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Earthquake paper v1.2 1
Earthquake paper v1.2 1
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Earthquake paper v1.2 1

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  • 1. THE DARFIELD EARTHQUAKE—DRAFTThe Darfield EarthquakeThe value of long-term research Today, four factors are used to assess seismic hazards: theIntroduction numbers and types of faults in an area, the history of earthquakes in a region, the characterisation of thoseIn January this year, a Magnitude 7.0 earthquake struck earthquakes, and how that translates into an intensity ofHaiti, leading to a calamity that killed over 200,000 people. shaking at the surface. New Zealand now has probabilisticIn September, a Magnitude 7.1 earthquake struck near assessments that combine these factors, at least at a regionalChristchurch, in which no-one was killed. In part, this level, into estimates of the chance of a given level of groundoutcome was down to luck. The timing of the Darfield shaking within the lifetime of a building. We understandearthquake (at 4.35 am on a Saturday morning) meant that how crusts are moving and how faults accommodate thatmasonry fell into empty streets. Despite this, it is movement, how that builds up strain energy, how likely itremarkable to have a Magnitude 7 earthquake near a city is that earthquakes will release that energy, and we have awithout any fatalities. Modern understanding of seismic good idea of how that energy release translates to shakingrisks, the corresponding construction standards and the at the surface.resulting performance of buildings in such an earthquakeare not a matter of luck. In this paper, the Royal Society of However, as the Darfield earthquake shows, there are stillNew Zealand explores how seismologists and earthquake surprises to be found. The most recent earthquake inengineers have developed the critical knowledge and Canterbury occurred on an unknown fault. The history weunderstanding of earthquakes, which has been have of recorded earthquakes is not long, by geologicalimplemented into the building practices that saved lives in timescales. While many small and deep earthquakes occurCanterbury. each week in New Zealand, since 1840 only seven large shallow earthquakes have occurred close to the CanterburyUnderstanding the seismic hazard region.For engineers to build earthquake-resilient buildings, theyneed to know what level of shaking is expected. Like manynatural phenomena, earthquake shaking is probabilistic, The development of seismic design philosophiesi.e., earthquakes that cause higher degrees of shakinghappen less often. However, determining the probability Making buildings strongand strength of earthquakes in the past was hampered by Building regulations in New Zealand started to includethe lack of understanding why earthquakes occurred. The earthquake resistance after the 1931 Hawke’s Bay1855 earthquake in Marlborough was one of the first ones earthquake (magnitude 7.8). Significant earthquakes haveto be used to show that earthquakes were related to faults. often served to drive changes in building practice, with theHowever, even by 1965 all that could be said was that Wairarapa earthquake in 1855 (of magnitude 8 or more)earthquakes were more of a hazard to buildings near the being responsible for the prevalence of wooden buildingsSouthern Alps and in the south and east of the North over masonry in Wellington. The damage from the 1931Island. earthquake, the political will for regulation arising from that damage, and burgeoning engineering profession allThe 1960s saw a revolution in geology with plate tectonics combined to produce, in 1935, the Standard Modelbecoming the theoretical basis of the field. In 1970, the Building By-Law setting requirements for the strengths ofRoyal Society of New Zealand ran a symposium on recent buildings. Earthquakes were presumed to cause horizontalcrustal movements that widely publicised the idea loads on walls and the philosophy was one of making thethroughout the local seismology community. The slow but building strong enough to resist those simple loads. To bemeasured deformations of the earth could then be fair, engineers and regulators at that time had littleunderstood and connected with the sudden slips at faults knowledge of the outcomes that earthquakes caused, howthat produce earthquakes. In parallel with that theoretical buildings shook, or the forces that shaking created.progress, our measurements of the earth’s deformationscontinued to improve, from traditional surveying to today’ssub-centimetre resolution GPS measurements.www.royalsociety.org.nz November 2010
