Shaken City

Two years after its big quake, Christchurch is still trembling.

Building totaled by the February 22, 2011, quake probably predates the more modern building codes.

Simon D. Pollard

Māori, New Zealand's indigenous Polynesian people, have a traditional explanation for earthquakes. When the Sky God was separated from his wife, the Earth, his tears flooded the land. So that the two would not see each other’s sorrow, their sons, who had separated them, turned their mother face downward. Her youngest son, Ruaumoko, was still at her breast, however, so when she was turned over, he was carried to the world below. The other sons gave him fire to keep him warm. Now, as he moves about, he sets off earthquakes and volcanoes.

Not an explanation that is much use for predicting when an earthquake will happen. But then, modern theories don’t score so well, either.

Map shows earthquake and aftershock epicenters near Christchurch, on the east coast of New Zealand’s South Island, from the magnitude 7.1 quake of September 4, 2010 (green star), to mid-December 2012. The worst event for the city was the magnitude 6.2 quake of February 22, 2011 (red star), with a death toll of 185. Two other major quakes occurred on June 13, 2011 (blue star), and December 23, 2011 (pink star). (Key: Green dots = 9/4/10–2/22/11; Red = 2/22/11–6/13/11; Blue = 6/13/11–12/22/11; Pink = 12/23/11–12/17/12; red lines show active faults, dotted yellow lines show subsurface fault ruptures.)

GNS SCIENCE

At 4:35 a.m. on September 4, 2010, my wife, Cynthia, and I were suddenly awakened because our house began to shake violently. As the shaking intensified, the house seemed like a doomed rat in the jaws of a giant terrier. Almost forty seconds later, the terrier decided that the rat was dead, and the house stopped being rattled. We had just experienced an earthquake whose epicenter was twenty-five miles west of our home in Christchurch and that measured 7.1 on the Richter scale.

New Zealand’s second largest city, with a population close to 400,000, Christchurch is situated on the east coast of the South Island, where the Canterbury Plains meet Banks Peninsula. The Canterbury Plains were formed from erosion washed east from the Southern Alps, the mountainous backbone of the South Island. The peninsula is the remnant of two volcanoes that emerged from the seabed 12 million years ago and became extinct some 6 million years later. The peninsula was an island until the plains, expanding like a scrambled egg poured onto a grill, reached the extinct volcanoes 25,000 years ago.

Major fault lines of New Zealand’s North and South Islands reflect the boundary between the Pacific and Australian tectonic plates. In the South Island, a series of major parallel faults join together southward to form the Alpine Fault, considered to be at considerable risk of producing a major earthquake in the next half century.

GNS SCIENCE
The spectacular landscapes of the South Island have a dark underbelly. New Zealand owes its existence to the grinding together of the Australian and Pacific tectonic plates. The Southern Alps, which arose around 25 million years ago from this tectonic pushing and shoving, rise as much as 12,300 feet, and would be taller if it were not for erosion. The stresses in the crust from the colliding plates has meant that much of New Zealand is riddled with fault lines, and continual earthquakes are part of the attendant baggage. The North and South Islands together experience about 15,000 earthquakes a year, and between 100 and 150 of these are large enough to be felt by people. A look at historical records beginning in the 1840s, when New Zealand was first significantly settled by Europeans, suggests that on average the nation will have a magnitude 6 earthquake every year, a magnitude 7 every 10 years, and a magnitude 8 every 100 years. Geological evidence and the records of Māori oral accounts provide a window on major prehistoric quakes as well.

Large detail of painting by Charles Blomfield shows the Pink Terraces,with the White Terraces in the distance. Formed by silica-rich hot springs, the lakeside terraces were a famous attraction on New Zealand’s North Island until 1886, when they were submerged following a volcanic eruption. The nation owes its very land to the tectonic activity along the boundary of the Pacific Plate, but volcanoes, like earthquakes, are an accompanying hazard in that zone.

Alexander Turnbull Library, Wellington, New Zealand

Another bonus of being on a plate boundary is volcanoes. New Zealand is part of the Ring of Fire, which is the boundary of the Pacific Plate and other tectonic plates and frames most of the coastline of the Pacific Ocean. Most of the Earth’s earthquakes and volcanoes occur in the region. Fortunately for Christchurch, no volcanoes are active in the South Island, but the North Island has numerous ones, some visibly active and others dangerously dormant. In June 1886, the island’s biggest eruption in the last 500 years happened when Mount Tarawera blew its top. One result was an eleven-mile fissure that ran through Lake Rotomahana, home to New Zealand’s first tourist attraction, the Pink and White Terraces. These glistening terraces were formed from the crystallization of silica-rich water flowing from hot springs, and the terraces cascaded into the lake like giant staircases. Described as the eighth wonder of the world, they held special spiritual significance to local Māori and dazzled visitors. After the eruption, a crater 330 feet high covered the terraces, and they were eventually submerged in 200 feet of water as a much deeper Lake Rotomahana filled up. In 2011, 125 years after they were thought to have been destroyed, parts of the terraces were rediscovered while the floor of the lake was being mapped. It appears that they may have survived the eruption, but are mostly blanketed in sediment.

