Geologists probe fault near Dunedin to improve knowledge of its past behaviour - 22/04/2016
Geologists working to improve the understanding of Dunedin’s potential exposure to earthquakes have opened up a fault south of the city in a bid to find out about its past rupture behaviour.
The Titri Fault is about 50km long and runs northeast from the Milton area to the southwestern outskirts of Dunedin City. At its closest, it comes within about 10km of the city centre.
Movement on the Titri Fault is vertical rather than horizontal, with upthrow on the southeastern side of the fault. Movements (ruptures) occur as a sudden jolt, following a long build-up of strain deep in the Earth’s crust. For each fault in the Otago region, the period of strain build-up between earthquakes is thought to typically last for thousands of years.
Another similar fault, the Akatore Fault, lies parallel to the Titri Fault but is closer to the coast. The Akatore Fault last moved in the recent prehistoric past, only about 1000 to 1200 years ago.
Innumerable earthquakes on the Titri Fault, spread over the past few million years, have uplifted the range of hills that separates the Taieri and Tokomairiro plains from the coast. This uplifted range of hills includes the Chain Hills, Saddle Hill, and Scroggs Hill.
Investigations carried out between the late 1990s and 2004 indicate the Titri Fault is potentially capable of producing an earthquake of as much as Magnitude 7.2.
There is currently not enough information on the fault for geologists to know with confidence when it last ruptured, or whether ruptures are fairly evenly spaced in time, or if they can occur in clusters.
Understanding and quantifying clustering on individual faults will greatly aid the accurate and meaningful estimation of earthquake hazard and risk in the Otago region, and perhaps more generally in New Zealand. It will also apply to other areas worldwide that have low to moderate earthquake activity
So three organisations - GNS Science, University of Otago, Victoria University of Wellington - won funding from the Earthquake Commission’s Biennial Grants Programme to investigate the Titri Fault by digging across it with the aim of identifying the size and age of its most recent movements.
Last week they dug a 4m-deep trench across the fault in two places – one near Milton and the other near Clarendon – and found evidence in the walls of the trench of the two most recent ruptures.
They estimate that the ruptures occurred several tens of thousands of years ago. They have collected sediment samples that will be analysed by two different age dating techniques – Optically Stimulated Luminescence and radiocarbon dating. Results are expected by early 2017.
The two most recent ruptures involved compressional (thrust) movement, totalling about 4m of vertical offset, on the southeastern side of the fault.
Project leader David Barrell, of GNS Science, said quantifying the behaviour of the Titri Fault and other active faults in Otago will lead to a more accurate estimation of earthquake hazard for the region.
This would have a number of positive spinoffs including better inputs for structural engineering, and an evidence-based approach to disaster preparedness and urban planning.
Mr Barrell said the project was consistent with the recommendation from the Canterbury Earthquakes Royal Commission that ‘Research continues into the location of active faults near Christchurch and other population centres in New Zealand to build as complete a picture as possible for cities and major towns’.
However, he said the challenge for geologists was to determine the degree to which the faults in the region were active and whether there are long intervals in between ruptures or if they occasionally have episodes where earthquakes cluster together.
Previous investigations of several other active faults in Otago have uncovered evidence of earthquake ‘clustering’ and geologists are keen to know if this behaviour is common to many faults in the region or if it is isolated to just a few faults.
“Understanding and quantifying clustering on individual faults will greatly aid the accurate and meaningful estimation of earthquake hazard and risk in the Otago region, and perhaps more generally in New Zealand. It will also apply to other areas worldwide that have low to moderate earthquake activity,” said Mr Barrell.
“Results of this investigation will be communicated to the Dunedin City Council, the Clutha District Council, and the Otago Regional Council. We will also make the information available to the mainstream media and stage at least one public lecture.”
Q & A
What effect do the initial findings of this investigation have on the earthquake risk of Dunedin?
The Titri Fault is not currently included in the National Seismic Hazard Model, which is used to calculate earthquake risk for New Zealand. But because initial results suggest that the rate of activity on the Titri Fault is relatively low, any increase to the earthquake risk in the Dunedin area will probably be very slight.
Why will the final results not be available until 2017?
Most of the dating will be done using the Optically Stimulated Luminescence (OSL) method, which has a long turnaround time because it requires accurate measurement of minuscule levels of background natural radioactivity emanating from trace amounts of naturally-occurring radioactive elements in the geological sediments.
Why are two different dating techniques being used in this investigation?
Radiocarbon is a more accurate dating method, but can only be done on plant or bird fossil material, and it can be hard to find suitable material to date. OSL dating is applied to samples from sand or silt layers, which are very common in geological sediments in the Otago region, but the measured ages have a larger plus or minus than with radiocarbon dating. We use radiocarbon wherever possible, but in Otago, OSL is the mainstay method because sand and silt material is so abundant.
Overall, how does the earthquake risk in Dunedin compare to other parts of New Zealand?
On average, near-coastal areas of the southeastern South Island, including Dunedin, are regarded as being one of the lowest risk areas of New Zealand for strong earthquake shaking. Only the northern third of the North Island is regarded as having a lesser risk.