Ask an Expert: Faultline fears and P-waves

Last updated 12:43 29/06/2011
Telegraph Road, near Burnham
Chris Rudge
September’s quake shifted a section of Telegraph Road, near Burnham, four metres to the right.

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We have been told the potential size of an earthquake is proportionate to the length of the faultline that caused it. The September earthquake of 7.1 was generated on a fault of an unknown length because it was not known it was there.

However, my granddaughters have joined up the consequent quake dots on Quakemap and find that the original fault may, in fact, now be at least four or five times longer. The September quake may have been a foreshock leading up to a rupture of the much longer fault.

This fault could now run from Methven to offshore Canterbury. What would be the potential quake size of a fault this long? - Ray

You have some excellent points, but also some missing information. You are right to say the maximum earthquake capable of being generated by a fault movement will be proportional to its length.

Concerning the Greendale Fault that ruptured to produce the major component of the magnitude-7.1 September 2010 earthquake (there were at least two other smaller faults that ruptured as well), we are fairly confident the fault is not much longer than what ruptured in September, and it is unlikely it can produce another major earthquake until the strain builds up again over many thousands of years.

At the western end of the Greendale Fault it comes up to other faults along the edge of the foothills, so cannot be much longer in that direction. At the eastern end, the big cloud of aftershocks around Rolleston strongly suggests the fault stops there, but to check this some seismic soundings have been taken to check for any underground extension.

We expect the results of those seismic soundings in about three weeks, and these will be reported when we have them. - Dr Kelvin Berryman, Manager, natural hazards research platform, GNS Science

Since September, the earthquakes or multiple faultlines have slowly but surely progressed towards the ocean. The fault or faults are now concentrating around Banks Peninsula, suggesting the volcanic strata has now been penetrated with shocks surfacing in Sumner, Taylors Mistake, Lyttelton, Godley Head, Boulder Bay and a mass of shocks within one nautical mile offshore.

This would suggest the strata of the peninsula is vulnerable to further larger shocks and will increase with momentum and intensity and progressively move further offshore.

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Given there is an extension of the Kaikoura sea trench leading south into Pegasus Bay, it seems a safe yet daunting assumption to predict is that the faultline or multiple faultlines are moving towards the sea trench.

Would this indicate a possible imminent continental-plate shift?

If Canterbury suffers a mass tectonic plate shift (as Japan), a tsunami wave of minimum height one metre will wipe out every coastal community along Pegasus Bay.

- Spencer

Each earthquake changes the stress on other faults nearby, which can lead to a sequence of earthquakes as has occurred recently. For the size of earthquakes that have occurred in Canterbury, these stress changes are mostly confined to the Christchurch/ Selwyn/Waimakariri districts onshore and Pegasus Bay offshore. Any stress changes on the boundary between the Pacific and Australian tectonic plates resulting from these earthquakes are extremely small. In the circumstances, the most plausible scenarios for further earthquakes in the current sequence occurring offshore is for events of similar magnitude and mechanism to those that have already occurred.

GNS Science has modelled examples of tsunami from such events and the modelled wave run-up heights were typically in the 0.5-2m range around Pegasus Bay.

However, funnelling effects at the heads of bays on the north coast of Banks Peninsula could cause wave run-up heights in the 2-4m range. Even relatively small tsunamis, such as those in these scenario events, are a danger to people on the coast and in small boats. There are other less likely scenarios (such as an earthquake- triggered submarine landslide, or an earthquake on a fault not linked to the current sequence) in which larger tsunamis are possible in Pegasus Bay. So it is always recommended to move to higher ground if you are close to the coast when a strong earthquake occurs. ECan has useful information about tsunami hazard in Canterbury at this link. http:/ /ecan.govt.nz/advice/ emergencies-and-hazard/ pages/booklets-q-files.aspx - Dr William Power, geophysicist/tsunami modeller, GNS Science

What are the different causes of the four effects: 1. Shaking sideways. 2. Slamming up and down. 3. Rolling waves of land. 4. Roaring sound. We've experienced all these at one time or another.

- Jim Riley

When an earthquake occurs, a sequence of waves are emitted. These waves travel in different ways and at different velocities, sometimes meaning their effects are separable.

The first waves that an observer feels after an earthquake are the P-waves. These are compressional waves, analogous to soundwaves. When they arrive, the observer will feel a punchy, quick movement.

Depending on the location of the observer relative to the earthquake's hypocentre, the waves will be felt in an up-down or inclined to-fro motion.

The second waves to arrive are the S-waves. These waves travel by a shear (or side-to-side) traction of the energy. In the Canterbury aftershocks, these are the dominant "sideways" shaking waves.

Following the P- and S-waves, the more slowly travelling surface waves arrive. They are experienced first with a mostly sideways "rolling" feeling. Later arriving surface waves are manifest as a combination of vertical and back-and-forth motion. These waves are analogous to ocean waves.

In the larger events, Christchurch has also experienced even later-arriving waves (that greatly extend the duration of shaking) that are caused by "basin" and "site" effects, including reflections of the energy off the bottom of the basin (at about 600m depth) and non-linear motions (eg liquefaction).

They are felt as pulses between about one and three seconds in duration. Particularly strong (local) waves are capable of generating soundwaves when there is enough "coupling" between the earth and the atmosphere. - Dr Bill Fry, seismologist, GNS Science

I have noted that when details of each earthquake are made known they relate to strength etc, but duration is never mentioned. I would believe that while any quake of higher magnitude can cause damage, the amount of damage caused would be relative to the length of time the quake is doing its thing. Is there a reason why the duration is never mentioned?- Ross Butler

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): 1. The type of building you are in and what floor you're on; 2. Your local geology; 3. The earthquake magnitude; 4. The distance between you and the earthquake epicentre; and 5. The depth of the earthquake.

Because there are so many variables, there is no one figure of "duration" that will be correct for every person who feels an earthquake. - Jennifer Coppola, GeoNet data technician, GNS Science

Will all these faultlines become dormant again for another 10,000+ years once things settle down? - L Lawrence

The sequence of quakes occurring in Canterbury at present is a rare event that might only happen once every 10,000 years or more. The present understanding is that when the stress in the crust under Canterbury finally equilibrates and seismic activity returns to pre-September 4 levels, the faults under Canterbury will become quiet again and start accumulating strain that will be released in another sequence of quakes a long way into the future. - (various scientists)

GNS scientist Dick Beetham has apologised for making incorrect statements last week regarding liquefaction in Kaiapoi.

"Several concerned residents from Kaiapoi have contacted me, as has the Mayor, to give me better information. I now understand from them that liquefaction has occurred three times in Kaiapoi: During the September 4 quake (I understand this quake caused the most severe liquefaction and lateral spreading in Kaiapoi); during the February 22 quake, but residents inform me this was not as severe and damaging as in September; and again during the June 13 earthquake, although a little less than in February," Beetham said.

* Email your questions to Vicki.Anderson@ press.co.nz

- The Press

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