Lake Grassmere Quakes

Following the earthquakes in southern cook straight, the GeoNet rapid response team left immediately to place seismometers around the area, to allow more detailed monitoring and get better information with which to model the fault ruptures.

This meant that when the Mag 6.6 occurred, the enhanced array of seismometers was already in place.

Here is a screen shot of the Mag 6.6 Grassmere Earthquake and immediate aftershocks over the following hours:


This GeoNet video gives an idea of the number and locations of aftershocks from the 16th to19th August

Earthquakes in Southern Cook Strait

This is a screen shot of the Wellington seismometer from very early on Monday morning 22 July

Following the earthquakes in Cook Strait over the weekend, it was impressive to arrive at work on Monday morning, and watch how the GeoNet team, many of whom had been busy right through the weekend, were in full action mode again. Ken Gledhill, the head of GeoNet, co-ordinated two meetings of the scientists and technicians during the day.

There are lots of different things involved with understanding earthquakes. These include getting accurate locations and magnitudes, modelling the position and orientation of the fault and the type of fault rupture from the seismic wave patterns of the aftershocks, working out the adjusted stress on nearby active faults and then trying to calculate probabilities of future quakes to inform an 'awakened' public...

Here is the aftershock map from Sunday evening that shows the relative magnitudes of the quakes and their initial locations in 2 dimensions. Up to today there have been nearly one thousand aftershocks already. As more and more occur and get analysed, a more precise 3D image of the fault(s) involved will be built up.

To help get more precise seismic data, the GeoNet fast response team are already in Marlborough, setting up some extra temporary seismometers at carefully chosen locations to 'fill in the gaps' between existing permanent stations. Here is a photo of one of the team yesterday, packing one of the seismometers for the trip. To see more photos of what these guys do, have a look here.


This is a computer simulation of the seismic waves from the M6.5 'quake propagating across the North Island and the adjacent sea floor:



New Zealand's background risk of earthquake probabilities has been calculated for the whole country. Once a reasonably large earthquake has occurred, these background risks of a larger quake increase for a while in the local area, and  a sequence of aftershocks follows that typically fits into a fairly predictable pattern of decreasing intensity over time.

In the video below Matt Gerstenberger explains how these calculations are made to produce probability tables and maps for future aftershocks.



For the latest information about the number and magnitude of aftershocks that have occurred in Cook Strait, as well as forecast probabilities for future quakes, have a look at this GeoNet page

Turakirae

The windswept coastline between Wellington Harbour and Palliser Bay forms the southern tip of the Rimutaka Ranges. These hills themselves are an extension of the axial ranges that stretch the length of the North Island. This is the view east from Baring Head towards Turakirae Head.

In good weather, this rugged environment is
the best place in the Wellington area for bouldering (low level rock climbing), but it is also a spectacular place to witness the effects of tectonic uplift on a coastline.

From the end of the Wainuiomata Coast Road, follow the track to Turakirae Head seal colony,  which is about 40 minute's walk. On the way you may notice that there are gentle steps in the landscape running parallel to the shoreline.

The lines that you can see in this aerial photo are ridges of washed up rocks that have been gradually piled up during many southerly storms. The reason that there are several storm ridges is that the coast has been uplifted by successive earthquakes, thus pushing the shoreline further out  and causing the creation of a new ridge after each event.

At least 5 ridges have been identified. Carbon dating of shells and plant material in the ridges shows that the oldest one (furthest from the sea) is over 7000 years old, with others (shown in the image) dating back to 5000, 2300, 158 years and present day. These are not thought to represent all the uplift events that have affected the area over that time, but simply the ones that have been well preserved.

The most recent uplift was during the 1855 earthquake. This involved  a massive rupture along the Wairarapa Fault that passes very close to Turakirae Head. It was New Zealand's largest historic 'quake, with an estimated magnitude of 8.2. It caused widespread damage, such as numerous massive slips in the Rimutakas, but fortunately few fatalities. A similar magnitude earthquake in Wellington nowadays might be a different matter simply because of the denser population and more developed infrastructure..

For more information on the Wellington earthquake hazard check out the GNS website here

Turakirae Head featured on the Coasters programme recently, hosted by Steve Logan who met me there with his film crew from Fisheye Films on his way along to Palliser Bay. The flat path like line extending into the middle distance is the top of  the pre 1855 storm ridge. Although very rocky it makes for a reasonable 4WD or walking track.

Steve came along on his pushbike and interviewed me about the geological features of Turakirae Head, as well as about its rock climbing attractions.
In case you missed the programme on TV1 on Saturday 22nd June, you can watch it online here.

As well as Steve and the crew from Fisheye Films, we were accompanied by Sophie and Frank (right in  photo) - two local Lower Hutt climbers who were part of the support team.