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Frequently Asked Questions about the Canterbury earthquakes.

 

  • Q: Why was it called the Darfield earthquake? And why don't you mention all the other localities when you report the aftershocks?
  • A: Although the magnitude 7.1 earthquake impacted much further afield than just Darfield, the scientific community named the earthquake after the nearest locality to the epicentre soon after the earthquake happened. It can now be argued that other communities or localities were in fact closer, but that is what was chosen on the day. We chose a unique name so that no confusion can arise with any previous earthquakes. In the past regional names were used, such as the Wairarapa earthquakes of 1942, but this forced us to number those earthquakes, leading to possible mix-ups if the numbers were left off. To the people of New Zealand, though, this was indeed the Canterbury earthquake.We have been specifically asked to send out the locations of aftershocks with reference to a single place, so that is is easier for people to visualise where they are occurring in relation to each other; it made sense to choose Darfield, and now Christchurch, for this. There is no intended slight to other communities that may indeed be closer to the epicentre of these aftershocks.We do not usually name aftershocks, but the overwhelming impact of the magnitude 6.3 Christchurch earthquake makes it exceptional.
  • Q: So why do you call the Christchurch earthquake an aftershock; surely it is a major earthquake in its own right?
  • A: Seismologically it is classed as an aftershock because of its relationship to the ongoing earthquake activity since September 2010. Its occurrence was always statistically possible, but the long time interval and slow decrease in general activity had made it less likely. Unfortunately, it did happen and in a location that brought possibly the worst result to the city of Christchurch.
  • Q: Why was the Christchurch earthquake so damaging; wasn't it smaller?
  • A: The city had been comparatively lucky with both the location 40 km to the west and the early morning timing of September 2010's magnitude 7.1 Darfield (Canterbury) earthquake; the location of this one within 10 km of the city and at a shallow depth of 5 km during the middle of a working day resulted in destruction, injuries and deaths.
  • Q: Was the Christchurch earthquake on an extension of the Greendale Fault?
  • A: Analysis of seismograph records of the devastating magnitude 6.3 earthquake show that it was just 9 km from the centre of Christchurch on a buried fault oriented roughly east-west. There is no obvious structure directly connecting the faults that ruptured in the September 2010’s magnitude 7.1 earthquake with the fault that generated the magnitude 6.3 event. On the contrary, precise aftershock relocations suggest that at least two north-east/south-west trending faults lie between the two and that there is no evidence from the earthquake data of an extension of the Greendale Fault.
  • Q: Where can I find a list of earthquakes in Canterbury, and if I fell an earthquake but isn't listed on your website what can I do?
  • A: You can look at the earthquakes that have occurred in the Canterbury region on our GeoNet Rapid website. You can look at earthquakes likely to have been felt in the region, or look at all earthquakes that have occurred in Canterbury.  You can also look at earthquake statistics for the region.
  • Q: Can I get traces of the drums for the 7.1 Darfield and 6.3 Christchurch earthquakes?
  • A: The images on the right show the Darfield and Christchurch earthquakes and some of their early aftershocks recorded at McQueen's Valley (MQZ) seismograph on Banks Peninsula. The seismogram is coloured red if it is clipped, i.e. the largest parts of the signal are not shown. If this was not done, then a large earthquake would obscure much of the seismogram from view - so if the signal is red, the real size is larger than shown. Lots of aftershocks can be seen on these images, with some of the biggest ones appearing out of the coda (dying away of shaking) of the mainshock.

  • Q: How long was that earthquake? Why don't you tell us the duration of it?
  • A: Defining the length of shaking is not routinely calculated. Seismometers are very sensitive instruments that will detect shaking for a much longer time than the human perception of shaking. How long shaking may be detected by a person will depend on (but is not limited to) several criteria:

    The type of building you are in and what floor you're on
    • Your local geology
    • The earthquake magnitude
    • The distance from where you are to where the earthquake is located
    • The depth of the earthquake

    Especially in earthquakes of this size, where aftershocks become so closely spaced in time (see image), the shaking appears to last a lot longer. It's not just one earthquake, but many strung together. So there is no single answer to, "How long did the earthquake last?".

