New Zealand through the eyes of astronauts
New Zealand through the eyes of astronauts
The earthquake that shook the southern South Island in the early hours of August 22 was the largest shallow quake in New Zealand since the destructive Inangahua shock of 1968. It had a magnitude of 7.1 and was centred near Secretary Island, on the west coast of Fiordland. Because of its remote location, it caused remarkably little damage to human infrastructure. The main reported impact was in Te Anau and Queenstown, where items were thrown off retailers’ shelves. In contrast, damage in the wilderness of Fiordland National Park was extensive, in the form of many landslides within 50 km or so of the epicentre. Strong aftershocks, some of magnitude 6.2, continued for many weeks following the main shock and caused considerable public anxiety. Scientists, however, greeted August’s seismic event with enthusiasm. Because the last two years have seen the initiation of the Earthquake Commission-funded GeoNet project, which upgrades geological-hazard monitoring in New Zealand, the Fiordland earthquake promises to supply more and better data than any previous quake. This will not only help geologists understand the forces at work beneath Fiordland, but also provide invaluable information for understanding and mitigating the effects of future earthquakes in New Zealand. Although the strongest tremors occurred in the southern South Island, people reported feeling the Fiordland quake as far away as Wellington, and even Auckland. More remarkably, it was felt on the east coast of Australia, particularly in Sydney, nearly 2000 km away. The energy from the earthquake was carried to Australia not via the earth’s crust but through the ocean. At about 1000 m depth is a region known as the SOFAR channel, where sound travels especially slowly. The higher temperature of shallower water and the greater pressure of deeper water both increase the speed of sound. As a result, sound energy that gets into the SOFAR channel essentially remains trapped within it and can therefore travel great distances. The Fiordland earthquake lifted a large area of the seafloor just off the coast, which “pumped” a lot of energy into the SOFAR channel, generating a pulse. This crossed the Tasman Sea, reaching the east coast of Australia about 20 minutes later. Some of the energy was then converted back into the shaking of continental rock, and this caused the motion that was felt in Sydney. The earthquake was recorded on seismometers around the world, and within a few hours locations for its epicentre and depth were being reported by both New Zealand and overseas scientists. The seismometer closest to the epicentre was about 50 km away, at Manapouri power station, and this instrument, together with a number of others in Fiordland, Southland and Otago, enabled scientists to pinpoint the location to within about 10 km. It is hard to be more precise that this, when seismic stations are as thinly spread as they are. Consequently, on August 22, Institute of Geological and Nuclear Sciences (GNS) scientists took nine portable seismographs to Te Anau and, over the next few days, deployed them close to the epicentre. Six of the seismographs were very sensitive instruments designed to record and locate even the weakest tremors, allowing the distribution in space and time of the many thousands of expected aftershocks to be mapped very precisely. Once completed, this will make it possible to identify which fault broke in the earthquake and to ascertain if there was any motion along other faults nearby. The other three instruments were so-called “strong-motion” devices, designed to record the shaking of only the largest aftershocks. The portable seismometers were left in position for several weeks, recording data in their internal memories. They have only recently been recovered from the field, and the mammoth task of calculating accurate locations for the thousands of aftershocks is just beginning. Immediately after the quake, reconnaissance was initiated to assess damage in the vicinity. More than 450 landslides were recorded. The largest dislodged more than 600,000 m3 of rock and nearly struck a Department of Conservation hut. Post-quake landslide surveys are important for several reasons. They provide an independent method for locating the source of an earthquake, as damage is usually concentrated nearby. They can also identify secondary hazards that result from landslides. For example, a slide can temporarily dam a stream, so that when the dam eventually breaks it releases a possibly devastating flood. The pattern, type and size of landslides can also indicate the kind of damage that future large earthquakes are likely to cause, which is useful for civil defence and other emergency planning. Apart from landslides, damage from the Fiordland earthquake was remarkably light: minor cracking of some concrete structures in and around Te Anau, a light degree of slumping along the shoreline of Lake Te Anau and brief closure of the Wilmot Pass road through the collapse of a cutting. The Manapouri power station, electricity transmission lines and communications were essentially unaffected. In a heavily populated area, damage would have been much more extensive. In the week after the earthquake, Global Positioning System (GPS) instruments that can fix positions to an accuracy of a few millimetres were set up at 11 geodetic survey monuments, or trigstations, within and around Fiordland. The positions