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Frequently asked questions 1. How many earthquakes happen in New
Zealand each year? 1. How many earthquakes happen in New Zealand each year? The Institute of Geological & Nuclear Sciences locates about 10,000 to 15,000 earthquakes in New Zealand each year. Most are too small to be felt and we only know they occurred because they are recorded by seismographs. Each year New Zealand has about 100 to 150 quakes that are big enough to be felt. 2. What is the biggest earthquake ever recorded? The largest earthquake recorded in the world in the last 200 years was the 1960 magnitude 9.5 earthquake in Chile. It caused 5700 deaths and created a large tsunami. It is the largest earthquake to be recorded by modern instruments and ruptured over 700km of faultline with slip of about 20m. It was the first real confirmation that the whole earth rings like a bell after a big earthquake. 3. What is the biggest known earthquake in New Zealand? The biggest known quake in New Zealand was the magnitude 8.2 Wairarapa earthquake of 1855. On an international scale, the 1855 earthquake is of major significance in terms of the area affected and the amount of fault movement. About 5000km2 of land was shifted vertically during the quake. The maximum uplift was 6.4m near Turakirae Head, east of Wellington. The maximum horizontal movement along the fault was 12m. The biggest New Zealand earthquake since instrumental recording began was the 1931 magnitude 7.8 Hawke’s Bay earthquake. 4. Can earthquakes be predicted? It is possible to estimate where big earthquakes are likely in the next 50 to 100 years, based on geological investigations and the historical record of earthquakes. However, it is not yet possible to accurately predict the time and location of the next earthquake. A number of physical changes have been observed before some earthquakes, but the problem is that so far, no particular change has been noted consistently. Some scientists have observed changes in the earth’s magnetic and electric fields, gas emissions, changes in water well levels, and changes in the levels of dissolved gases in groundwater. Other scientists have noted changes in the frequency and location of small earthquakes. A very small number of earthquakes have been successfully predicted. The most notable success was near Haicheng, China in 1975, where 90,000 people were evacuated a few hours before an earthquake that destroyed 90 percent of the buildings. The prediction was based on unusual animal behaviour and a greatly increased number of small earthquakes (foreshocks) that suddenly stopped. One of the animal observations was that snakes came out of hibernation and died due to the cold. It is now thought that this was caused by unseasonably warm weather. However, scientists wrongly predicted a major quake in Kwantung Province, and for two months millions of people lived in tents before authorities decided the prediction was wrong. Later in 1976, an unpredicted quake, magnitude 7.8, in China’s Tangshan Province took 250,000 lives. It was the most disastrous earthquake this century. Since then, China has moved its resources away from earthquake prediction and into improving the earthquake resistance of buildings. 5. What are foreshocks? Foreshocks are earthquakes that occur up to days or weeks prior to a larger earthquake. Scientists are currently unable to tell the difference between foreshocks and normal background seismicity until the large earthquake has happened. However, the fact that many large earthquakes do have foreshocks, indicates that something unusual is happening before the large events and gives us hope that one day we will be able to detect what this unusual activity is. 6. What is the relationship between past and future earthquakes in the same place? In the past scientists believed that a similar-sized earthquake happens on the same fault at regular intervals. However, earthquakes are not that simple. They do not occur like clockwork in the same place. There is a margin of uncertainty in recurrence intervals between ruptures on an active fault. For example, intervals between ruptures on the South Island’s Alpine Fault range from 270 years to 500 years. 