the inter- and intra-event residuals as a function of various predictor variables. The developed GMPEs include specific features as evident in the NZ database; consistent scaling for parameters not well constrained by the NZ database; and pseudo-spectral amplitudes for vibration periods from 0.01 to 10 seconds. Hence, these models represent a significant advance in the state-of-the art for empirical ground motion prediction in NZ.
On 4 September 2010, a magnitude Mw 7.1 earthquake struck the Canterbury region on the South Island of New Zealand. The epicentre of the earthquake was located in the Darfield area about 40 km west of the city of Christchurch. Extensive damage was inflicted to lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers and wetlands throughout Christchurch and Kaiapoi. Unreinforced masonry buildings also suffered extensive damage throughout the region. Despite the severe damage to infrastructure and residential houses, fortunately, no deaths occurred and only two injuries were reported in this earthquake. From an engineering viewpoint, one may argue that the most significant aspects of the 2010 Darfield Earthquake were geotechnical in nature, with liquefaction and lateral spreading being the principal culprits for the inflicted damage. Following the earthquake, an intensive geotechnical reconnaissance was conducted to capture evidence and perishable data from this event. This paper summarizes the observations and preliminary findings from this early reconnaissance work.
Research Report: 2010-02The objective in writing this report is to provide a guide to structural engineers on how to assess the potential seismic performance of existing hollow-core floors in buildings and the steps involved in the design of new floors. Hollow-core units in New Zealand do not contain stirrups within the precast concrete section. This is due to the way that they are manufactured. The only reinforcement in the great majority of hollow-core units consists of pretensioned strands that are located close to the soffit. A consequence of this is that hollow-core units have a number of potential brittle failure modes that can occur when adverse structural actions are induced in the units. These adverse actions can be induced in a major earthquake due to the relative vertical, horizontal and rotational displacements that occur between hollow-core units and adjacent structural elements, such as beams or structural walls. A number of large scale structural tests backed up by analytical research has shown that extensive interaction occurs between floors containing prestressed precast units and other structural elements, such as walls and beams. The constraint that prestressed units in a floor can apply to adjacent beams can result in an increase in strength of the beams to a considerably greater strength than that indicated in editions of the New Zealand Structural Concrete Standard published prior to 2006. The extent of this increase is such that it could in some cases result in the development of a non-ductile failure mechanism instead of the ductile failure mechanism assumed in the design. Prestressed floor units tie the floor bays together leaving a weak section where the floor joins to supporting structural elements. The restraint provided by the prestress restricts the opening of cracks within the bay. In the event of an earthquake this restraint can result in wide cracks developing at some of the boundaries to floor bays. These cracks may have a significant influence on the performance of the floor when it acts as a diaphragm to transfer seismic forces to the lateral force resisting structural elements in the building. The report contains details of; 1. The different failure modes, which may be induced in hollow-core floors, and the failure modes that may develop in a buildings due to the presence of hollow-core units in the floors; 2. Criteria that may be used to assess the magnitude of the design earthquake which may be safely resisted by a hollow-core floor in a building; 3. Details of how construction practice related to the use of hollow-core floors in New Zealand has changed over the last five decades. This highlights particular aspects that need to be considered in carrying out an assessment of existing hollow-core floors; 4. Information on how a new hollow-core floor may be designed to be consistent with the Earthquake Actions Standard, NZS1170.5: 2004 and the Structural Concrete Standard, NZS3101: 2006 (plus Amendment 2); 5. A review of the research findings relevant to the behaviour of New Zealand hollow-core floors under earthquake conditions. Research that was used to develop the assessment and design criteria is described together with details of how the different criteria were developed from this work.
On 4 September 2010, a magnitude Mw 7.1 earthquake struck the Canterbury region on the South Island of New Zealand. The epicentre of the earthquake was located in the Darfield area about 40 km west of the city of Christchurch. Extensive damage occurred to unreinforced masonry buildings throughout the region during the mainshock and subsequent large aftershocks. Particularly extensive damage was inflicted to lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers and wetlands throughout Christchurch and Kaiapoi. Despite the severe damage to infrastructure and residential houses, fortunately, no deaths occurred and only two injuries were reported in this earthquake. From an engineering viewpoint, one may argue that the most significant aspects of the 2010 Darfield Earthquake were geotechnical in nature, with liquefaction and lateral spreading being the principal culprits for the inflicted damage. Following the earthquake, a geotechnical reconnaissance was conducted over a period of six days (10–15 September 2010) by a team of geotechnical/earthquake engineers and geologists from New Zealand and USA (GEER team: Geo-engineering Extreme Event Reconnaissance). JGS (Japanese Geotechnical Society) members from Japan also participated in the reconnaissance team from 13 to 15 September 2010. The NZ, GEER and JGS members worked as one team and shared resources, information and logistics in order to conduct thorough and most efficient reconnaissance covering a large area over a very limited time period. This report summarises the key evidence and findings from the reconnaissance.