of these sites had been accurately measured using GPS during February 2001 in a joint Otago University GNS project. After making a small correction to allow for tectonic movement between 2001 and 2003, scientists could deduce the displacement of the 11 sites during the earthquake by noting the difference between the February 2001 and late-August 2003 positions. The largest horizontal displacement recorded was 17 cm in a west-northwesterly direction, in the Museum Range, combined with subsidence of about 10 cm. From the pattern and size of the displacements it is possible to calculate a model of the position and orientation of the fault surface that ruptured during the earthquake, as well as the amount and direction of slippage between the two sides of the fault. Data indicate that the fault surface is an area about 35 km long by 20 km wide, located on the dipping interface between the subducting Australian plate and the overriding Pacific plate. Almost 2.5 m of displacement occurred between the two sides of the fault, with the Pacific plate sliding upwards and west‑northwestwards relative to the Australian plate. Although the largest vertical displacement observed at any of the GPS stations was 10 cm, the model predicts considerably greater uplift elsewhere nearly 50 cm offshore of Secretary Island. This sudden and substantial raising of the seafloor explains how so much energy entered the SOFAR channel. Such movements of the seafloor commonly generate tsunami, and, indeed, a small tsunami with wave heights of 10–20 cm was observed at the Jackson Bay tide gauge and at tide gauges on the east coast of Australia. While the greatest uplift occurred offshore, the model predicts as much as 40 cm of upward movement along the outer coast of Secretary Island, and lesser displacements to the north and south. Nature provides a useful measure of the size and distribution of this kind of coastal uplift in the form of sessile intertidal organisms such as barnacles, mussels and red algae, which survive in only a narrow range between low and high tide. After a sudden raising or lowering of the coastline, these organisms slowly die before re-establishing themselves in their preferred locale. Now that several months have elapsed since the Fiordland earthquake, scientists are poised to start looking for evidence of this phenomenon. The damage caused by an earthquake depends on a number of variables. One is the strength of the quake; another is the composition of the ground—soft soils, for example, tend to amplify the shaking more than solid rock. A third is the decrease in shaking with increasing distance from the source the so-called “attenuation” of a quake. Attenuation can be estimated by measuring the amplitude of the shaking with strong-motion seismometers at varying distances from an earthquake’s source, and also by analysis of “felt reports,” in which individuals give their personal assessment of the strength of the shaking. Over a thousand felt reports have been received following the Fiordland quake and are being analysed at GNS. But by far the most information is coming from the nearly 200 strong-motion seismometers that have been upgraded or newly deployed throughout the country over the past two years as part of the GeoNet project. The earthquake was recorded by 60 of these, at locations ranging from Manapouri to Taumarunui. The combined data so acquired constitute the best set of information on attenuation ever recorded in New Zealand, and will almost certainly lead to improvements in earthquake-hazard estimation and building codes. The question most often asked after a big earthquake is: does this increase the likelihood that other large quakes will follow? This is not an easy question to answer. While it is certainly true that a major shake can affect the stress, or pressure, acting on other faults nearby, this can make an earthquake on one of those faults either more or less likely. Over the past decade, scientists have come up with ways of calculating the changes in stress an earthquake produces on neighbouring faults. This does not mean they can predict earthquakes, but it does mean they can assess whether a quake is more or less probable. In the case of the Fiordland event, we are sure from the GPS results and the well-located aftershocks that it occurred on the subduction interface. The question immediately arises, therefore: what effect has it had on the nearby Alpine Fault. Preliminary calculations suggest that the likelihood of an earthquake on either the eastern or western branch of the Alpine Fault has been increased. The earthquake threat popularly known as “the big one” would be made manifest by a major slip—of perhaps 4 to 8 m along several hundred kilometres of the Alpine Fault. The last time this happened is believed to have been in 1717. The Fiordland event has made an Alpine Fault earthquake slightly more likely.
This is the first in a series of earth-science articles by Purakanui geologist Mark Walrond. For his debut column he examines a landmark close to his Otago home: the famous Moeraki boulders.
A pair of boomerangs unassuming commas of wood—are displayed to one side of the exhibition’s entrance. Above looms the 7.2 m plywood-andfabric wing of a glider. Opposite is the feathered pinion of one of nature’s supreme wind-riders—a southern royal albatross, Diomedes epomophora. Video screens repeat a three-minute sequence showing gliders, both diomedian and human, taking off, soaring and returning to earth.
