Ask an Expert: Alpine Fault fears

Last updated 09:26 06/07/2011

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Quake Science

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When the Alpine Fault goes, and, surmising it is a sub-8 rupture, what would the effect be on buildings and soil in Christchurch? Would it trigger faults to rupture here? - Luke

The best estimate for the rupture of the southern and central section of the Alpine fault (from south of Milford Sound to around Inchbonnie), which appears to have been the extent of past ruptures, is estimated to be near to a magnitude 8 earthquake.

The effects of such an earthquake have been discussed for many years. The consensus is that the strength of shaking would be much less than has been experienced in the recent Canterbury sequence (because of the 100km distance between the fault and Christchurch), but, because of the much larger magnitude, the low-level shaking would probably go on for a long time and could be expected to last for four to five minutes for an Alpine Fault quake. This would very likely induce widespread liquefaction in susceptible sediments (and was taken into account in recent government decisions about retreating from the most susceptible suburbs).

The impact of such an earthquake on well-engineered buildings would not be a problem. The earthquake and its shaking characteristics are incorporated into building code requirements. - Dr Kelvin Berryman, manager, Natural Hazards Research Platform, GNS Science

In the 1960s, there was only open farmland west of Christchurch, with irrigation supplied by above-ground races fed from the Waimakariri. The 1970s started the population move westward and the carve-up of farmland into hundreds of 10, 20 and 30-acre blocks, all with water bores. More recent years have seen large-scale irrigation on to dairy conversions, also taking groundwater. The September quake was just a matter of time, with underground channels now becoming waterless fractures/fissures, linking together, the beginning of a domino effect, land stability-wise. Can you please explain the effect water pressure has re earthquakes? - Anonymous (Australia).

Earthquakes induce changes in groundwater level and scientists observed significant changes in groundwater levels in Christchurch City and Central Canterbury areas after the Darfield earthquake in September. Groundwater level changes caused by the Darfield earthquake were also observed as far afield as Blenheim and Northland.

Conversely, groundwater pressure may impact on earthquakes. The effects of human activity on earthquake activity is termed 'induced seismicity'. Seismicity can be induced by:

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Water storage reservoirs (where large water storage dams impact on groundwater pressures below the dam)

Ground subsidence associated with extraction of fluids (e.g. petroleum and groundwater)

Reinjection of geothermal fluids (where geothermal fluids are pumped into the ground after use for power generation). Typically the magnitude of induced earthquakes is small. These effects are not always observed and sometimes links between cause and effect are difficult to prove. The groundwater system under Canterbury is being "worked harder" by the urban population and rural users. Summer groundwater levels have been declining over time as the city population increases. Drainage of much of Christchurch City since the early days has reduced shallow groundwater levels and groundwater extraction for agricultural use has increased over time.

However, these effects on groundwater volume are unlikely to induce seismicity. This is because these changes in groundwater levels (about 1 to 2m) are a small proportion of the total resource volume as the groundwater system is hundreds of metres thick under the Canterbury Plains. - Dr Paul White, groundwater scientist, GNS Science

I asked a GNS scientist about the effects of the February 22 earthquake on Sumner and surrounding hills. He supported my theory that energy had been released in the area and occurrence of another earthquake of similar magnitude in Sumner would be unlikely (rupture area would move away from original point); the June 13 earthquake proved my theory to be incorrect. My questions are: Since June 13 the likelihood of a magnitude 6 to 7 has gone up - can you explain why? What is the relationship with the epicentre? Can we expect another large magnitude quake around the Sumner area? Professionals say there is still a lot of energy to be released in a series of aftershocks, is this true? - Gavin

The February 22 earthquake epicentre was about 6km west of Sumner, and primarily released energy in a broad area stretching from beneath Cashmere across to beneath Southshore. Some aftershocks of the February 22 earthquake, mostly on the same day, further released stress beneath the Port Hills, between 3 and 5km from Sumner. The June 13 magnitude 6.3 earthquake ruptured close to the eastern end of this earlier aftershock activity, we believe possibly in a NNW-SSE direction, just 2km west of Sumner.

The June 13 magnitude 6.3 earthquake had its own family of aftershocks which help to redistribute stress following the earthquake rupture. However, based on statistical and physical models there is a small possibility for any aftershock to trigger a larger earthquake, which, taken together with the continuing effect of the September earthquake, slightly increases the likelihood of a magnitude 6 to 7 for the region. - Dr Stephen Bannister, seismologist, GNS Science

What is happening in terms of fault lines and splinters beneath Banks Peninsula, particularly the outer areas in an arc from Port Levy right round to the Lake Forsyth outlet? - Evan

A good question. There appear to be several more fractures and fault splinters at depth (the earthquakes originate at depths of 5-10km) than we can see at the ground surface. Although we have have conducted some geophysical surveys to look for some of the faults deep below the surface, we have not surveyed over Banks Peninsula. Remember that not all faults we can (or cannot) see deep in the bedrock will produce a large earthquake in the near future. - Dr Kelvin Berryman, manager, Natural Hazards Research Platform, GNS Science

Some earthquakes have very few aftershocks, especially those that occur in a downgoing subducting slab. Some crustal earthquakes have many aftershocks, and the reasons can vary. One possibility, not necessarily for the Christchurch earthquake, is an earthquake sequence, where aftershocks rattle on for years.

For example, the 1952 Arvin-Tehachapi earthquake of magnitude 7.3 in the California Great Valley is still having aftershocks more than a half century later. Is it possible we could be experiencing aftershocks in 50 years? - Bill

The exact end of an aftershock sequence is difficult to define and depends on various things such as the level of earthquakes that occurred in the region prior to the start of the sequence. What we can say is that the aftershock sequence is likely to go on for decades. However, the rate of aftershocks will decrease significantly; that means that the sequence will not continue as it is now and "felt earthquakes" will occur less and less often. We cannot say the exact numbers for sure, but within one year there may be around one or two magnitude 4 quakes per month and they will continue to become less frequent. - Dr Matt Gerstenberger, seismologist/geohazard modeller, GNS Science

* Email your question to Vicki.Anderson@press.co.nz Because of the volume of questions posed, see press.co.nz for more questions and answers.

- The Press

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