  • 2. THE DARFIELD EARTHQUAKE—DRAFTFigure 1 : Fault map of the Darfield earthquake. Courtesy of Dr Mark Quigley, University of CanterburyIn 1940 the El Centro earthquake in California was During the 1960s and onwards, seismic design began torecorded by an accelerometer. These data provided much- focus on the idea of using plastic deformation to absorb theneeded information about the shaking that earthquakes energy of shaking. John Hollings began the developmentscause and revealed that ground shaking could be much that led to what is now called “capacity design”. For large,greater than assumed by simple loading analysis. This multi-storey buildings, the columns and floors can beinformation, the deployment of accelerometers in New thought of as rigid, but joined by flexible hinges. As theZealand, and better mapping of active seismic faults was columns sway from side to side, the steel reinforcement atused by Dr Ivan Skinner and others to update the seismic the hinges bends but does not break, and absorbs energyloading code published in 1965. This new code introduced while it does so.the idea of different risk zones, with Wellington in thehighest, Christchurch in the medium risk zone, and For this approach to succeed, a great many questions had toAuckland in the lowest risk zone. The zones implied be thoroughly researched. The Department of Engineeringnothing about the probability of earthquakes, but did at University of Canterbury led efforts to understand anddescribe the expected degree of shaking for each area. quantify the shaking expected for a given ground movement, both through testing on shake tables and through modelling, beginning in 1966 with RobinMaking buildings ductile – capacity design Shepherd’s work using an “electronic digital computer”.The growing understanding of the strength of earthquake Bob Park and Tom Paulay led much of this research,forces showed that buildings could be built strongly including addressing the critical questions of how muchenough, but only at great cost. Driven by this, the approach plastic deformation could be survived by reinforcing bar inchanged to one of ductility - absorbing energy by plastic corner joints between columns and floors; how plasticdeformation within a structure’s steel frame or reinforcing. zones could extend away from joints and into columns,Much like crumple zones in cars, which can absorb energy floors, beams and walls; and how reinforcing bars shouldin a crash, buildings were designed using plastic be tied to other reinforcing bars.deformation zones.www.royalsociety.org.nz November 2010
  • 3. THE DARFIELD EARTHQUAKE—DRAFTFigure 2 : Peak ground accelerations during the Darfield earthquake. We acknowledge the New Zealand GeoNet projectand its sponsors EQC, GNS Science and LINZ, for providing this image. Wooden houses, brick chimneysProfessor Tom Paulay, FRSNZ, FIPENZ & Professor BobPark, FRSNZ, FIPENZ, FEng A similar tale can be told for wooden houses. Early earthquakes created concern, resulting in some changes ofProfessors Tom Paulay and Bob Park helped build the practice, but effective regulation did not develop until theglobal reputation of the University of Canterbury’s Depart- basic research had been done and a thoroughment of Civil Engineering, developing large-scale models of understanding of how houses behave in earthquakes hadstructures, and promoting the idea that it was not enough been developed.for buildings to be strong – they should also have the capac-ity to deform to absorb the shaking that earthquakes induce. The Marlborough and Wairarapa earthquakes in 1848 andThe world-wide influence of their work was recognized in 1855 (magnitude 7.8 & 8.2) showed the benefit of timber2008 when Tom Paulay became the only New Zealander to construction over unreinforced masonry. Similarly, afterbe elected as a “Legend of Earthquake Engineering” by the the magnitude 7.8 Murchison earthquake in 1929, C.International Association of Earthquake Engineering. Dixon of the State Forest Service published an article noting the damage that wooden houses suffer: movement between the foundations and superstructure, lack ofIn 1975, Park & Paulay’s combined research culminated in bracing in lower walls, and falling chimneys. He madethe publication of the book “Reinforced Concrete recommendations about the design and construction ofStructures”, now translated into Spanish and Chinese and wooden houses and the Hawke’s Bay earthquakein regular use today across the world, and the updated emphasised these, but the resulting bylaws were notbuilding loading code in 1975 and revised Concrete Code published until 1944 and even then, they had significantin 1982. These standards, and the work behind them, gaps. It was not until 1978 that a code of practice wasrevealed the earthquake vulnerability of numerous older published that was backed by substantial research andbuildings in New Zealand. In Wellington alone, over four sound engineering. The 1987 Edgecumbe earthquakehundred buildings needed strengthening or replacing, provided a test of this code, with houses built to the codedriving redevelopment across New Zealand’s cities in the undamaged, except for movement between the foundations1980s. and superstructure, a noted omission in the code of practice. Numerous chimneys also failed in that quake, although more from poor construction than poor requirements.www.royalsociety.org.nz November 2010