Two issues ago we ran a provocative Viewpoint article by Chris de Freitas, of Auckland University, which questioned the currently accepted notion that global warming caused by human activity is abruptly changing Earth’s climate. We invited this Viewpoint to highlight the fact that, despite regular media reports attributing a great range of phenomena to anthropogenic (i.e. human-induced) global warming, there is still ongoing scientific debate on the issue—a matter which is rarely reported in the popular media. That climate change triggered by human agency will lead to dire future consequences—a view promoted strongly by the United Nations Intergovernmental Panel on Climate Change (IPCC) has attained mantra status in some sectors, particularly with lobby groups and policy makers. The Kyoto Protocols are framed by the authority of this dominant theory. Our government has enacted, and signalled for the future, legislative changes to meet various commitments prescribed by New Zealand’s agreement to these protocols. Three National Institute of Water and Atmospheric Research (NIWA) scientists, David Wratt, Brett Mullan and Dave Lowe, who have been engaged in climate research for 20 years and who contributed to the 2001 IPCC report, recently sent us a rebuttal of de Freitas’s Viewpoint. The position of the three NIWA scientists is that of the majority of the scientific establishment. The full text of both the de Freitas Viewpoint and the NIWA rebuttal can be viewed on our website (www.nzgeographic.co.nz/ viewpoint). Readers are encouraged to examine the arguments and form their own conclusions. Here we merely summarise the NIWA response. To de Freitas’s claim that atmospheric carbon dioxide levels are being stabilised by increased plant growth and other feedback mechanisms, Wratt, Mullan and Lowe present a graph depicting steadily rising carbon dioxide levels measured at Baring Head between 1971 and 2002, and claim that atmospheric levels of the gas are increasing steadily. Worldwide surface temperature rises are real, they say, and not due to urban effects, as de Freitas argued. They are a little ambivalent about whether satellite temperature records (as opposed to Earth-based measurements) show no warming trend, as de Freitas strongly claimed. While de Freitas put the view that any warming trend of the past 50 years is inconsequential once climate fluctuations over the last 1000 years are considered, Wratt and his colleagues see things very differently. According to their sources, the past 50 years have seen warming on a scale unparalleled in the preceding millennium. Answering the point that climate change over the past century may have been driven by changes in solar output, the NIWA team quote the IPCC, which concedes there may be something to the solar argument but still considers that greenhouse-gas emissions are five to eight times as significant. Is the sea level rising due to global warming? De Freitas noted it has been rising since the end of the last ice age. The NIWA group doesn’t directly disagree, but suggests global warming will cause thermal expansion of the ocean, producing rising sea levels for centuries to come. Not surprisingly, the NIWA scientists disagree with de Freitas’s concluding comment that global warming will be unlikely to exceed 1º C over the next century. They state, “The 2001 IPCC Working Group I report, with its projection of a globally-averaged temperature increase of 1.4 to 5.8° C from 1990 to 2100, is a truly balanced assessment, drawing on input from a large number of experts and reviewers.” What are we to make of these conflicting views? It is important to realise that the collection and interpretation of scientific data is often difficult and complex. Consider the question of whether satellite temperature measurements of the atmosphere show a trend over time. The question is simple. But in reality a number of satellites have probably collected such data—some French, some American, some Russian, perhaps Japanese. They have probably been in different orbits in different years over a 30-year span, measuring with different instruments at different times of the day or night, at different levels in the atmosphere. How accurately calibrated were the instruments? Did orbital decay affect the measurements? How comparable was the data they produced? How meaningful is it to average that data over the whole world—for a decade—to come up with, say, a 0.1º temperature increase? Scientists can have the best intentions and still produce different sets of data that, in the context of the climate debate, may suggest profoundly different outcomes when extrapolated far into the future. In science, there are usually several different sets of ideas being evaluated on any topic at any time, and dissent is normal. In the field of climate change, the cautions and caveats of scientists collide with the bedrock conviction of environmental activism. There is pressure on science to come up with evidence quickly to avert supposed global catastrophe. The difficulty with climate change is to distinguish between a run of aberrant weather and changing climate. In the same way that occasionally you can throw heads five or six times in a row, a run of hot years may occur entirely by chance. There is no substitute for time when struggling to distinguish between a string of coincidences and the start of a trend. It is certain that climate has undergone