  • Q: How many aftershocks have there been? When will they stop?
  • A: It depends! We're working through our data to locate all the magnitude threes; this will take some months. As the numbers of earthquakes go up by roughly a factor of 10 for each of the ranges, we expect to locate 1000-2000 magnitude threes over the course of the entire aftershock sequence. So that means there are probably up to 20,000 magnitude twos! We have to strike a balance between the people we have to do the work and what gives us the best scientific outcome. As we already locate 15,000 earthquakes in a typical year without a Darfield-type aftershock sequence, we are setting the bar at locating magnitudes 3.0 and above. Bear in mind also that prising out individual earthquakes from very busy looking seismograms makes the job even slower than normal.

  • Q: I heard a loud rumble, but felt no shaking.
  • A: When a small earthquake is very shallow and very close, people can sometimes hear them without feeling them. The first wave arriving from an earthquake is a compressional wave (P-wave). When this wave hits the ground surface, it can produce a sound wave in the air which people hear. This effect is more pronounced when hills are around where the sound can reverberate. For larger, close earthquakes, this same effect accounts for people sometimes hearing earthquakes coming before feeling them. In this case, the P-wave causes the noise, and the slower shear wave (S-wave), which carries more energy, causes the shaking.
  • Q: Akaroa and Lyttelton Volcanoes sit on the Banks Peninsula. What are the chances that the earthquakes will trigger these volcanoes to erupt?
  • A: The Banks Peninsula volcanoes are long extinct with the last eruptions around 6 or so million years ago. There is no chance of them erupting again as the plate boundary conditions that allowed them to form in the first place no longer exist. 6 Million years is a long time in a volcano sense and volcanism that once existed along the east coast of the south island has long finished. All current and future volcanic activity in NZ will be located in the North Island.

    If volcano unrest does occur we see quite different types of earthquakes from what we are seeing now. During volcano unrest we see earthquakes that indicate magma movement which are quite distinct from the tectonic earthquakes we see now.

    All the scientific modelling of the data over the months shows conclusively that the Canterbury earthquakes are due to fault movement only. There is no volcanic component to them at all. The ground deformation has also been modelled and this shows only fault related movement and nothing to suggest intrusion of magma beneath the Peninsula. Again if this were the case it would produce a quite distinct signature much different to what has been recorded.

    Any hot springs in the area are also not volcano related, again water that is part of a volcanic system has quite different chemical make up to "non volcanic" hot springs and the two can be easily distinguished. The fact that there are old volcanoes near to where the earthquakes are located is really just coincidence.

  • Q: What are the risks from any tsunami that may be caused by earthquakes in this region?
  • A: There was no tsunami associated with the M6.3 aftershock because it happened on land. 

    Many of the aftershocks from the September 2010 earthquake are occurring to the east of the Greendale fault (this includes the M6.3 quake). There is the potential for an earthquake to occur off the coast from Christchurch, however in order to generate a tsunami, an earthquake would generally have to be larger than M6.5 (although the tsunami generated would not necessarily be destructive). Preliminary water modelling by GNS tsunami scientists shows that likely wave run-ups at Christchurch beaches from the M7.1 and M6.3 events, had they occurred offshore, would be no more than 2m and 0.5m, respectively. In unfavourable situations, in particular two north-facing bays on Banks Peninsula, these waves could have run-ups as high as 3-4m and 2m, respectively, had the earthquake been offshore.

    It is very unlikely that any of the aftershocks around Christchurch would be big enough to cause a tsunami because they wouldn't cause displacement of the sea floor.

    Authorities will give warnings about tsunami if they are coming from far away. Turn on your radio and follow instructions. Take essential items if you are told to evacuate, and do not go to the beach to watch the waves. If you are near a beach and feel a strong earthquake or notice the water receding unusually, move to higher ground immediately. If you cannot go to higher ground, go at least 1.5 km inland. Do not return for at least one hour or until advised by authorities.

 

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