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THE DARFIELD EARTHQUAKE—DRAFT




The Darfield Earthquake
The value of long-term research
                                                                Today, four factors are used to assess seismic hazards: the
Introduction                                                    numbers and types of faults in an area, the history of
                                                                earthquakes in a region, the characterisation of those
In January this year, a Magnitude 7.0 earthquake struck
                                                                earthquakes, and how that translates into an intensity of
Haiti, leading to a calamity that killed over 200,000 people.
                                                                shaking at the surface. New Zealand now has probabilistic
In September, a Magnitude 7.1 earthquake struck near
                                                                assessments that combine these factors, at least at a regional
Christchurch, in which no-one was killed. In part, this
                                                                level, into estimates of the chance of a given level of ground
outcome was down to luck. The timing of the Darfield
                                                                shaking within the lifetime of a building. We understand
earthquake (at 4.35 am on a Saturday morning) meant that
                                                                how crusts are moving and how faults accommodate that
masonry fell into empty streets. Despite this, it is
                                                                movement, how that builds up strain energy, how likely it
remarkable to have a Magnitude 7 earthquake near a city
                                                                is that earthquakes will release that energy, and we have a
without any fatalities. Modern understanding of seismic
                                                                good idea of how that energy release translates to shaking
risks, the corresponding construction standards and the
                                                                at the surface.
resulting performance of buildings in such an earthquake
are not a matter of luck. In this paper, the Royal Society of
                                                                However, as the Darfield earthquake shows, there are still
New Zealand explores how seismologists and earthquake
                                                                surprises to be found. The most recent earthquake in
engineers have developed the critical knowledge and
                                                                Canterbury occurred on an unknown fault. The history we
understanding of earthquakes, which has been
                                                                have of recorded earthquakes is not long, by geological
implemented into the building practices that saved lives in
                                                                timescales. While many small and deep earthquakes occur
Canterbury.
                                                                each week in New Zealand, since 1840 only seven large
                                                                shallow earthquakes have occurred close to the Canterbury
Understanding the seismic hazard                                region.
For engineers to build earthquake-resilient buildings, they
need to know what level of shaking is expected. Like many
natural phenomena, earthquake shaking is probabilistic,         The development of seismic design philosophies
i.e., earthquakes that cause higher degrees of shaking
happen less often. However, determining the probability         Making buildings strong
and strength of earthquakes in the past was hampered by         Building regulations in New Zealand started to include
the lack of understanding why earthquakes occurred. The         earthquake resistance after the 1931 Hawke’s Bay
1855 earthquake in Marlborough was one of the first ones        earthquake (magnitude 7.8). Significant earthquakes have
to be used to show that earthquakes were related to faults.     often served to drive changes in building practice, with the
However, 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 buildings
Southern Alps and in the south and east of the North            over masonry in Wellington. The damage from the 1931
Island.                                                         earthquake, the political will for regulation arising from
                                                                that damage, and burgeoning engineering profession all
The 1960s saw a revolution in geology with plate tectonics      combined to produce, in 1935, the Standard Model
becoming the theoretical basis of the field. In 1970, the       Building By-Law setting requirements for the strengths of
Royal Society of New Zealand ran a symposium on recent          buildings. Earthquakes were presumed to cause horizontal
crustal movements that widely publicised the idea               loads on walls and the philosophy was one of making the
throughout the local seismology community. The slow but         building strong enough to resist those simple loads. To be
measured deformations of the earth could then be                fair, engineers and regulators at that time had little
understood and connected with the sudden slips at faults        knowledge of the outcomes that earthquakes caused, how
that produce earthquakes. In parallel with that theoretical     buildings shook, or the forces that shaking created.
progress, our measurements of the earth’s deformations
continued to improve, from traditional surveying to today’s
sub-centimetre resolution GPS measurements.




www.royalsociety.org.nz                                                                                     November 2010
THE DARFIELD EARTHQUAKE—DRAFT




Figure 1 : Fault map of the Darfield earthquake. Courtesy of Dr Mark Quigley, University of Canterbury