  • 4. THE DARFIELD EARTHQUAKE—DRAFTSeparating buildings from the ground - Lead rubber Three other trends are influencing current research. Thebearings and seismic isolation first is to recognise the increasing reuse and refitting ofIn the 1970s, earthquake researchers began to consider how buildings as our cities change away from development onbuildings could absorb the shaking caused by an greenfield sites to more urban densification. The second isearthquake without damage to the building itself. Rather the increasing use of more sustainable materials such asthan use steel hinges within the building to localize timber for large buildings. The third is the increasing usedamage and prevent collapse, the idea grew of using of pre-fabricated components and assemblies in buildings.separate dampers to protect the building from any damage Key to this work are the facilities such as those at theat all. Lead was used to absorb large amounts of energy University of Canterbury, where they proudly say that “wethrough substantial plastic deformation without being can build anything; we can bust anything”. Another keydamaged itself. Unlike steel, lead’s low melting point tool is New Zealand’s world class network of millimetre-means that the material can be hot-worked at room precision continuous GPS recorders. The first pay-off fromtemperature – when deformation changes the grain this tool has been the discovery in New Zealand of slowstructure, the lead can recrystallise and retain its strength. earthquakes that occur over weeks. What these slowThe use of lead rubber bearings was developed by Bill earthquakes mean for earthquake and other hazards is aRobinson and Ivan Skinner at the Physical and matter for ongoing research, but our ever-higher-Engineering Laboratory of the DSIR. resolution measurement of the earth’s deformation will result in further discoveries that inform our understandingThese new designs were rapidly taken up by the Ministry of earthquakes.of Works and lead rubber bearings are now responsible forprotecting thousands of buildings and bridges, includingTe Papa and the retro-fit to Parliament. Conclusion Improvements in the earthquake resilience of cities comesIn Christchurch, lead rubber bearings protect the at a cost, but nothing compared to that of a damagingChristchurch Women’s Hospital. This building is designed earthquake without such improvements. Throughto function after an earthquake as a stand-alone crisis unit. foresight and luck, New Zealand has been spared such aIt came through the September earthquake with no trial. Our earthquake resilience has been driven by thestructural damage. results of substantial research, into understanding our seismic hazards, the threat to our cities, how we can create resilient buildings and infrastructure, and how to prepareSaving money as well as saving lives – current for earthquakes when they do occur. Forethought and longand future research investment, rather than reaction, has led to Christchurch’sResearch over the past fifty years has focused on saving escape from disaster.lives. We have seen the success of this in Christchurch.However, while no lives were lost, the city still sufferedseveral billion dollars worth of damage. Research over thelast decade has shifted to reducing the cost and communitydisruption caused by earthquakes by making buildings Contributors and Reviewersmore resilient at lower cost and by more specificpredictions of ground shaking. Professor Andy Buchanan, Alistair Cattanach,We can now create structures that should keep their Dr Tim Haskell, Dr Mark Quiggley, Professoroccupants alive through an earthquake. However, after Martha Savage, Professor Euan Smith, Adamstrong shaking those buildings may require expensive Thornton, Dr Peter Woodrepair or even demolition. The resilience of ourcommunities is increased by designing buildings to surviveshaking and then require less or easier repair, and buildingdesigns that make it easier to assess post-quake damage. For further information, please contactIncreased understanding of how shaking is transmitted Dr Jez Westonfrom the bedrock through local soils and to buildings Jez.weston@royalsociety.org.nzallows for increasingly site-specific risk assessment. Thesurface effects of an earthquake, such as liquefaction, canvary on an almost street-by-street basis, depending uponlocal features of the soil and underlying geology. Resolvingthese details allows building designers to make investmentsin seismic resilience that are matched to the site-specifichazard rather than overbuilding for the worst seismichazard in a particular region. Except for figures & the RSNZ logo, Emerging Issues papers are licensed under a Creative Commons 3.0 New Zealand Licence.www.royalsociety.org.nz November 2010

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