7. Do big earthquakes come in clusters? Big earthquakes are not evenly spread. Between 1929 and 1934 New Zealand was hit by five major earthquakes of magnitude 7 or more. This is 10 times greater than the long-term average for earthquakes of magnitude 7. There is evidence to suggest that big earthquakes can accelerate or retard the arrival of another earthquake. It is thought they do this by shifting the balance of stresses in the Earth’s crust. 8. So how often can we expect a big earthquake? Records dating from the 1840s show that, on average, New Zealand can expect several magnitude 6 earthquakes every year, one magnitude 7 every 10 years, and an 8 every century. But large earthquakes are not evenly spaced, and they sometimes arrive in bunches. 9. Is there a relationship between magnitude and the number of aftershocks? Seismologists have established a relationship between magnitude of the main event and the number and size of aftershocks. The relationship is indicative only. In general large, shallow earthquakes produce felt aftershocks. Deep earthquakes produce fewer aftershocks that are less likely to be felt. Several hundred aftershocks occurred after the magnitude 6.3 Edgecumbe earthquake in 1987. More than 20 aftershocks greater than magnitude 6 followed the 1964 magnitude 9.2 earthquake in Alaska. Each of these aftershocks was a considerable quake in its own right. The strain released in earthquakes has typically been accumulated over 100’s to 1000’s of years, and cannot be smoothly released in just a few seconds. Aftershocks occur as the rocks try to readjust to the state they were in before those stresses started to accumulate, and also to readjust around the edges of the piece of fault that ruptured. The rate of aftershocks initially decreases very rapidly with time, but for a large earthquake they can continue at a low level for years. 10. Does an earthquake produce different types of wave? An earthquake produces three kinds of waves — P waves, S waves, and surface waves. The first waves to arrive are the P waves, sometimes called sound or compression waves, which are often heard rather than felt. They travel at 4–8 km/sec (14,000–28,000 km/h) in the earth’s crust. They will often hit a house with a bang or a boom. P waves do not generally cause a lot of damage except in the biggest earthquakes. Next to arrive are the S waves, travelling at 2.5–4 km/sec (9000–14,000 km/h). The S waves pack the bigger punch because they are bigger and move at right angles to the direction of travel. This side-to-side motion (like a snake wriggling) is what causes the most damage to structures. S waves cannot travel through liquids because liquids have no shear strength and therefore don’t support the mode of travel of S waves. Seismologists use the difference in arrival time between P and S waves to calculate the distance between the earthquake source and the recording instrument. Even with computers, this can take up to 15 minutes, although much of that time is spent in collecting data and accurately timing the arrival of the waves — the actual calculation takes the computer only a few seconds. Surface waves travel around the surface layers of the earth and are the slowest of the earthquake waves. Surface waves from large earthquakes travel around the whole earth many times before they become too small to record on seismographs. 11. How long does an earthquake last? People who have just felt an earthquake often ask how long it lasted. What they are really wanting to know is how long they felt the shaking. This depends on the size of the earthquake and their distance from it, because earthquake waves spread out as they travel, but also become weaker. A magnitude 6 earthquake several hundred kilometres away can be often be felt for 30–40 seconds. The actual duration of slip on the earthquake fault is usually quite brief — just a few seconds for a magnitude 6 for example. This is because the fault rupture spreads very quickly (at about the S wave speed), so the whole process of faulting is over very quickly. During the very largest earthquakes, fault rupture can continue for up to 5 minutes as the rupture spreads over a length of say 1000km. For these earthquakes very high levels of aftershocks mean that the ground can be felt shaking continuously for some hours. 