In 1940 the El Centro earthquake in California was             During the 1960s and onwards, seismic design began to
recorded by an accelerometer. These data provided much-        focus on the idea of using plastic deformation to absorb the
needed information about the shaking that earthquakes          energy of shaking. John Hollings began the developments
cause 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 be
information, the deployment of accelerometers in New           thought of as rigid, but joined by flexible hinges. As the
Zealand, and better mapping of active seismic faults was       columns sway from side to side, the steel reinforcement at
used by Dr Ivan Skinner and others to update the seismic       the hinges bends but does not break, and absorbs energy
loading code published in 1965. This new code introduced       while it does so.
the idea of different risk zones, with Wellington in the
highest, Christchurch in the medium risk zone, and             For this approach to succeed, a great many questions had to
Auckland in the lowest risk zone. The zones implied            be thoroughly researched. The Department of Engineering
nothing about the probability of earthquakes, but did          at University of Canterbury led efforts to understand and
describe 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 Robin
Making 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 much
enough, but only at great cost. Driven by this, the approach   plastic deformation could be survived by reinforcing bar in
changed to one of ductility - absorbing energy by plastic      corner joints between columns and floors; how plastic
deformation 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 should
in a crash, buildings were designed using plastic              be tied to other reinforcing bars.
deformation zones.




www.royalsociety.org.nz                                                                                   November 2010
THE DARFIELD EARTHQUAKE—DRAFT




Figure 2 : Peak ground accelerations during the Darfield earthquake. We acknowledge the New Zealand GeoNet project
and its sponsors EQC, GNS Science and LINZ, for providing this image.

                                                                Wooden houses, brick chimneys
Professor Tom Paulay, FRSNZ, FIPENZ & Professor Bob
Park, FRSNZ, FIPENZ, FEng                                       A similar tale can be told for wooden houses. Early
                                                                earthquakes created concern, resulting in some changes of
Professors Tom Paulay and Bob Park helped build the
                                                                practice, but effective regulation did not develop until the
global reputation of the University of Canterbury’s Depart-
                                                                basic research had been done and a thorough
ment of Civil Engineering, developing large-scale models of
                                                                understanding of how houses behave in earthquakes had
structures, 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 and
The world-wide influence of their work was recognized in
                                                                1855 (magnitude 7.8 & 8.2) showed the benefit of timber
2008 when Tom Paulay became the only New Zealander to
                                                                construction over unreinforced masonry. Similarly, after
be 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 of
In 1975, Park & Paulay’s combined research culminated in        bracing in lower walls, and falling chimneys. He made
the publication of the book “Reinforced Concrete                recommendations about the design and construction of
Structures”, now translated into Spanish and Chinese and        wooden houses and the Hawke’s Bay earthquake
in regular use today across the world, and the updated          emphasised these, but the resulting bylaws were not
building loading code in 1975 and revised Concrete Code         published until 1944 and even then, they had significant
in 1982. These standards, and the work behind them,             gaps. It was not until 1978 that a code of practice was
revealed the earthquake vulnerability of numerous older         published that was backed by substantial research and
buildings in New Zealand. In Wellington alone, over four        sound engineering. The 1987 Edgecumbe earthquake
hundred buildings needed strengthening or replacing,            provided a test of this code, with houses built to the code
driving redevelopment across New Zealand’s cities in the        undamaged, except for movement between the foundations
1980s.                                                          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
THE DARFIELD EARTHQUAKE—DRAFT