The volcanoes that formed Banks Peninsula may be dead, but they nevertheless pose a hazard to Christchurch. A large basalt plug beneath the peninsula has played its part in creating seismic havoc in the region.

The September 4 earthquake was felt through most of the South Island and the southern part of the North Island. It resulted from activity in a complex array of previously unknown shallow faults in the crust of the Pacific Plate, which produced a rupture on the land surface eighteen miles long and a horizontal displacement in places of up to thirteen feet. Fences and hedges in the path of the rupture developed odd-looking kinks. Because of New Zealand’s history of earthquakes, however, strict building codes have meant that housing and commercial buildings are constructed to withstand large quakes. Even many historic buildings have been retrofitted with earthquake strengthening. Most of the architectural casualties were unreinforced brick buildings and brick chimneys. Still, the impact on the region was enormous, and the repair bill was estimated at US $2 billion.

Christchurch was actually built on a swamp and has a very high water table. In some of the city suburbs, sandy silt deposits saturated with water sit close to the surface. Because water is incompressible, vigorous shaking can cause these deposits to rise to the surface in a process called liquefaction. In those suburbs, roads, gardens, and houses filled up with saturated silt. About 100,000 tons of silt had to be removed from these areas.

Coincidentally, one of the most damaging earthquakes to strike Christchurch in the nineteenth century also occurred in early September a few minutes after 4:00 a.m. While the Anglican ChristChurch Cathedral, a focal point of the city, had only superficial damage in the September 2010 earthquake, on September 1, 1888, the shaking caused the heavy cross on top of the 100-foot spire to sway so much that the upper portion of the spire snapped off and crashed to the ground. The cross, which had been held in place by supporting wires, dangled precariously against the remains of the spire. Aside from the damage to the cathedral, the description of the earthquake as reported in The Sydney Morning Herald on September 3, 1888, could have expressed how, 122 years later, people responded to the September 2010 earthquake:

At Christchurch its effects were most alarming. The first shock occurred at four minutes past 4, and was followed at intervals by four others extending over half an hour. The whole city was aroused. People rushed from their houses into the streets, momentarily expecting the rocking buildings to collapse. The greatest commotion prevailed. Wall pictures were dashed to the ground and there was an immense destruction of glass and crockery ware. The bells at the cathedral were made to toll by the rocking spire, and immediately afterwards about 26ft. of the spire came crashing to the ground. A large number of chimneys also fell, and many buildings were cracked. The cathedral itself does not appear to have suffered much damage, but it has been decided not to hold service there tomorrow.

A photograph of the cathedral showing the damaged spire became an iconic image of Christchurch’s vulnerability to earthquakes, especially after the September 2010 quake.

Photograph of ChristChurch Cathedral, above, shows the damaged spire following an earthquake on September 1, 1888.

Alexander Turnbull Library, Wellington, New Zealand

More extensive damage from the February 22, 2011, quake raises doubts that the historical structure can be restored and made safe.

© NEW ZEALAND DEFENCE FORCE

No one died from the 1888 earthquake or the September 4, 2010 quake, although the latter was the most damaging earthquake since 1931, when a magnitude 7.8 quake struck Napier on the North Island and killed 256 people. In 2010, by contrast, two people were seriously injured, and both recovered. While many people did have damage to their houses and had to endure the effects of liquefaction, the prevailing feeling was, because there was no death toll, that we had dodged a bullet. After all, only nine months earlier, on January 12, 2010, there had been a magnitude 7.0 earthquake in Haiti, which, like the September Christchurch earthquake, was shallow with an epicenter close to a large city. Current estimates put the death toll as a result of the Haiti earthquake at between 40,000 and 80,000.

Roadway cracked by September 4, 2010 quake

Simon D. Pollard
Bath’s law states that the largest aftershock from a large seismic event is about one order of magnitude less than the main event. An aftershock of 5.9 that occurred nine minutes after the September 4 quake gave hope that the worst was over. For the next four days, however, Cantabrians endured a large number of big aftershocks, often being awakened during the night because their beds were shaking so much. Then on the 8th, at 7:50 in the morning, there was a magnitude 5.1 aftershock. I was sitting at the dining room table with a cup of coffee. This aftershock didn’t feel like a terrier with a rat, but rather like something with one kick had tried to knock the house off its foundations. It was a short sharp jolt and very violent. It occurred on a previously unknown fault under Banks Peninsula, only about six miles from the city center. It was an ominous warning, but nobody could know that.

