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NIWA to investigate possible marine faults near Chch

NIWA’s research vessel Kaharoa set sail from Wellington today, destined for Lyttelton and equipped with seismic survey gear to survey an area of southern Pegasus Bay.

The seismic survey work being undertaken by NIWA is part of Project Offshore, a part of the effort by the Natural Hazards Research Platform in supporting Response and Recovery to the Christchurch earthquake. The survey results will assist in the understanding of future earthquake in the Canterbury region, in preparation for rebuilding the city.

The scientists will identify whether there are any faults in the coastal marine area, that could lead to undersea earthquakes off the Canterbury Coast. The survey will help with understanding the broader fault zone along which the Christchurch faults have ruptured.

NIWA’s principal scientist and voyage leader Dr Philip Barnes says that the scientists “are trying to identify if there are any faults that represent, or are part of, this fault zone, that potentially extends off the coast in the Banks Peninsula area.

On Thursday, the scientists will begin a seismic survey of southern Pegasus Bay, in the Banks Peninsula area. They will survey out to 40 km east of the New Brighton Coast, up to 30 km north of Lyttelton heads. The six scientists, including an Otago University scientist, and six crew will be at sea for up to 7 days.

The scientists are determining what is in the area, and the likelihood of rupture occurring on an offshore fault.

“We don’t know what we will find. We could find no faulting. There is no evidence for a fault zone off shore, based on existing information,” says Dr Barnes. The area has not yet been comprehensively surveyed, and there is very little marine geological information around Christchurch, Banks Peninsula, and Sumner. Dr Barnes conducted the last survey work in Pegasus Bay nearly twenty years ago.

On board Kaharoa, the scientists will be collecting seismic reflection data using multi-channel seismic gear and a seismic boomer. Both seismic acoustic systems project sounds down into the water and then look at the sounds reflected back from the seafloor and from layers (immediately) underneath.

The seismic boomer is designed for looking at shallower depths, up to 100 metres, with high resolution. “If there was a fault that had ruptured up to the sea floor, we can see it in the seafloor images,” says Dr Barnes.

“The multi-channel seismic equipment enables us to look deeper into the sea floor – 1 to 1.5 kilometres down, to see any hidden faults, buried beneath the sea floor, that we wouldn’t see with just the surface imagery,” says Dr Barnes.

The combination of multi-channel and seismic boomer enables the scientists to look at the upper 1.5 km zone, at faults that haven’t been activated or reactivated, for some time. “If the fault has not ruptured in the past tens of thousands of years, we won’t see anything,” says Dr Barnes.

The faults that have been rupturing in the recent Christchurch earthquakes are very old faults that have been reactivated.

Dr Barnes says that if the scientists “find a new fault, we will try and focus in on that to map it accurately. The new data will be combined with data held by GNS Science and partners – to look at the likelihood of future events.”

By merging this information with similar records from on land, the scientists can learn how the different faults relate to each other. A history of ‘when, where and how often’ can be developed for each fault. This can be used to model earthquake risk.



steve on 18/03/2011 11:33am

Quote: The faults that have been rupturing in the recent Christchurch earthquakes are very old faults that have been reactivated.

I’m intrigued by this statement. What were the 1869 Christchurch Earthquake and the 1870 “Lake Ellesmere” Earthquake but not reactivation of these same or very similar fault lines. Possibly slightly smaller events, but from the same essential locations.

Surely stating any other case as above is but a splitting of hairs.

When is this issue to be addressed by the various professions, for up to now there seems to be a consistent effort to almost deny the very existence of these historical earthquakes.

June 5 1869, “Christchurch” earthquake1
Since 1853, when organised European settlement of Christchurch and Lyttelton began, ten or so earthquakes are known to have shaken Christchurch city strongly enough to cause chimney and minor building damage and/or significant contents damage (i.e. caused shaking intensities of MM 6 or more). Among these are large earthquakes, centred some distance from Christchurch.
Only one earthquake is known to have caused pervasive building and chimney damage throughout the CBD and nearby suburbs, encompassing Avonside, Linwood, Fendalton and Papanui. This event was the 1869 June 5 8:00 am (local time).
The highest intensity of shaking was clearly confined to within a few kilometres of Christchurch CBD. Contents damage occurred in many shops and homes, and many chimneys and chimney tops fell or were cracked. The stone spire of St John’s church in Latimer Square was cracked from top to bottom, reportedly the worst damage in the city. In the Government buildings, the tops of two chimneys came down, plaster was cracked, and several stones displaced. Similar damage occurred in some other brick and stone buildings, including Matson’s building, the NZ Loan & Trust building and the NZ Insurance building. Avonside and more generally, the area north of the Avon River and east of and along Papanui Road, were most affected with many chimneys damaged or down. The side of one brick house collapsed in Manchester Street. The relatively low level of masonry damage in the city is consistent with MM 7 rather than the MM 7 Р8 assigned by Dibble et al. (1980).
Outside the city intensities rapidly decreased, reaching MM 5 at Kaiapoi and Halswell. However, a few chimneys and household contents were damaged at Lyttelton.
The short duration of shaking, small spatial extent of MM 7, the occurrence of aftershocks and the rapid attenuation are consistent with an earthquake at very shallow depth within a few kilometres of the CBD.
August 31 1870, “Lake Ellesmere” earthquake1
The 1870 earthquake, on August 31 at 6:53 pm (local time) was more widely felt than the 1869 earthquake, with the highest intensities around Christchurch, at locations on Banks Peninsula and South Canterbury, but felt at least as far south as Dunedin, on the West Coast and North Canterbury.
In Christchurch city, there was damage to household and shop contents as well as isolated chimney damage, e.g. one or two chimneys fell in Avonside, one in Chester Street and another in Papanui Road. The upper parts of several chimneys were rotated near the railway station. There was also isolated minor structural damage. At St John’s church in Latimer Square the stone cross fell, causing damage to several roof slates, and some old cracks opened in the stonework. Old cracks in the Town Hall also moved. The damage is consistent with MM 6 at most. Several chimneys, as well as household goods, were damaged at Lyttelton, about Banks Peninsula and in South Canterbury as far south as Timaru, the intensity not exceeding MM 6, except possibly at Temuka, and the eastern side of Lyttelton Harbour.
The spatial distribution of intensities indicates an earthquake at greater depth than the 1869 earthquake. This is consistent with the reported longer duration of strong shaking, the occurrence of only one or two slight aftershocks and observations of the two shocks separated by a short interval that are almost certainly P and S waves.
1extracts courtesy of Downes & Yetton (in prep.)

DazzaMc on 16/03/2011 10:16pm

I have being a little concerned about the risk of a offshore quake. If you look at the aftershocks you’ll note that they tend to (by large) stop at the beach, it’s almost like they have hit a ‘wall’. It will be great to get some data on it!

steve on 18/03/2011 11:54am

Yes indeed. Upon the predicate that earthquakes and faultlines are co-existent, and that several aftershocks have had epicentral positions located offshore, what we need to know is whether any of these recently defined earthquake/microfaulting events have the potential to run on further out to sea. I guess this is what NIWA are basically investigating.

Guest on 23/03/2011 6:11am

…Ken ring told them to look off the coast….GNS probly got there guess for the “aftershock” off his homepage too…….

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