Separating buildings from the ground - Lead rubber                Three other trends are influencing current research. The
bearings and seismic isolation                                    first is to recognise the increasing reuse and refitting of
In the 1970s, earthquake researchers began to consider how        buildings as our cities change away from development on
buildings could absorb the shaking caused by an                   greenfield sites to more urban densification. The second is
earthquake without damage to the building itself. Rather          the increasing use of more sustainable materials such as
than use steel hinges within the building to localize             timber for large buildings. The third is the increasing use
damage 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 the
at all. Lead was used to absorb large amounts of energy           University of Canterbury, where they proudly say that “we
through substantial plastic deformation without being             can build anything; we can bust anything”. Another key
damaged 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 from
temperature – when deformation changes the grain                  this tool has been the discovery in New Zealand of 'slow
structure, the lead can recrystallise and retain its strength.    earthquakes' that occur over weeks. What these slow
The use of lead rubber bearings was developed by Bill             earthquakes mean for earthquake and other hazards is a
Robinson 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 understanding
These new designs were rapidly taken up by the Ministry           of earthquakes.
of Works and lead rubber bearings are now responsible for
protecting thousands of buildings and bridges, including
Te Papa and the retro-fit to Parliament.                          Conclusion
                                                                  Improvements in the earthquake resilience of cities comes
In Christchurch, lead rubber bearings protect the                 at a cost, but nothing compared to that of a damaging
Christchurch Women’s Hospital. This building is designed          earthquake without such improvements. Through
to function after an earthquake as a stand-alone crisis unit.     foresight and luck, New Zealand has been spared such a
It came through the September earthquake with no                  trial. Our earthquake resilience has been driven by the
structural 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 prepare
Saving money as well as saving lives – current                    for earthquakes when they do occur. Forethought and long
and future research                                               investment, rather than reaction, has led to Christchurch’s
Research 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 suffered
several billion dollars worth of damage. Research over the
last decade has shifted to reducing the cost and community
disruption caused by earthquakes by making buildings              Contributors and Reviewers
more resilient at lower cost and by more specific
predictions of ground shaking.                                    Professor Andy Buchanan, Alistair Cattanach,
We can now create structures that should keep their
                                                                  Dr Tim Haskell, Dr Mark Quiggley, Professor
occupants alive through an earthquake. However, after             Martha Savage, Professor Euan Smith, Adam
strong shaking those buildings may require expensive              Thornton, Dr Peter Wood
repair or even demolition. The resilience of our
communities is increased by designing buildings to survive
shaking and then require less or easier repair, and building
designs that make it easier to assess post-quake damage.
                                                                  For further information, please contact
Increased understanding of how shaking is transmitted             Dr Jez Weston
from the bedrock through local soils and to buildings             Jez.weston@royalsociety.org.nz
allows for increasingly site-specific risk assessment. The
surface effects of an earthquake, such as liquefaction, can
vary on an almost street-by-street basis, depending upon
local features of the soil and underlying geology. Resolving
these details allows building designers to make investments
in seismic resilience that are matched to the site-specific
hazard rather than overbuilding for the worst seismic
hazard 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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Earthquake paper v1.2 1