great change in the past without any assistance from humans. And it is also certain that if humans could be removed from the Earth, and all anthropogenic effects undone, climate would continue to change. How, then, when we focus on a change in temperature over no more than 50 years, can we be sure that, first, the climate is changing and, second, that humans are responsible? Some commentators seem to think that our climate must be preserved in its present balance at all costs, inferring that it was at some magical optimum a few moments before the start of the industrial revolution. This is utter nonsense. Earth’s climate is dynamic and cyclic. A mere 9,000 years ago, barely a blink in the cosmic scheme of things, our world was easing out of glaciation. Aucklanders of the time could have enjoyed a weekend stroll to Waiheke across the Hauraki Basin, thanks to dramatically lower sea levels. Around 40 glacial and interglacial cycles have occurred in the past two million years alone. Seen in the light of this bigger picture, other variables—which humans have no hand in—must have played a dominant role in earlier eras. For instance, several recent studies suggest that galactic cosmic rays could be the main driver of global climate change. These rays are thought to ionise molecules in the atmosphere that then act as condensation nuclei, promoting cloud formation. Clouds reflect sunlight, leading to cooling. Every 140 million years, our solar system passes through the spiral arms of the Milky Way, home to a lot of star-formation activity and a source of cosmic rays. The authors of these studies note that Earth’s temperature oscillations over the past 500 million years have a period of about 135 million years and conclude that “cosmic ray flux alone can explain [approximately] 66 per cent of the total variance in the temperature data.” In this scenario, carbon dioxide has played a minor role at best. And it is worth remembering that the main greenhouse gas is not carbon dioxide—it is water vapour, and humans have only an indirect impact on its concentration in the atmosphere. Warming caused by human activities promotes evaporation, and more water vapour in the air could translate into more clouds, which then reflect incoming sunlight back to space and lead to cooling of the atmosphere. Incorporating these sorts of possibilities into the climate models that form the basis for global-warming predictions is far from straight foward. In a complex system, change in one small element can precipitate a gross change, and this is where the spectre of global warming seems most potent. As climates change, some species become less viable, and extinction may threaten. But even extinction has its upside. After one major extinction event caused by a meteor strike and a massive climate shift, mammals moved from being lizard food to having a more promising outlook. A certain variant of simian became the dominant species. Global warming was responsible for the rise of Homo sapiens—we are an unforeseeable by-product of the greenhouse effect. Now we must consider to what extent we are responsible for another round of global warming.
In April 2000, New Zealand honeybees received a death threat in the form of the varroa mite, an insect parasite which, if left uncontrolled, is capable of destroying hives and wiping out bees from entire regions. Once inside a hive, the mites multiply rapidly, weakening the honeybee colony and making it susceptible to disease and hive robbers such as wax moths—the culprit behind the destruction of comb in this hive on apiarists Tony and Jane Lorimer's Waikato property. Though confined at present to the North Island, varroa is predicted to colonise the entire country, decimating wild honeybee populations everywhere.
On Friday October 3, a deepening low moved south-east across the Tasman Sea towards New Zealand. An extensive area of upward atmospheric motion ahead of the low created a broad, deep cloud sheet which unleashed heavy rain over most of the North Island and the top of the South Island, bringing rivers into flood and damaging roads with slips and washouts. In parts of the Tararua Range and on Mount Taranaki, over 300 mm of rain fell in 24 hours. The ranges of Nelson and Marlborough received around 100 mm. On the Coromandel Peninsula, a car was carried hundreds of metres down a flooded stream when its driver attempted to cross a ford in the middle of the night. Two men in the car survived, but a young woman drowned. The town worst affected by the downpour was Paekakariki, just north of Wellington, where rain released a massive landslide from a steep hill above the town. Tonnes of rock and mud buried cars and motel units to roof level as debris swept down over State Highway 1 and the main trunk railway line. One train driver, on finding his locomotive surrounded by flowing water, stopped and reversed back to the station. Water dammed up behind the landslide, forming a large pond which flooded 21 houses and poured into the Paekakariki Hotel, where patrons were watching the NPC rugby match between Wellington and Otago. Mud blocked the door, so they had to escape through the windows. State Highway 1 was blocked from Friday night to Saturday afternoon. Traffic backed up for kilometres in both directions, and many people had to spend the night in their cars. The reason for the exceptionally heavy rain in the Paekakariki area was that a convergence