12. How do we know which fault is most likely to rupture next, say in Wellington? The reality is that we don’t know for sure which fault is going to rupture next. We talk of probabilities based on continuous monitoring and our knowledge of fault rupture histories. We know there are five major faults in Wellington. The Wairarapa Fault ruptured in 1855 generating an earthquake of about magnitude 8.2. This fault has a recurrence interval of 1150–1200 years. The Ohariu Fault ruptured about 1100–1200 years ago, and has a recurrence interval of 1500–5000 years. The Wairau Fault last ruptured more than 800 years ago and has a recurrence interval of 1000–2300 years. Shepherds Gully Fault last ruptured about 1200 years ago and has a recurrence interval of 2500–5000 years. The Wellington Fault last ruptured between 300 and 500 years ago producing an earthquake of about magnitude 7.6. This fault produces a large earthquake about every 500 to 700 years. This is how we deduce that the Wellington Fault has the highest probability of rupturing next in the Wellington region. 13. Do earthquakes occur only along faultlines? As far as seismologists understand, all but the very deepest earthquakes (deeper than 600km) occur on faults. Seismic waves are generated when the two sides of the fault rapidly slip past each other. For most earthquakes, the faults do not break the surface, so the faults can be "seen" only through analysing the seismic waves. Faults can be anywhere from metres to a thousand kilometres long. Seismologists still have much to learn about the mechanism that causes the deepest earthquakes. At 600km, the earth is probably too warm for faults to be brittle like glass, so some sort of chemical change might occur very rapidly. 14. Why are geologists and seismologists always eager to record aftershocks? Aftershocks produce some of the highest quality earthquake data for scientists to study. Aftershocks can be used as "echo sounders" to study the local structure of the earth. By accurately plotting aftershocks, seismologists and geologists can find the orientation of the fault plane, which helps enormously in characterising the earthquake, and the stresses and strains within the earth that caused it. For these reasons seismologists and geologists are often the first people to arrive at the site of an earthquake so they can deploy portable seismographs. Data collected by these instruments becomes extremely valuable and is sometimes shared among the international scientific community for analysis. In the four weeks after a big earthquake, scientists can record as many earthquakes as they usually get in a year. 15. What’s the difference between magnitude and intensity? Magnitude is a measure of the size of an earthquake and can be related to the amount of energy released at the focal point. It can be likened to the power of radio or television waves sent out from a broadcasting station. Intensity is how well you receive the signal, which can depend on your distance from the energy source, the local conditions, and the pathway the signal has to take to reach you. Intensity is measured on the Modified Mercalli scale — a 12-point scale that represents the intensity of ground-shaking, or destructiveness of an earthquake. Mercalli, an Italian seismologist, developed the scale in the early 20th century. It was later modified to suit building standards in California. New Zealand scientists have modified it further to suit New Zealand conditions. It is used by scientists, engineers, architects, planners, and insurance companies who need to know the relationship between the strength of shaking at ground level and the degree of damage. There is no linear relationship between magnitude and intensity. Magnitude is a quantitative measure and intensity is qualitative. There is roughly a 30-fold increase in seismic energy for each step up in magnitude. A magnitude 5 earthquake releases as much energy as the Hiroshima atomic bomb — the equivalent of 15 kilotons of TNT. A magnitude 6 is equivalent to 30 Hiroshima bombs. Alternatively, a magnitude 7 quake releases about a million times more energy than a magnitude 3 quake.
16. What’s the difference between quake-proof and quake-resistant? Reports that talk about a quake-proof structure
are usually confusing quake-proof with quake-resistant. It is too
expensive to make a structure such as a bridge totally quake-proof.