  • 1. THE DARFIELD EARTHQUAKE—DRAFT The Darfield Earthquake The value of long-term research Today, four factors are used to assess seismic hazards: the Introduction numbers and types of faults in an area, the history of earthquakes in a region, the characterisation of those In January this year, a Magnitude 7.0 earthquake struck earthquakes, and how that translates into an intensity of Haiti, leading to a calamity that killed over 200,000 people. shaking at the surface. New Zealand now has probabilistic In September, a Magnitude 7.1 earthquake struck near assessments that combine these factors, at least at a regional Christchurch, in which no-one was killed. In part, this level, into estimates of the chance of a given level of ground outcome was down to luck. The timing of the Darfield shaking within the lifetime of a building. We understand earthquake (at 4.35 am on a Saturday morning) meant that how crusts are moving and how faults accommodate that masonry fell into empty streets. Despite this, it is movement, how that builds up strain energy, how likely it remarkable to have a Magnitude 7 earthquake near a city is that earthquakes will release that energy, and we have a without any fatalities. Modern understanding of seismic good idea of how that energy release translates to shaking risks, the corresponding construction standards and the at the surface. resulting performance of buildings in such an earthquake are not a matter of luck. In this paper, the Royal Society of However, as the Darfield earthquake shows, there are still New Zealand explores how seismologists and earthquake surprises to be found. The most recent earthquake in engineers have developed the critical knowledge and Canterbury occurred on an unknown fault. The history we understanding of earthquakes, which has been have of recorded earthquakes is not long, by geological implemented into the building practices that saved lives in timescales. While many small and deep earthquakes occur Canterbury. each week in New Zealand, since 1840 only seven large shallow earthquakes have occurred close to the Canterbury Understanding the seismic hazard region. For engineers to build earthquake-resilient buildings, they need to know what level of shaking is expected. Like many natural phenomena, earthquake shaking is probabilistic, The development of seismic design philosophies i.e., earthquakes that cause higher degrees of shaking happen less often. However, determining the probability Making buildings strong and strength of earthquakes in the past was hampered by Building regulations in New Zealand started to include the lack of understanding why earthquakes occurred. The earthquake resistance after the 1931 Hawke’s Bay 1855 earthquake in Marlborough was one of the first ones earthquake (magnitude 7.8). Significant earthquakes have to be used to show that earthquakes were related to faults. often served to drive changes in building practice, with the However, 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 buildings Southern Alps and in the south and east of the North over masonry in Wellington. The damage from the 1931 Island. earthquake, the political will for regulation arising from that damage, and burgeoning engineering profession all The 1960s saw a revolution in geology with plate tectonics combined to produce, in 1935, the Standard Model becoming the theoretical basis of the field. In 1970, the Building By-Law setting requirements for the strengths of Royal Society of New Zealand ran a symposium on recent buildings. Earthquakes were presumed to cause horizontal crustal movements that widely publicised the idea loads on walls and the philosophy was one of making the throughout the local seismology community. The slow but building strong enough to resist those simple loads. To be measured deformations of the earth could then be fair, engineers and regulators at that time had little understood and connected with the sudden slips at faults knowledge of the outcomes that earthquakes caused, how that produce earthquakes. In parallel with that theoretical buildings shook, or the forces that shaking created. progress, our measurements of the earth’s deformations continued to improve, from traditional surveying to today’s sub-centimetre resolution GPS measurements. www.royalsociety.org.nz November 2010
  • 2. THE DARFIELD EARTHQUAKE—DRAFT Figure 1 : Fault map of the Darfield earthquake. Courtesy of Dr Mark Quigley, University of Canterbury In 1940 the El Centro earthquake in California was During the 1960s and onwards, seismic design began to recorded by an accelerometer. These data provided much- focus on the idea of using plastic deformation to absorb the needed information about the shaking that earthquakes energy of shaking. John Hollings began the developments cause 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 be information, the deployment of accelerometers in New thought of as rigid, but joined by flexible hinges. As the Zealand, and better mapping of active seismic faults was columns sway from side to side, the steel reinforcement at used by Dr Ivan Skinner and others to update the seismic the hinges bends but does not break, and absorbs energy loading code published in 1965. This new code introduced while it does so. the idea of different risk zones, with Wellington in the highest, Christchurch in the medium risk zone, and For this approach to succeed, a great many questions had to Auckland in the lowest risk zone. The zones implied be thoroughly researched. The Department of Engineering nothing about the probability of earthquakes, but did at University of Canterbury led efforts to understand and describe 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 Robin Making 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 much enough, but only at great cost. Driven by this, the approach plastic deformation could be survived by reinforcing bar in changed to one of ductility - absorbing energy by plastic corner joints between columns and floors; how plastic deformation 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 should in 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—DRAFT