Life returned somewhat to normal in Christchurch for the Christmas season. On the night following Christmas Day, however, a series of aftershocks, some directly under the city, awakened many people. December 26 was a public holiday, and the city center was mostly open. Day-after-Christmas shoppers filled the city, even as aftershocks continued. The largest of these was a magnitude 4.9 that, because of its close proximity, caused further damage to weakened buildings and rattled people’s nerves. Shops were evacuated, and a wall from a building collapsed through the roof of a neighboring restaurant. Fortunately the restaurant was closed that day; otherwise, people would have died.

Then at 12:51 p.m. on Tuesday, February 22, 2011, a fault very close to the one that ruptured on September 8, only six miles from the city center, caused a magnitude 6.3 earthquake. Because of the hardness of the region’s underlying crust the quake’s shaking was exceptionally intense. Banks Peninsula may also have acted as a “seismic lens” that reflected the shock waves back toward Christchurch. The impact on people and buildings, as measured by the Modified Mercalli Intensity (MMI) scale, was much higher than would have been expected from the magnitude. It registered on average around IX, “violent,” with one characteristic being “general panic.” (On the MMI scale, I is “instrumental”—not generally felt by people—and the maximum, XII, is “catastrophic.”)

In a valley on Banks Peninsula and close to the epicenter, the peak ground acceleration (PGA) was 2.2 times that of gravity, and people were thrown up in the air. It was the highest PGA ever recorded in New Zealand and among the highest ever recorded globally. As the prime minister of New Zealand, John Key, remarked when he visited Christchurch only hours after the earthquake, “It is just a scene of utter devastation. We may well be witnessing New Zealand’s darkest day.”

There were 185 fatalities in the February earthquake, 115 occurring in a six-story building that collapsed and caught fire. The collapse of a four-story building killed 18 people. People also died as a result of falling masonry and in landslides and cliff collapses on Banks Peninsula. But given that it was in the middle of a weekday, when the city was full of people in buildings and on the streets, it is fortunate that the death toll was not higher.

Crushed cars following the quake on February 22, 2011

Simon D. Pollard

While many people had stories of narrowly avoiding being killed or injured in the February quake, one of the most remarkable examples of survival was inadvertently captured by a security camera. In a quiet street near the city center, the camera was monitoring the street view of a photographer’s studio. When the earthquake started, a man in the middle of the road can be seen running to take cover under the doorframe of a two-story historic building with a masonry façade. Within a couple of seconds the side of the building collapses, followed by the decorative parapet two stories above the doorframe, and the view of the building disappears in a cloud of dust. After about twenty seconds, the dust settles and the man is seen walking with determination out from under the doorway, over the rubble, and onto the road, before disappearing from view. The man was a lecturer at the University of Canterbury, and earthquake drills since the September earthquake had drummed into him the importance of taking shelter. He said that his determined-looking gait was so he could quickly get to colleagues he was about to meet when the earthquake struck, to tell them that he was not hurt.

At least half of the region’s 200,000 houses were damaged. Around 10,000 houses are being demolished, and it will take a number of years before all the others are repaired. For people in vulnerable suburbs, liquefaction surfaced with a vengeance, and around 590,000 tons of silt needed to be removed. Some suburbs will have to be abandoned because the land is too damaged to rebuild on. There was unprecented damage to the city infrastructure, with more than 90 miles of water mains and sub-mains and 190 miles of sewer pipes needing to be replaced. More than 550 miles of roads around the city also need to be repaired, and even two years after the February earthquake, driving in parts of Christchurch reminds me of rough roads in Kenya.

The Madonna in the window of the Roman Catholic Cathedral of the Blessed Sacrament rotated outward during the February 22, 2011, earthquake. Many people took comfort in the way she now faced the shattered city.

Simon D. Pollard
The two iconic cathedrals in the city, the ChristChurch Cathedral and the Roman Catholic Cathedral of the
Blessed Sacrament, were both severely damaged in the February earthquake, and it is unlikely they can be repaired. When I first heard that ChristChurch Cathedral had lost its spire in the earthquake, I pictured the 1888 event. The reality was that the entire spire had toppled along with half of the supporting tower.