Quake-resistant is about as good as we can get for structures, such
as bridges, as there is no guarantee against the massive forces
generated by earthquakes. The duration of strong shaking can also
be an important factor in building damage, as prolonged shaking
can set up resonances and also exceed a building’s ability
to absorb the earthquake energy. Seismologists have investigated the effect of the moon’s gravity for many years. The short answer is that while the moon does deform the earth slightly in a 12-hour cycle called the solid earth tides, it doesn’t seem to have an effect on the time an occurrence of big earthquakes. There are difficulties in understanding the effects of tidal forces because they are relatively small. However, if a fault or region is ready to rupture, it wouldn’t take much to tip the local stress field to the point of rupture. It’s worth noting that there is a much better correlation between the earth’s gravitational pull on the moon and moonquakes. Yes, seismographs have been taken to the moon and have recorded between 300 and 600 ‘moonquakes’ per year. 18. What do tectonic plates have to do with earthquakes? The earth’s surface is made up of 15 huge, rigid plates of rock anywhere from 15 km to 100 km thick. These plates are constantly moving and at their edges they are constantly bumping and grinding into each other. The constant movement causes stresses to build up in the brittle, upper layers of the plates. When the brittle rock finally breaks, it generates an earthquake. Plates, such as the Pacific plate, that carry a limited amount of land mass move the fastest. Under New Zealand, the Pacific plate is moving at about 50 mm a year — about the same rate that your fingernails grow. The entire plate interaction zone is potentially a source of moderate to large earthquakes. The rim of the Pacific Ocean, where the Pacific plate sinks under or slides past other plates, is one of the most active of all the earth’s plate boundaries. Plates don’t jostle haphazardly – they move only in one direction, at least in the short geological time frame. 19. Does the earth really ring like a bell after a big earthquake? Seismic waves from the biggest earthquakes (over magnitude 8.3) can bounce around inside the earth for up to a month. This makes the earth "ring". However, you need special instruments to hear the ring because the tone is very low — about 1 cycle per hour. Compare this with the 256 cycles per second of middle C on the piano 20. What constitutes an active fault? Geologists believe that if a fault shows evidence of having moved at least once in the past 100,000 years, it should be regarded as a potential source of earthquakes. If it has moved at least once in the past 5000 years, then it should be considered a potential source of damaging earthquakes to any settlement within a radius of 50km. Once a major fault has formed, future earthquakes are generated along the same line, and after hundreds of thousands or million of years of movement, increasingly large vertical and horizontal separations of land occur. Repeated earthquakes and their associated fault movements have formed the major mountain ranges of New Zealand. 21. What’s so special about New Zealand’s Alpine Fault? The Alpine Fault, which runs for about 600km up the spine of the South Island, is one of the world’s major geological features. It’s the "on-land" boundary of the Pacific and Australian Plates. It has ruptured four times in the past 900 years, each time producing an earthquake of about magnitude 8. Approximate rupture dates are 1717AD, 1620AD, 1450 AD, and 1100AD. Its rate of horizontal movement is about 30m per 1000 years — very fast by global standards. Each time it has ruptured, it has also moved vertically, lifting the Southern Alps in the process. In the last 12 million years the Southern Alps have been uplifted by an amazing 20,000m, and it’s only the fast pace of erosion that has kept their highest point below 4000m. So the fault has been responsible for building some of New Zealand’s most spectacular scenery. The fault has a high probability of rupturing in the next 40 years. The rupture will produce one of the biggest earthquakes since European settlement of New Zealand, and it will have a major impact on the lives of many people. In between earthquakes, the Alpine Fault is locked. All these things make the Alpine Fault special. 22. Is a fault line always one big long line? Faults can be as short as a few metres and as long as 1000km. The fault rupture from an earthquake isn’t always a straight, or continuous line. Sometimes there can be short offsets between parts of the fault, and even major faults can have large bends in them. 23. Does the direction of rupture influence the size of the impact in different places? Yes. This phenomenon has been observed many times in New Zealand and overseas. Seismic energy gets focused in the same direction as the direction of rupture – a kind of Doppler shift. So if you are unlucky enough to be in the line of fire, a magnitude 6.5 earthquake may hit you with the force of a much bigger quake. Other factors such as topography and rock type can also focus seismic energy in different ways. Many New Zealand faults trend northeast/southwest. This dictates the likely direction of rupture for these faults. Predicting the likely impact of a given magnitude quake at a particular site involves extremely complex three-dimensional mathematics. 24. Is there such a thing as earthquake weather? For many years there have been suggestions that earthquakes occur more often during warm, still, humid weather. This observation is anecdotal and has no basis in science. International records show that earthquake occurrence is spread over all weather conditions, during all seasons of the year, and during both day and night. Scientists have examined the "earthquake weather" proposition many times and have been unable to find a link between weather and earthquakes. . |