Figure 2 : Peak ground accelerations during the Darfield earthquake. We acknowledge the New Zealand GeoNet project and its sponsors EQC, GNS Science and LINZ, for providing this image. Wooden houses, brick chimneys Professor Tom Paulay, FRSNZ, FIPENZ & Professor Bob Park, FRSNZ, FIPENZ, FEng A similar tale can be told for wooden houses. Early earthquakes created concern, resulting in some changes of Professors Tom Paulay and Bob Park helped build the practice, but effective regulation did not develop until the global reputation of the University of Canterbury’s Depart- basic research had been done and a thorough ment of Civil Engineering, developing large-scale models of understanding of how houses behave in earthquakes had structures, 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 and The world-wide influence of their work was recognized in 1855 (magnitude 7.8 & 8.2) showed the benefit of timber 2008 when Tom Paulay became the only New Zealander to construction over unreinforced masonry. Similarly, after be 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 of In 1975, Park & Paulay’s combined research culminated in bracing in lower walls, and falling chimneys. He made the publication of the book “Reinforced Concrete recommendations about the design and construction of Structures”, now translated into Spanish and Chinese and wooden houses and the Hawke’s Bay earthquake in regular use today across the world, and the updated emphasised these, but the resulting bylaws were not building loading code in 1975 and revised Concrete Code published until 1944 and even then, they had significant in 1982. These standards, and the work behind them, gaps. It was not until 1978 that a code of practice was revealed the earthquake vulnerability of numerous older published that was backed by substantial research and buildings in New Zealand. In Wellington alone, over four sound engineering. The 1987 Edgecumbe earthquake hundred buildings needed strengthening or replacing, provided a test of this code, with houses built to the code driving redevelopment across New Zealand’s cities in the undamaged, except for movement between the foundations 1980s. 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—DRAFT Separating buildings from the ground - Lead rubber Three other trends are influencing current research. The bearings and seismic isolation first is to recognise the increasing reuse and refitting of In the 1970s, earthquake researchers began to consider how buildings as our cities change away from development on buildings could absorb the shaking caused by an greenfield sites to more urban densification. The second is earthquake without damage to the building itself. Rather the increasing use of more sustainable materials such as than use steel hinges within the building to localize timber for large buildings. The third is the increasing use damage 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 the at all. Lead was used to absorb large amounts of energy University of Canterbury, where they proudly say that “we through substantial plastic deformation without being can build anything; we can bust anything”. Another key damaged 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 from temperature – when deformation changes the grain this tool has been the discovery in New Zealand of 'slow structure, the lead can recrystallise and retain its strength. earthquakes' that occur over weeks. What these slow The use of lead rubber bearings was developed by Bill earthquakes mean for earthquake and other hazards is a Robinson 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 understanding These new designs were rapidly taken up by the Ministry of earthquakes. of Works and lead rubber bearings are now responsible for protecting thousands of buildings and bridges, including Te Papa and the retro-fit to Parliament. Conclusion Improvements in the earthquake resilience of cities comes In Christchurch, lead rubber bearings protect the at a cost, but nothing compared to that of a damaging Christchurch Women’s Hospital. This building is designed earthquake without such improvements. Through to function after an earthquake as a stand-alone crisis unit. foresight and luck, New Zealand has been spared such a It came through the September earthquake with no trial. Our earthquake resilience has been driven by the structural 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 prepare Saving money as well as saving lives – current for earthquakes when they do occur. Forethought and long and future research investment, rather than reaction, has led to Christchurch’s Research 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 suffered several billion dollars worth of damage. Research over the last decade has shifted to reducing the cost and community disruption caused by earthquakes by making buildings Contributors and Reviewers more resilient at lower cost and by more specific predictions of ground shaking. Professor Andy Buchanan, Alistair Cattanach, We can now create structures that should keep their Dr Tim Haskell, Dr Mark Quiggley, Professor occupants alive through an earthquake. However, after Martha Savage, Professor Euan Smith, Adam strong shaking those buildings may require expensive Thornton, Dr Peter Wood repair or even demolition. The resilience of our communities is increased by designing buildings to survive shaking and then require less or easier repair, and building designs that make it easier to assess post-quake damage. For further information, please contact Increased understanding of how shaking is transmitted Dr Jez Weston from the bedrock through local soils and to buildings Jez.weston@royalsociety.org.nz allows for increasingly site-specific risk assessment. The surface effects of an earthquake, such as liquefaction, can vary on an almost street-by-street basis, depending upon local features of the soil and underlying geology. Resolving these details allows building designers to make investments in seismic resilience that are matched to the site-specific hazard rather than overbuilding for the worst seismic hazard 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