As I write this, more than two and a half years after the September 2010 earthquake, the Canterbury region within a thirty-mile radius of the epicenters of that and the other major earthquakes has had over 13,000 aftershocks, including 1 magnitude 7 quake, 3 magnitude 6 quakes, 57 magnitude 5 quakes, and 467 quakes of magnitude 4. On June 13, 2011, for instance, the fault six miles from the city center let loose a magnitude 5.9 quake at 1:00 p.m., a 6.4 at 2:20 p.m., and a 5.2 at 2:21 p.m. These were rated on the MMI scale as VIII, IX, and VI, respectively—destructive, violent, and strong. About a month later the city was blanketed with a foot of snow from a winter storm. The media referred to this added hardship as “the icing on the quake.”

Building rubble from the December 26, 2010, earthquake

Simon D. Pollard
A series of six quakes between magnitudes 5.1 and 6.2 occurred on December 23 and 24, 2011. They caused further damage to vulnerable buildings and sealed the fates of both cathedrals, as well as making stressed citizens wonder if it was ever going to stop. I was in a supermarket for the magnitude 6.2 quake, and as cans fell on me I watched the contents of two aisles empty onto the floor. The supermarket was evaculated, but then it reopened two hours later. People quickly accepted that wine-stained linoleum was the new look for supermarkets in Christchurch.

We have grown used to living with aftershocks and try to accomodate them into our daily lives. Conversations with strangers usually start by talking about how their respective houses have survived the earthquakes and what progress is being made in having them repaired or rebuilt. For the last ten years, I have lived a fifteenminute drive from the city in a suburb on the slopes of Banks Peninsula. Our wooden house sits on twenty poles driven into clay. It is an excellent shock absorber and we have been very lucky.

People have become very vigilant about carrying car keys, cell phones, and other emergency supplies with them at all time. You did not want to be evacuated from your workplace and realize that your car keys and phone were inside the building you had been forced to leave and might not be able to enter again for some time. Pantries were stocked with food to last at least a week, and old landline phones that did not require power to work reappeared in many houses. After the initial earthquake, when most of Canterbury lost power, many people fumbled around in the dark looking for the flashlight they knew they had somewhere in the house. Now, it would be hard to find a house in Christchurch that did not have one or two flashlights beside beds and often in other rooms in the house.

I have lived most of my life in Christchurch, including twenty-two years close to the city’s center. When the news programs started showing the devastation to the city, I always knew what part of the city I was looking at. Now, with more than half of the 220 buildings over five stories, and a quarter of all buildings in the business district, having been demolished or in the process of being demolished, it is easy to get lost in this strange landscape. The cost of the earthquakes has been estimated at US $18 billion, around 10 percent of New Zealand’s Gross Domestic Product. The complete rebuild will take ten to twenty years, and the new normal is pockets of normality in a landscape reminiscent of postwar Europe. However, it is an opportunity to rebuild a seismically safe (as well as energy-efficient “green”) twenty-first century city.

As a friend of mine says, “We live in a seismically active area, and currently it is seismically active.” Everybody plays the game of guessing the magnitude of an aftershock, and everybody has a quick link to the website Canterbury Quake Live (www.canterburyquakelive.co.nz). It is often difficult to know how big an aftershock is because a magnitude 3 that had an epicenter a mile away may feel like a magnitude 5 located ten miles away. A more reliable indicator is to see how quickly the number of visitors to the website increases after an aftershock. In minutes, it can go from a usual background of around 200 visitors to more than 4,000, suggesting that the aftershock was a big one.

While it has been a few months since we have had an aftershock that was big enough to get the adrenaline going, it is very likely they will continue to pop up and remind us of what what lies beneath for many years to come. As the American historian Will Durant said in 1946, “Civilization exists by geological consent, subject to change without notice.”

If there is an elephant in the room—or a bull in the china shop—it is the Alpine Fault, which runs almost the entire length of the western side of the Southern Alps and can even be seen from space. When it ruptures, it is likely to produce a magnitude 8 earthquake that will have a devastating impact on people living on the west coast of the South Island and in those towns close to the Southern Alps. It has ruptured on average every 330 years, with a range of between 140 and 510 years. And the last time the Alpine Fault went off was in 1717. It is estimated that there is a 30 percent chance that it will rupture in the next 50 years.

But located sixty miles from that fault, the inhabitants of Christchurch are likely to experience less violent shaking than occurred during the September 4, 2010, and February 22, 2011, earthquakes, although it is likely to go on for much longer. And because the recent earthquakes have destroyed most of our vulnerable buildings, and building codes for new structures are even more stringent, it is likely the city may be one of the best places to be in the South Island to ride out this fateful seismic event.

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