zone developed in the lower atmosphere where northeast winds blowing down the coast from Paraparaumu met north-westerlies blowing across Cook Strait. What meteorologists call the “line of convergence” between these airstreams was oriented perpendicular to the coast and pointing at Paekakariki. Cumulus clouds formed along the length of the convergence zone and showed up on the MetService radar as a line of strong echoes characteristic of heavy rain cells. These moved rapidly down the length of the zone under the influence of a deep northwest flow in the middle layers of the atmosphere. Because the zone was almost stationary for a time, the cells all tracked over the same place on the coast, a phenomenon US meteorologists call “train echoes.” As the cumulus clouds slammed into the steep hills behind Paekakariki, the air was forced rapidly upwards, which made the rain even heavier. Most floods in New Zealand are caused by weather systems that bring relatively warm, moist air down to our latitudes from much further north. On this occasion, a ship in the Tasman Sea reported a temperature of 17º C in the airstream headed for the North Island. Since this air had a humidity of close to 100 per cent, it contained around 12 grams of water vapour per kilogram. We tend to forget that air has mass, as it parts so effortlessly as we move through it, but the air in an average-sized house weighs about 400 kg. At 17º C and 100 per cent humidity, this much air would contain about 5 kg of water vapour, which, if it were all condensed, would make 5 litres of liquid. As the humid air was lifted by the combined effects of the convergence zone and the hills, it experienced lower surrounding pressure, which caused it to expand. When any gas expands, its temperature falls. Since cold air is able to carry less water vapour than warm air, some vapour condenses, too, forming the liquid droplets that clouds and rain are made of. When this process is rapid, the rain becomes heavy. Given the devastation that occurred at Paekakariki, it was lucky there were no deaths or serious injuries. However, the rain clouds may have played a role in the crash of a freight plane and the loss of its two pilots off the Kapiti Coast on the Friday night. Among the hazards cumulus clouds present to aircraft are turbulence and icing. These clouds can contain water droplets with a temperature below zero. When the droplets touch the metal surface of an aircraft, they can freeze onto it, altering its aerodynamic properties and possibly jamming the plane’s controls. Whether or not these weather hazards were a factor in this crash remains to be explored by the accident investigators. As the low moved away to the east of New Zealand, south-westerly winds blew cold air from over the Antarctic sea ice up and over the country. Snow fell to sea level in Otago and Southland, killing thousands of lambs. Icy conditions closed roads near Dunedin and Gore. The Desert Road was also closed after ice caused two accidents, and a few snow flurries were even seen in the hill suburbs of Wellington. The heaviest snowfall was in the Catlins region of Otago, where snow accumulated to a depth of 15 cm in places. A rule of thumb for comparing snowfall to rainfall is that each centimetre of snow is equivalent to one millimetre of rain. Although this can vary depending on how slushy the snow is, it is clear that the precipitation in the cold south-westerlies was only a fraction of that which occurred over the North Island in the warmer north-westerlies. One reason for this difference is that cold air contains much less water vapour than warm. Even at 100 per cent humidity, air with a temperature a couple of degrees above zero contains only about 4 grams of water vapour per kilogram—one third the amount in the air that headed for Paekakariki. Further, there were no slow-moving convergence lines focusing the precipitation in one spot over Otago. Rather, the showers were travelling fast, thereby spreading precipitation over a large area. As with so many dramatic events in human history, the Paekakariki story has an unexpected twist. The rugby game being watched by patrons of the Paekak pub that fateful Friday included Christian Cullen, one of the finest fullbacks ever to pull on an All Black jersey, playing in what was likely to be his last home game for Wellington before leaving to play rugby in Ireland. Some said the rain that fell was the sky gods weeping to see the last of the player whose nickname is the “Paekakariki Express.”
Twenty-six years old this year, the QEII Trust has helped nearly 1700 landowners establish convenants, all of which seek to preserve the natural environment for future generations.
The dance hit “I Will Survive” battled its way through the sound system at Te Marua Speedway—and I really, really hoped that I would. Once I finished my sausage-on-a-stick it would be my turn to head out onto the track and drive in my first speedway race. I had to use all my willpower not to turn to the woman next to me and tell her what a moment she was about to see. Instead, I ate my sausage in silence, grinning like a maniac.
Wherever it is too dry, cold, wet or rocky for trees to prosper—as at Lake Heron, in inland Canterbury—expect to find tussock. It is as intrinsic to the high country as sand is to Ninety Mile Beach. But who should own and manage the tussock lands?
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